VK4ZXI

Commercial DVB-T Amplifiers and filters and implications for DATV (first draft)

Introduction

Commercial solid-state DVB-T amplifiers use about 250 W "pallet" amplifiers; a pair of amplifiers, each using a pair of LMDOS transistors in a single package. The circuit boards are mounted on a piece of thick copper then on a heat sink. To get higher power, 1 to 50 KW, many pallet amplifiers are used in parallel with a system of splitters and combiners. The amplifier output passes through a series of filters, to stay within a standardized spectrum mask to limit adjacent channel interference. To conclude I note some implications for DATV.

Pallet amplifers

A typical pallet amplifier is pictured below (bought on eBay). The gold rectangle on the left splits the input to a pair of Doherty amplifiers, while the one on the right combines the amplified signal to the output. The combiner uses a type of circulator to dump RF to a dummy load, the black rectangles on either side of the combiner, that doesn't go to the output, so it does not go to the other amplifier.

The white rectangles are the LMDOS transisters in this case BLF888A. Each contains a matched pair of Mosfets. Each amplifier is a modified form of class B, with a pot to adjust the quiescent current for each to improve linearity. Each amplifier has impedance matching on the input and output. A fairly conventional RF amplifier widely used in amateur radio.

LMDOS Transistor

A snip from the data sheet for a modern form of the same transistor is below. It is specifically designed for broadcast TV, with three versions for the three TV bands and runs on 50 V.

Of significant interest is the 150 W at 50 percent efficiency on DVB-T. This would appear to be an anomaly as it is rated as a 750 W device. The anomaly comes down to how the power is measured, a long-standing misunderstanding as to how DVB-T power is measured. I will discuss this in another post, but the 750 W is for single CW carrier, compared to the RMS value of the nearly 8000 carriers in the envelope of an 8 MHz DVB-T channel.

The PAR (peak-to-average ratio) value of 8 dB is the operating parameter of the amplifier, allowing only 8 dB of Crest factor before clipping. Crest factor can be as high as 38 dB. The amplifier deliberately clips the crests to 8 dB. The reason for this is that the Crest Factor is statistical. With a large number of pallet amplifiers, the crests add up potentially to give extremely high peaks, with voltages that would damage RF equipment after the amplifier. A PAR of only 8 dB will create significant intermodulation skirts that need to be filtered out.

Complete transmitter

The block diagram of a high power amplifier using multiple pallet amplifiers and drivers. in this case there are 8 pallets per block and 8 blocks, giving 64 pallets in all. The output would be about 64 x about 250 W or probably 15 kW total.

There is a filter to remove the shoulders or skirts to an acceptable level. The filters use up to 8 cavity filters of different types, I believe including some notch filters per my 70 cm amplifier filter.

There is also a harmonics filter as cavity filters are resonant at odd harmonics of the main signal. Power and cooling are major issues.

TV stations run with at least one reserve amplifier, sometime another at a different site, such as at another channels tower.

https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_news_from_rs/172/n172_mpeg2.pdf

Spectrum masks

Spectrum masks are mandated for commercial TV stations to limit adjacent channel interference, as shown below.

The spectrum masks are achieved using the cavity filters discussed in the last section. There are two masks, Critical masks with a 50 dB limit for skirt for stations that have adjacent channels, as is common in metropolitan areas. Uncritical masks are less strict at 40 dB for skirts, and are used for isolated stations common in rural areas where the interference is less of a problem.

Measuring sideband emissions of T-DAB and DVB-T transmitters for monitoring purposes Rec. ITU-R SM.1792 1, RECOMMENDATION ITU-R SM.1792

Implications for DATV

There are a number of implications for DATV particularly those using DVB-T.

DVB-T amplifier efficiency is higher than commonly understood because of the different way it is measured, just like the difference between measurement of CW and SSB amplifiers.

Commercial DVB-T amplifiers are driven much harder than most DATV amplifiers, with low PAR to clip crests, but create bigger shoulders or skirts. This can be done with DATV amplifiers, provided filters are used.

Filters to remove intermodulation skirts or shoulder are mandatory. They could be used in DATV to reduce interference and to get more power. They are probably necessary for multiple lower bandwidth channels, 2 MHz, adjacent in a band.

Power measurements of DVB-T transmitters- first draft

Introduction

I have been puzzled for some time about how power of DVB-T amplifiers were measured relative to other modes. This is an important issue in DATV as DVB-T has been criticized as inefficient compared to DVB-S. It would appear that some of the debate comes down to how the power is measured. Most amateurs use simple diode power meters that do not give an accurate reading for DVB-T.

In this post, I outline the theory and practice of measuring DVB-T power correctly with either a thermal power sensor meter or envelope power using a spectrum analyser. It is important that the correct measurements be used otherwise it is comparing apples with oranges. Different power measurements are accepted in amateur radio, vis, CW (peak power, key down) compared to SSB (peak envelope power).

I wonder if another measurement unit for digital modes may be more appropriate, such as the data rate to the DC power input and spectrum width, bits/sec Watt Hz perhaps?

Theory of power measurement of DVB-T transmitters

Others have written this better than I can, so I use a direct quote:

“The output signal of a DVB-T transmitter consists of thousands of carriers modulated in phase and amplitude. Therefore it resembles a Gaussian noise signal. It should be noted, however, that very high peaks of the sum signal are limited due to effects in the process of generating and amplifying the signal. The only simple way to define the power of a COFDM signal like DVB-T is an RMS definition. It is also closely linked to the theoretical system analysis.

As the number of carriers of a given DVB-T system (either 2k or 8k) is constant and all carriers have defined power, the total power of a DVB signal is the sum of all carrier power values. In practice only the total power can be measured. In principle one symbol is insufficient for assessing the power. With thermal power meters the integration time constant is much larger than a symbol period allowing valid measurements”. Implementation guidelines for DVB terrestrial services; Transmission aspects, Digital Video Broadcasting (DVB); RTR/JTC-DVB-304

The actual number of number of carriers for 2k is 1,705 carriers and for 8k, 6,817 carriers.

Practical power measurement of DVB-T amplifiers

Power measurement is a very complex topic. It is covered in many books and application notes available on the web. It is also very important. Try reading about satellite or cell phone power and efficiency!

Simple diode power measurements used by amateurs in SWR meters are designed for measuring CW or SSB, not complex wide-band digital TV signals, including both DVB-T and DVB-S.

Again, I use a direct quote on the practical measurement of DVB-T power:

“Power measurements on DVB-T transmitters: Mean power measurements

In the case of analog transmitters, signal power is determined by measuring the peak power of the sync pulse floor of the modulated CCVS signal. The sync pulse floor is always the reference in analog TV because this signal component must be transmitted without compression or distortion.

In DVB this is different: the “Sync 1 Inversion and Randomization” block of the DVB modulator... ensures constant mean power of the transmitter output signal. In DVB, therefore, it is not the peak power that is measured, based on the crest factor, but the mean output power. Three methods are available:

1. Mean power measurement with Power Meter NRVS and thermal power sensor (FIG 27)

Thermal power sensors supply the most accurate results if there is only one TV channel in the overall spectrum, which is nearly always the case at the DVB-T transmitter. Plus, they can easily be calibrated by performing a highly accurate DC voltage measurement, provided the sensor is capable of DC measurement.

2. Mean power measurement with Spectrum Analyzer FSEx or FSP

A frequency cursor is placed on the lower and another one on the upper frequency of the DVB channel. The spectrum analyzer calculates the power for the band between the cursors (FIG 28). The method provides sufficient accuracy as in DVB-T normally no signals are put on the air in the adjacent channels.

3. Mean power measurement with DVB-T Test Receiver EFA”

“Measurements on MPEG2 and DVB-T signals (4)” in News from Rohde&Schwarz Number 172 (2001/III)

I have left out the detail of test receivers, as most amateurs would not have access to one. A power meter and the spectrum analyser are pictured below.

The simplest way to illustrate the difference in power measurement is by commercial specification of a modern DVB-T LMDOS power transistor. The CW power is 750 W, while the DVB-T power is 150 W, with an efficiency of around 50 percent. The DVB-T power measurement would have been done with a thermal power meter. The CW (key-down) and DVB-T power represents different modes and different ways of measuring power. However, a thermal power meter can measure any type of signal, including DVB-S, and give the RMS value.

Even the CW figure is misleading as Morse is a digital mode with a low duty cycle, the RMS figure would be much lower.

Operating parameters and apparent "efficiency"

The operating parameters of an amplifier will affect apparent "efficiency". The transistor above, uses a PAR (peak-to-average ratio) of only 8 dB. Amateurs have typically used a much higher PAR (30 dB?) in order to reduce the intermodulation skirts. While this will reduce power input as well as output, there has been much debate over the achieved power out compared to the maximum possible power out, usually using the CW output.

As I have discussed in earlier posts, it is possible to use filters to reduce skirts and achieve higher power outputs, comparable to device specifications. I discovered this based on intuition and experience building, modifying and tuning cavity filters for voice FM repeaters. I have subsequently found that they are routinely used in broadcast DVB-T TV.

Using a thermal power meter and operating parameters akin to broadcast TV, the achievable power efficiency can be better understood.

DATV power efficiency; W/W or something else; b/sWHz?

To a point, power efficiency, no matter how defined or measured, is probably not what we really want to know. Power in, data rate, spectrum band-width and probably cost are what we really want. So a new unit of efficiency is data rate (bits/second) divided by power in (Watts) and band-width (Hz); b/s W Hz! There is probably a unit name for it, but I don't know what it is. I am serious and will discuss it in another post with modes and numbers.

Let's do it!

So far has been "book learning". The next step for me is to do it. I had sufficient interest in the DVB-T power puzzle to buy a used thermal power meter and a spectrum analyser capable of measuring envelope power. New power meters and sensors are very expensive (~$10K), but I imported a used HP 438A power meter and HP 8482H thermocouple thermal sensor for about $500. The spectrum analyser is a Siglent SSA3021X. Calibrated dummy loads and attenuators are also needed.

I also have a SWR power meter, a cheap AD8307 power meter and access to a Bird power meter, all of which use diode sensors. I should use a HP diode sensor to suit my meter, but I can't afford it!

I certainly don't advocate everyone buying this gear, but I want to investigate the puzzle.

Testing power output from digital signal power amplifiers is not simple. My bench tests will be the subject of another post.

Conclusion

to be done- see introduction

RF tap for panadaptor/second SDR after IC-7300 bandpass filters (First draft)

Introduction

It is possible to tap into the IC-7300 after the bandpass filters, just before the ADC. There is a coax connector that allows an SDR tap to receive the filtered RX signal. It is shared with the low-level TX path, but the levels are low, giving a TX monitor too. Using CAT controls, the SDR can be controlled by the TRX. It can also operate as a second RX in the operating band. The RF tap is an alternative to the INRAD RX7300-receive antenna cable, with some advantages and disadvantages.

Finding the RF tap

In an SDR-based TRX there is no IF tap point, but on the IC-7300, there is a RF tap point after the band pass filter and RF amplifier, where an SDR has the good filtering of the IC-7300, but can access the whole operating band.

The RF tap point J1431 is shown in a portion of the RF unit schematic from the service manual. The schematic shows the RX, TX and power paths in green, brown and red respectively.

The connector on the board is clearly marked and accessible. There is plenty of room and a frw spots to tap 12 V if an isolation amplifier is used.

As can be seen from the schematic, the tap point is shared by both the RX and TX signal paths. However, the TX signal level is low, easily handled by a spectrum analyser of an SDR, an SDRPlay in my case. I just used an oscilloscope probe on a spectrum analyzer then the SDRPlay both working well. I forgot to take photos but will do so in due course.

I have been waiting on getting a TMP plug and socket to make a proper connection, together with an isolation amplifier, like those from Clifton labs; common for IF taps (but are no longer available). See http://vk4zxi.blogspot.com.au/2013/11/sdrs-at-first-if-of-trx-as-panadator.html

TMP- Taiko Denki Connectors https://www.therfc.com/taiko.htm $1-50 each

Given that the tap is shared between RF and TX, I would be interested in the views of RF engineers of any potential problems with RX and TX level controls, as the tap could affect the circuit.

Possibilities!

The RF tap can be used in many ways:

The post obvious is as a panadaptor for the TRX. It is protected by the band pass filters of the IC-7300, as many SDRs don't have much, if any, input filtering. However, in means the SDR can only be used in the operating band. Using CAT controls and the likes of HDSDR, the SDR will track the IC-7300 and allow control from either device as per the post above.
The SDR can be used as a second receiver in the same band. One could be set narrow, the other wide. With SDR software it is usually possible to have multiple receivers, so you can have as many as you like.
The SDR can act as a small-signal monitor of the TX signal, as it is in the TX path as well.

An RF tap can be done on most TRX. I originally considered the idea for my IC-7100.

Discussion

The RF tap seems pretty much the ideal way of getting a panadaptor/CAT control for the IC-7300.

The RF tap does not require modification of the TRX, given the cable connections, so it can be taken out if the TRX needs to go for service.

Like the INRAD RX7300-receive antenna cable, it requires a splitter arrangement of some sort, as both are in the direct RX line.

Japanese TRX use JIS not Phillips screws

Ever wonder why it is so easy to damage a screw on a Japanese made radio using a Phillips head screwdriver? Obscure, but simple, they use JIS screws not Phillips!

I haven't checked, but I think they are on made-in-Japan cars, certainly lots of other gear.

JIS screws, which pre-date Phillips by 20 years, usually have a dot or other indent on the head.

The solution is simple, buy a set of JIS screwdrivers, about $25 on eBay.

http://themtblab.com/2016/01/tools-vessel-jis-screwdriver.html

MiniTiouner DVB-S receiver build (draft)

Unique hardware: https://www.batc.org.uk/shop/minitiouner
https://wiki.batc.tv/MiniTiouner_hardware_Version_2
https://wiki.batc.tv/MiniTioune_software
https://www.videohelp.com/software/LAV-Filters
All codecs: https://www.videohelp.com/software/sections/codec-packs
https://wiki.batc.tv/File:Notes_on_building_the_BATC_v2_Minitiouner.pdf

Decontis DVB-T measurement, analysis and monitoring software (draft)

Introduction

There has been a lack of good DVB-T monitoring software for both TX monitoring and RX measurement, unlike DVB-S that has Tutioune. I recently came across a commercial grade package from decontis that is relative inexpensive and uses a cheap USB-T dongle. While comprehensive, it is not particularly easy to use as it is network-based. I am still working on getting it all going for TX monitoring. My favorite element is a proper constellation chart.

The purpose of the post is to alert others to its availability and hopefully help get it all working. Networks are not my favourite area.

Other software and hardware

The available DVB-T measurement, analysis and monitoring software is limited. CrazyScan2 for terrestrial/cable DVB-tuners https://sourceforge.net/p/crazyscan/wiki/Info/ uses PCTV USB tuner. The other alternative is to use a TV tuner, which gives MER and BER, but not constellation charts.

Lots for DVB-S excellent Tutioune http://www.vivadatv.org/page.php?p=tutioune-en CrazyScan

Decontis

http://www.decontis.com/

dtvTools – DVBBundles: DVB
SAMalyzer, SAMcorder, SAMitor, SAMbuddy-RF, SAManalog, SAMager-Agent, SAMrack: 50 €

TV Tuner

For compatable tuners: http://www.decontis.com/components/sambuddy-rf/

PCTV TripleStick 292e USB

http://www.pctvsystems.com/Products/ProductsEuropeAsia/Hybridproducts/PCTVtripleStick/tabid/308/language/en-GB/Default.aspx

http://blog.palosaari.fi/2014/04/naked-hardware-15-pctv-triplestick-292e.html

Silicon Labs Si2157 tuner

LimeSDR running DATV Express DVB-S TX software (1st draft)

With the MiniTioune DVB-S RX, I have begun trying different TX using DATV Express software under Windows 10. The logical first hardware would be the DATV Express hardware TX, but having shifted rooms in the house, I have not been able to find; I know exactly where it was in the other room!

The LimeSDR is a popular recent SDR dual duplex transceiver by Lime Microsystems using a new version of their own chip. Cost is about US$250, but they have just announced a mini version for about US$150. It replaces the popular BladeRF; I sold mine to by the new model.

DATV Express TX software is available for the LimeSDR; https://discourse.myriadrf.org/t/windows-based-dvb-s-s2-t-transmitter-for-limesdr/1348 (https://wiki.myriadrf.org/LimeSDR-USB) . It worked well without any hitches on 23 cm, with both TX and RX running on the same computer. I am currently using my main PC, but will move it to my fast Dell laptop for project work. With all the test gear, its a real kitchen table job. I might try the table in my new room, just need to tidy it!

One of the purposes of running DVB-S is to compare power measurement techniques with it and with DVB-T. I have discussed power measurement of wide TV signals in earlier posts.

Another purpose is to investigate cavity notch filters with DVB-S as it seems to have problems with "spread" when the power amplifier is driven too hard. I have done some work with cavity filters and DVB-T, see earlier posts.

More photos when I get it running on my laptop.

Modifying cavity filters for DATV TX or for repeaters

Introduction

I am currently doing further work on using notch cavity filters for DATV DVB-T transmitters. My earlier efforts were with what I had at hand and not knowing the solution; I (re)discovered that notch filters clean up DVB-T TX very well. However, it was at low power, 10 W, and high losses, >6 db because of the six cavities in a mobile duplexer. Here, I will report on modifying high power >100 W individual filters. In the next post I will report on using them and determining is just one pair are sufficient. The other goal of this post is to show how easy it is to modify older commercial filters for DATV or repeater use.

Modifying cavity filters

Old commercial filters are relatively easy to modify as the only thing that changes is the coupling loop, provided they are on frequency (not too hard to change that too!). Notch filters are the simplest as they use a single simple coupling, just a loop of metal. Old commercial filters are usually made very well, often silver plated. On UHF, they are relatively cheap; $100 for a four cavity duplexer.

Other than the coupling, the RF design of a cavity filter is simple, a quarter wave resonator (antenna) in a box, usually a cylinder. With a notch filter, the cavity is connected to the TX coax line with a single coax "T". The cavity absorbs the RF at the resonator's resonate frequency; an antenna in a box! The impedance is determined by the ratio of the cylinder to the resonator, like coax, about 3:1 for 50 Ohm.

The mechanical design is more complex, particularly with a variable length resonator to change frequency. The Q should be as high as possible, which is why many are silver plated brass, although can be copper plated or aluminium. The adjustment screw is an non-magnetic, low thermal expansion alloy of steel, Invar, with finger stock for a very good connection to the movable part of the resonator. There are "tricks" with the couplings to get good results without high cost. Some cavities use a capacitive "hat", to change frequency, as is done with antennas.

Couplings are mechanically simple but very complex for RF. There is virtually nothing in textbooks, most of it is proprietary, but most types are covered in: http://www.repeater-builder.com/antenna/pdf/ve2azx-duplexer-info.pdf. Black magic!

The key point of resonators here is that the closer to the resonator, 2-3 mm, the higher the coupling and the deeper the notch. However, as coupling increases, losses increase.

Making modifications

I have made a new coupling for a pair of large aluminium cavities, 150 mm diameter and about 400 mm long. The process of doing it is fairly easy, remove the original coupling, a loop soldered to an N connector. Unsolder the end of the loop attached to the connector pin and cut the earthed end to allow the new coupling to be soldered to it.

Make a sketch of how the coupling is mounted in the cavity and measure all the critical dimensions, particularly the connector center pin to the resonator and the same for the earth point. A small measure can be made by cutting a rectangle of grid paper. Then do a 1:1 drawing of the location. The new notch coupling is about 20 mm parallel to the resonator and 2 or 3 mm from it. The coupling can be made from a strip of copper about 5 mm wide and 1 mm thick, or a larger diameter piece of copper wire. The coupling is bent with a pair of long nosed pliers so that it matches the drawing. See Photo 1 of my drawing.

Once the coupling is accurately bent, solder it to the connector and adjust the shape as needed. The only part that is critical is that the piece of the coupling closest to the resonator must be parallel.

Photo 1 Sketch of new coupling, as described. I was originally going to solder the earth leg to the coax connector, OK if PTFE, but soldered it to a tag I cut from the old coupling instead. Both arrangements are drawn. The top plate was 10 mm thick, making things a little awkward.

Photo 1.5 The modified loop. The earth is soldered to part of the old coupling rather than to the connector as originally planned. The earth screw is a bit corroded, I should clean it.

With the resonator screwed back in place, its RF response can be shown with spectrum analyser. Spectrum analysers for DATV can be improvised using an SDR and a noise source for about $200 vs >$1500 for a Chinese one (which are very good). See http://vk4zxi.blogspot.com.au/search/label/noise%20source

Photo 2 The response of the new coupling, a sharp asymmetric notch and about 22 db deep with less than 1 db loss. It initially was about 20 db, but bending the coupling closer to the resonator, a small increase was obtained.

Notch filters are limited to about 25 db. For repeater cavities, I would chase that, but it is not that critical for a DATV skirt/splatter filter.

For a DVB-T filter, a sharp rectangular response is desired. Notch filters have it on the high frequency side, but a shallower response on the low frequency side. As a DVB-T signal is a 7 MHz wide rectangle, made up of nearly 8000 carriers, another notch filter is needed on the high frequency side, but the response reversed. This can be done with a quarter wave length cable between the coax T and the cavity.

For initial DATV testing. I will only do one side, so I can compare it directly with the unfiltered response on the other side of the signal.

Other UHF cavity filters

I bought a four cavity repeater duplexer a couple of days ago that I might use if I need two cavities per side for DVB-T.

I connected up one of the cavities and had a look at how it worked. Wow! An excellent pass reject cavity for a 70 cm amateur repeater. I opened one cavity and was surprised by two things. First that it was copper plated brass (not silver) that was still working well after about 30 years. The second, was how far the coupling loops were from the resonator, >20 mm. This was significant for me as I had struggled with pass reject cavities for 2 m. I tried to put the coupling near the resonator, as per notch cavities, but may have introduced too much induction with long wires. The other problem is when the connectors are opposite each other from the resonator, common with pass-band cavities.

Photo 3 The test one I have been discussing earlier on the right and the old cavity just noted on the left. Size matters for cavity filters as the surface area is proportional to Q, as well as power handling; the bigger the better.

Photo 4 The response of the old pass reject cavity, a huge 50 db! Great for a repeater but no use for a DATV skirt/splatter filter.

Photo 5 The assembled duplexer, a Motorola T1500 series, and unassembled cavity .

Photo 6 A close-up of the resonator and coupling loops, note the large spacing from loop to resonator. It is configured as a pass reject with a variable piston capacitor between coupling loops. May be original but looks like a modification; not mentioned in the 1983 Motorola brochure. Mounting the capacitor can be mechanically difficult as it must be insulated and accessible for adjustment outside.

Photo 7 The pass reject response can be changed to notch, by unsoldering a wire from one coupling loop, then using a coax T on that connector. A very disappointing 10 db because the loop is not closely coupled, being so far from the resonator. The response can be improved by making a new loop that is 2 - 3 db from the resonator, as described in the main article. The cavity can be converted to pass-band by removing both wires and increasing coupling.

Conclusion

It is relatively simple to modify used cavity filters suitable for use as a DATV DVB-T filter. The next step is to set it all up to see how well one high power cavity will work.

Polyphase harmonic rejection mixer: AirSpy HF+

Introduction

Can you get excited about a new mixer, usually boring devices that haven't changed in decades? Yes, the new polyphase harmonic rejection mixer in the AirSpy HF+ is almost as revolutionary as SDRs and will have a major influence on their design.

The big advantage of a polyphase harmonic rejection mixer is that it acts as a RF filter for the selected signal, as well as suppressing harmonics and other aliases of the mixing process and local oscillator. It means that the mixer can virtually be connected to the antenna. Typically, a polyphase harmonic rejection mixer converts down to an ADC at base-band. It seems they can be used for both RX and TX.

The post covers how the AirSpy HF+ works, and gives references to what I have been able to find out about polyphase harmonic rejection mixers. They are new and still covered by recent patents. A link to a PowerPoint gives general technical details of the mixer.

AirSpy HF+

The AirSpy HF+ is rather unique for modern SDRs as its main purpose is to cover the HF bands, although it does cover VHF as well, although it only covers 200 kHz. And costs just $199. Most new SDRs start at VHF and go to daylight, well 3 or 6 GHz! They are intended for wide band mobile phone type applications, with coverage up to 30 MHz. The new LimeSDR (and $99 mini) and transverter ($299) covers up to about 10 GHz, but has limited RF band-pass filtering.

The unassuming appearance of the HF+ is shown in Picture 1 and the basic architecture of the HF+ is shown in Picture 2, clipped from https://airspy.com/airspy-hf-plus/.

Picture 1 AirSpy HF+

Its maker's description: "Airspy HF+ achieves excellent HF performance by means of a low-loss preselection filter, high linearity LNA, high linearity tunable RF filter, a polyphase harmonic rejection (HR) mixer that rejects up to the 21st harmonic and multi-stage analog and digital IF filtering.

The 6 dB-stepped AGC gain is fully controlled by the software running in the DSP which optimizes the gain distribution in real time for optimal sensitivity and linearity. Harmonic rejection is a key issue in wide band HF receivers because of the large input signal bandwidth of the input signal. The output of the IF-filter is then digitalized by a high dynamic range sigma delta IF ADC for further signal processing in the digital domain."

Picture 2 The basic architecture of the HF+

Polyphase harmonic rejection mixer

The way the new mixer works is not simple, it uses multiple phases (16?) of the local oscillator to use phasing to reject its harmonics, but at the same time, and because it is to a 200 kHz base-band, it rejects everything else too.

The big advantage is not needing a large number of band pass filters like a direct sampling SDR; the IC-7300 has 15!

The best explanation I have found is a slide show; http://icd.ewi.utwente.nl/temp_files/158b39412cff88a4181bfec0f4449c24.pdf. It is also subject to patent; https://www.google.ch/patents/US20110298521?hl=de. One of the authors wrote the slide show.

The mixer is an analogue CMOS device, STA709 from ST Microsystems, but the full datasheet is currently only available under NDA (non-disclosure agreement). So, no point taking RF cover off the HF+, too hard to remove anyway!

The new mixer is not entirely new, as stated in the patent, it relies on existing harmonic rejection mixers and other patents.

From AirSpy group: "You can see it as a "super Tayloe mixer". The problem with the original Tayloe Mixer is the harmonic responses at multiples of the LO frequency. The fix is to mathematically suppress these responses by adding more phases. The LO will no longer look like a square wave, but rather like a quantized sine wave. Basically, the more phases you add, the more harmonics you cancel.
This method is combined with narrow band filtering at the mixer itself. There is a switched-capacitor N-Path filter built into the mixer that is tuned using the same LO phases, which provides additional selectivity.
When you see it, all the ingredients required to implement this architecture can be implemented using CMOS silicon, and have a very good "horizontal" and "vertical" scalability: Horizontal with more phases (hence, less harmonics); Vertical with better fab processes (better linearity and NF).
The icing on the cake: This same technology can also work for TX."

Performance of AirSpy HF+

The HF+ is still very new, I only received mine in the last couple of weeks. The HF+ gives some performance results. There have been a number of comparative reviews against other SDRs, such as the new $99 RSP1a, by radio amateurs and shortwave listeners. However, there has not been a full technical review by the ARRL or RSGB.

However, with the limited testing the HF+ seems to have a high dynamic range and superior ability with weak signals near large signals, as would be expected from the design.

Conclusion

The polyphase harmonic rejection mixer of the Airspy HF+ is a significant development in radio design and is likely to rival other technologies over the coming years.

Appendix 1

Summary incorporating comments from AirSpy IO Group

Hi All

I asked the Airspy group about the workings of the HF+, and have summarized my own findings and comments from the group:

The HF+ uses a very modern and novel architecture, primarily a polyphase harmonic rejection mixer. See https://airspy.com/airspy-hf-plus/

As best I can work out, when converting to base-band, it is an effective filter for the desired signal and rejects even strong signals close by with virtually no filtering ahead of the mixer.

It uses multiple (16?) phases of the local oscillator to use phasing to reject its harmonics, but at the same time, and because it is to a 200 kHz base-band, it rejects everything else too. A bit like the old phasing SSB modulators, that used two phases.

The big advantage is not needing a large number of band pass filters like a direct sampling SDR; the IC-7300 has 15!

The best explanation I have found is a slide show; icd.ewi.utwente.nl/publications/get_file.php?pub_id=563. It is also subject to patent; https://www.google.ch/patents/US20110298521?hl=de. One of the authors wrote the slide show.

From AirSpy group: "You can see it as a "super Tayloe mixer". The problem with the original Tayloe Mixer is the harmonic responses at multiples of the LO frequency. The fix is to mathematically suppress these responses by adding more phases. The LO will no longer look like a square wave, but rather like a quantized sine wave. Basically, the more phases you add, the more harmonics you cancel.

This method is combined with narrow band filtering at the mixer itself. There is a switched-capacitor N-Path filter built into the mixer that is tuned using the same LO phases, which provides additional selectivity.

When you see it, all the ingredients required to implement this architecture can be implemented using CMOS silicon, and have a very good "horizontal" and "vertical" scalability: Horizontal with more phases (hence, less harmonics); Vertical with better fab processes (better linearity and NF).

The icing on the cake: This same technology can also work for TX."

Apparently the mixer is a CMOS device, STA709, but the full datasheet is currently only available under NDA. So, no point taking RF cover off the HF+, too hard to remove anyway!

Regards Drew VK4ZXI

High power UHF DVB-T amplifier, filters and testing- very draft

I have a used 150 W pallet amplifier from a scrapped DVB-T transmitter via eBay. It is bolted to a heat sink from a satellite transmitter.

I am basically following the 1 kW CW UHF amplifier from W6PQL. I have the low pass filter for the amplifier. I temporarily soldered some SMA connectors to test its frequency response. Down about 1 db at 500 MHz, but -40 db at the third harmonic; very good.

It has been suggested to use a pass band cavity filter duplexer. I had one on hand that I had just tuned for a repeater, trying to get a narrow pass-band.

Just using three cavities, I varied two of the three cavities to try to get a wider response. No joy, little wider, but more importantly 10 db loss.

To be continued...

2m Duplexer design repair tune pager-reject

Introduction

My local radio club's main 2m duplexer had an intermittent fault. The duplexer was fixed and retuned to a new channel, 1600, not 600 kHz split, partly to move the RX further from a Pager TX.

The Telewave TPRD-1556 Pass-Reject duplexer has an unusual coupler that I had not seen before that works very well, 45 db reject each. They should be possible to DIY. The fault was a defective piston capacitor used to tune a cavity.

An unexpected benefit of the pass-reject RX cavities is a significant 72 db reject of the 148 MHz pager TX. More is possible, if needed from notch cavities at 25 db reject each.

Tuning high performance cavities, 45 db each, highlights the limitations in instrumentation, particularly the dynamic range, 85 db, with the reject response lost in the noise floor.

Design of the Telewave TPRD-1556 Pass-Reject duplexer

The Telewave TPRD-1556 Pass-Reject duplexer looks like any other six cavity pass-reject duplexer, except that the cavities only have one coax connector not two.

The mystery is in the design of the coupler. The coupler use semi-rigid coax as capacitors as well as the main part of the coupling. There is a piston capacitor to adjust the spread. The design allows a spread from 400 kHz to over 1600kHz, whereas another conventional pass-reject cavity could only go out to 1100 kHz spread.

The couplings are different lengths in order to mirror the RX-TX responses, something that can be difficult to do. I don't know how the do it, presumably with phasing.

Telewave TPRD-1556 Pass-Reject duplexer

RX coupler and detail; ~150 mm long

RX coupler detail showing one end of the centre conductor connected to the coax connector pin but the other earthed. One end of the shield is connected to the capacitor and the other end is connected to the centre pin

TX coupler; ~ 130 mm long

Repair- intermittent piston capacitor

The piston capacitor, while quite reliable, had failed as the ceramic was loose in one end.

Fixing it was simple. It was not possible to just unsolder the capacitor as that would disturb the coupling. I removed the inner piece, then using bolt cutters, crushed the ceramic. Then the bottom portion could be unsoldered and the top part removed. I only had a shorter, smaller value capacitor, but as the screw of the original was most of the way out, I thought it would work, as it did.

New capacitor and original one, showing construction with concentric cylinders.

Tuning the duplexer

Tuning is straight forward with a spectrum analyser and tracking generator. Repeater RX 145.6 MHz, TX 147, 1600 kHz split. The cavities are done one at a time. The length of the resonator is set first and shouldn't need to be readjusted. The spread is then adjusted. The cavities work very well giving 45 db reject and less than 0.5 db loss; within specification per data sheet.

Having done all of one side, I then join two, check, then join the third. The photo shows two cavities, with the reject a bit lost in the noise. The reject should be 90 db, but the analysers dynamic range is only about 85 db. The second photo in each pair is the SWR for three cavities; not sure were the second dip comes in the RX chain, but of no consequence. Between the pairs, the mirroring can be seen.

Two RX cavities showing TX reject, better than 85 db.

SWR of three RX cavities, odd second dip.

Two TX cavities showing RX reject, better than 85 db.

SWR of three TX cavities

Pager TX rejection

The repeater site is near two 2 kW pagers at about 148 MHz. There was a major concern for desensing the repeater's RX. Fortuitously, the club had this faulty pass-reject duplexer that could be fixed and retuned for the site.

An unexpected outcome is that the pass-reject duplexer has about 72 db reject on RX and similar for TX. This can be supplemented by notch filters on the pager frequency to give a further 25 db reject for each cavity, if necessary.

Perhaps obvious, but I was not aware of the potential for pass-reject duplexers to attenuate unwanted signals other than the desired RX and TX for the repeater. This is a significant advantage over other designs, such as notch cavities that reject either RX or TX, but let everything else through.

The repeater's licensed split was changed from the standard 600 kHz to 1600 kHz to further separate the the RX and pager. Ironically, the pager rejection for be 600 kHz split would be 20 db or more, the reject being in the noise and outside the spectrum analyser's dynamic range of about 85 db.

I will report more on this when the repeater is installed. I will probably do another post on this subject as it is an important topic, but incidental to the main purpose of this post.

148 MHz pager rejection for RX 145.6 MHz, 72 db with pass-reject duplexer

Home-made pass-reject cavity filters

I had been experimenting with notch filters for home-made duplexers, very easy to make, whereas I will now try to make pass-reject couplings based on those used in this duplexer. They are particularly good in that they use a single connector , simplifying construction. They seem to be out of patent and should not be a problem copying them, not that I want to sell any. More in another post.

Limitations of instrumentation tuning pass-reject duplexers

Tuning duplexers highlights the limitations in instrumentation, particularly dynamic range. Each cavity has about 45 db reject. Two give about 90 db reject, but the instrument shows noise at the reject frequency, indicating it is outside the instruments dynamic range. All three cavities give about 135 db reject, way down in the noise.

The response can be improved a little using a low resolution band width (RBW), 1 kHz in the last photo. But it takes some time to plot a 10 MHz span at 1 kHz and is only useful for static devices.

I use a Siglent SSA3021X spectrum analyser, a "cheap" Chinese instrument and it has about 85 db of dynamic range. Name-brand ones do maybe 10 db better; not a lot. The specification of the instruments give the noise floor, about 150 db, but don't give the dynamic range. The dynamic range is largely determined by the analogue to digital converter (ADC).

One way of checking the TX reject at the RX is to use the repeater's TX, with the antenna port connected to a dummy load, and measure TX reject signal at the RX port. It is prudent to use a cheap power meter and a variable attenuator to first check that the signal is low enough. The spectrum analyser can then be connected to the RX port and the TX leakage measured; notionally 145 db down on TX output. To a point, the check needs to be done as the repeater's RX is to be connected to the port. The TX reject signal is more indicative than accurate as there can be stray RF from leaky cables and enclosures. Test leads and the repeater cables should be double shielded coax to minimise this.

The other check is the TX output of the repeater and through the duplexer to make sure losses are within expected losses.

I will do another post on this subject as it is important and include some work with wide-band software-defined radios (SDRs) as real-time improvised spectrum analysers.

DIY 2m single connector pass-reject coupling

Introduction

In my last post I describe the repair and tuning of a high performance 2 m duplexer that uses an unusual single connector pass-reject coupling. In this post I describe how to make one for about $30 that achieves the same level of performance.

First, I explain what seems to be the theory of the pass-reject coupling as a parallel tuned circuit, using the coax as the inductor and part of the capacitor. The coax conductor is the other part. The variable piston capacitor is in parallel with the coax capacitor to allow tuning. It also gives the necessary mirroring for RX and TX responses.

Then I describe how I made one that achieves about the same performance as the original.

I am very pleased as I have been working on DIY pass-reject couplers for some time. It allows the construction of a complete high performance, six cavity VHF duplexer for about $300, less the cost of connectors and cabling.

Theory of the tuned coupling

The coupling is a pass-reject type using a parallel tuned circuit, DC from coax connector to earth. Looking at the centre pin, it is connected to the shield of the coax and the conductor is earthed, then the coax is bent into the coupling shape. At the other end, the shield is not terminated but the the conductor is connected to earth. This is a tuned circuit, with the shield both an inductor and part of a capacitor. The capacitor is from the capacitance of the coax at 98 pF per metre. The piston capacitor (1-10 pF) is in parallel with the coupling capacitance, allowing tuning.

The coupling is similar, but the reverse of a magnetic loop antenna. In a magnetic loop antenna, the antenna loop is the tuned circuit and the coupling an un-tuned loop to couple the RX/TX coax to the tuned loop; a transformer. In the cavity filter, both the resonator and coupling are tuned, explaining the pass and the reject. The resonator determines the pass frequency and the coupling the reject frequency, which is how they are tuned. Resonator first for the pass, then the capacitor reject.

The TX coupling is shorter and tunes to a higher frequency, not clear why.

The mirroring comes from the adjustment of the trimmer. more gives RX shape, pass then reject, whereas less gives the TX shape, reject then pass. Not sure why this occurs, some interaction with the resonator I suppose.

The Q of the resonator, cavity and coupling determines the depth of the notch. The Q is high, in the order of 2000 to 5000.

The base of the resonator, effectively a quarter-wave antenna is primarily a magnetic field, whereas the other end, electrical. Being near the base, the coupling is magnetic, as per an air-cored transformer.

Pass-reject couplings are particularly good as they give high out of band rejection as well very sharp pass and reject for RX and TX.

Effective circuit, ignoring resistances.

Process

As with other coupling construction, as I have describe a careful drawing is made of the commercial coupling on grid paper. In this case, I just drew the outline with the coupling sitting flat on the paper. The coax sizes are drawn too big, but my coupling just needs to fit inside the drawing.

The first photo shows my copy and the commercial coupling, the second, the paper tracing.

Materials

The biggest problem was getting a minimum 32 mm disk for the base of the coupling. I couldn't think of how to make them; the hole is not in the middle. Then I thought of getting them made on a lathe; expensive. But then I thought laterally. The first was to use large coins that could be soldered. The 1966 Australian 50 cents is 32 x 2 mm and 80% silver; so I ordered some at $10 each. Then I tried searching eBay; stamped brass blanks were available locally at about $1 each; a bit thin at 1.2 mm but 32 mm diameter is ok. They arrived first and I used them.

The coax is semi-rigid RG402 copper tube shied, PTFE and silver/copper/steel inner, available from Element 14 at $20 per m.

I had some piston capistors. They are available on eBay from "element 13" in Bulgaria, or from usual suppliers at high prices. 1-14 pF is ok.

Making

The holes are drilled with a step bit in a drill press. A Dremel or similar is useful for cutting/sanding/stripping.

I extended the centre pin by slipping on a piece of the coax shield to give more length. Bending the coax is easy with the right diameter dowel. Soldered the coax to the pin, cut the other end to length soldered it to the capacitor and pin again.

Tuning

Checked it would tune to the same frequency as the original and tried it in the cavity; top wasn't perfect fit but it is to go into another cavity anyway. Very close to the original in performance, even with the 1600 MHz split; excellent.

Original coupling

Copy; near identical. Low loss and high reject.

Mirroring from adjusting the trimmer capacitor

Free air resonance

Conclusion

After many attempts at an easy to build pass-reject coupling, finally success. The couplings are for a fairly big tuning range, so dimensions are not too critical.

I will build a copy of the shorter TX coupling and see if it performs in a similar manner. To a point, the one I have made can act as RX or TX by adjusting the trimmer.

"Plastic Fantastic" Magnetic Loop- Linear actuator drive

Introduction

I have constructed a linear actuator drive version of VK5JST's trombone capacitor tuned "Plastic Fantastic" magnetic loop antenna for 20 m. The linear actuator drive considerably simplifies the construction and waterproofing.

The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed.

The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment.

Tuning the antenna while operating a SDR TRX is possible as the noise is visibly higher in a spectrum scope waterfall; Win4IcomSuite on IC-7300. The noise on the S meter rises from S2 to S6.
.

The design

The antenna uses the underground plastic gas pipe now widely available, including Bunnings. The pipe has a PTFE inner, a layer of aluminium then an outer of yellow plastic. With the PTFE inner, 19 mm copper pipe can be used to create a trombone capacitor. See Tregellas 2017 for details of the design.

I have used a 200 mm linear actuator for tuning rather than the geared motor screw drive of the original. These cost little more than the geared motor arrangement, but simplify construction and waterproofing. The cost is about $47 for the 200 mm 12 V actuator.

The linear actuator is positioned so as not to push the trombone capacitor out; they have internal limit switches. However, the actuator will contract too far and needs an external limit switch at maximum capacitance.

https://www.ebay.com.au/itm/12V-750N-100-200mm-Linear-Actuator-Electric-Motor-Opener-Heavy-Duty-Lifting-AU/332524444416?epid=15013316556&hash=item4d6c005b00:m:mSjXeORzrUMEAZG23JbL-pw

The design is further simplified using a plastic chopping board as the main mount, together with lots of zip ties.

The completed antenna

Detail of the linear actuator drive for the trombone tuning capacitor, mounted on a chopping board.

The coupling loop, which largely determines SWR.

The linear actuator was too fast at 10 mm/s. A 12V pulse width modulation (PWM) was used to slow it down to a usable speed. I used a PWM speed controller from Jaycar at $35. They are available on eBay from about $5 and up.

The SWR is dependent on the coupling between the drive loop and the main loop. While 1.1 is meant to be possible, I achieved 1.5 with some adjustment.

Performance

The antenna produces a sharp dip in SWR across the 20 m band with the linear actuator working well.

Tuning the antenna while operating a SDR TRX is possible as the raised noise is visibly higher in a spectrum scope waterfall. I used my IC-7300 with the new Win4IcomSuite.

Unfortunately 20m has been very quiet during the day, just an odd bit of CW.

Conclusion

The "plastic fantastic" magnetic loop antenna works well with a linear actuator, provided its speed is reduced by a PWM speed controller. As a proof of concept, usable RX performance can be obtained, with some room for optimisation. It is a pity that 20m is so quiet at the moment to allow further on-air testing.

References

Jim Tregellas VK5JST, "The Plastic Fantastic: a Magnetic Loop costing around $54 for 40 metres"Amateur Radio (Australia) Sept 2017 pp 14-17.
http://www.ahars.com.au/uploads/1/3/9/8/13982788/plasticloop1.pdf

VK5KLT paper: http://www.ahars.com.au/uploads/1/3/9/8/13982788/article-antenna-mag-loop-2.pdf

??

Cavity filters for Earth Moon Earth communications

Introduction

Cavity filter for EME

I haven't seen a cavity filter used in EME, but few know about them. It could be quite desirable to have a pass band cavity filter very close to the antenna and before the broadband low noise amplifier.

The LNAs are usually mounted at the antenna, but the loss of a couple of metres of coax (-0.3 db) to a cavity filter (-0.5 db) may be justified to give 30 db of out of band noise attenuation. This would greatly help in reducing overload of the LNA by such noise. With a filter it may/should be possible to use much more gain in the LNA. Further, two cavities could be used in series to give 60 db of noise attenuation.

A high Q 70cm pass band filter is a little taller than a quarter wave length, 250mm, and up to 200mm diameter to give high selectivity. The same cavity can be tuned to its third harmonic to work on 23cm with high Q and selectivity, and possibly at higher bands.

I tried a pass-reject filter as the selectivity is higher than pass-band, but the out of band dips around the pass, but rises significantly a little way from it, making them unsuitable. The reject frequency is immaterial in this application.

Cavity filter and harmonic response

Cavity filters are essentially a quarter wave antenna, the resonator, in a box, the cavity. At resonance they produce a peak or a notch depending on how they are coupled to a driving signal.

The Q of the filter is very high, but varies with design and quality. The larger the diameter of the cavity, the larger the surface area and a higher Q. With silver plating the Q is increased over aluminium, copper or brass. The design of the coupling also effects Q. However, these are all compromises with cost and other factors. Typically the cavity is aluminium, the resonator silver plated and the coupling a copper loop with an N or BNC connector.

For a pass band filter there are two loop couplings for in and out. The fundamental response is at the quarter wave length of the resonator.

As with most antenna, the resonator is resonate on odd harmonics. This harmonic response can be used to filter at high frequencies, but with the simpler mechanical construction of lower frequencies. I will show how a 2m filter can be used on 70 cm and 23 cm. I can't go to a higher band because of the limit of my spectrum analyser, but this post is to show the general principle.

I had a 2m passband cavity at hand, but not a particularly good one, so results are indicative, but surprising good in this application.

The first photo shows the fundamental response at 145 MHz, better than 30 db, with about 0.5 insertion loss.

The next shows the third harmonic at about 415 MHz, still impressive. The exact multiplier seems to be effected by the cavity's geometry. It is just a matter of re-tuning to the desired frequency.

Even the ninth harmonic on 23 cm is still very good. The insertion losses and artefacts are likely influenced by the coupling design, UHF not N connectors and the cable. Further, I did not normalise the analyser for this shot. Again this is indicative. It would be better to use a 70cm filter on its third harmonic for 23cm and perhaps above.

Cavity filters in EME use.
One effect of a highly resonant antenna, like the magnetic loop of

The cavity filter as used

The 2m cavity filter I used, not the best even for 2m. Pass band filters aren't selective enough for the 2m 600 kHz amateur repeater split. However, they can be enough for 70cm with the wider split. It is possible that this filter came from a 70cm or commercial repeater. I bought four as surplus.

The coupling with its nasty UHF connector. The loop seems small for 2m and may have been used on its third harmonic. However, coupling design is not particularly critical at 2m or 70cm. The earthed long side is close to the resonator. The rotation of the coupling influences selectivity and insertion loss. For high selectivity and higher losses, still about 0.5 db, I adjusted it to be closest to the resonator.

The top of the cavity showing the placement of the coupling loops and the adjustment screw. The rod is invar to minimise temperature changes. However, in this application it is not critical as the peak is quite broad at the resonant frequency. The filter looks very selective in the photos, but I am using a wide span.

Earth Moon Earth communications- very draft

Introduction

A post on EME thoughts, mainly as a record of what I have found.

Antenna systems

Mast and rotators

Locate the antenna at the club; keep partner and neighbours happy.

The club has a new tilt-mast for UHF/VHF. Needs to be erected, about $600 to drill hole and concrete. https://www.nbsantennas.com.au/

The club has a new G-5500 Yaesu EL/AZ rotator. It needs the AL and EZ rotators to be mounted separately to suit the carriage of the mast.

The club has a computer rotator controller. https://ea4tx.com/en/tienda/antenna-rotator-system/ars/

I also have the same mast, two of the EL/AZ rotators and a EA4TX controller. The mast base is in, but I need to erect the mast. I have the controller working with the rotators. I was building them for satellite tracking.

While EME and satellite tracking can be done at ground level, having them on a mast gets them above trees, buildings and other obstacles for a 360 degree view of the sky.

Antenna

The antenna depends on the bands used. For 70 cm, a Yagi array is the norm. At 23 cm a dish can be used.

Yagi array

When I first got back in radio, I researched building high-performance 2m and 70cm Yagi. I have all of the components to build both, I was following the design in the ARRL handbook that uses insulated elements for low noise. The antenna uses 25mm square section boom, nylon top hat washers and push-nuts to secure the elements. (ref??)

For satellite tracking, the Yagi can have have both vertical and horizontal polarisation on the same boom, with phasing to get circular polarisation. The small satellites tumble, so both polarisations are necessary.

I had two 2400mm fibreglass poles made locally at $80 each. They are to mount two dual polarisation Yagi for 2m and 70cm. The satellites are repeaters with different band for up and down. The poles simplify mounting as they are insulators and do not interfere with the antenna, allowing centre of gravity mounting. Such mounting reduces the load on the EL rotator.

For EME, the same poles could be used to create the H to mount four Yagi. The EME Yagi are single polarisation and easier to make. I may need to get some more antenna components, but they are not expensive, less than $100.

Parabolic Dish antenna

I used to play with satellite TV as I am into home theatre. Prime focus C band 2300mm dishes are cheap new, about $250, but are also available for about $100 used on Gumtree. New is probably easier as collecting used ones are a hassle.

http://www.satking.com.au/satellite-tv/satellite%20tv-%20cband%20dishes/satking-2-3m-standard-c-band-dish

The dishes use a polar mount and can track the geo-stationary TV satellites by rotating in one plane using a linear actuator. The actuators have a reed switch so a controller can follow the tracking.

For EME on 23cm, the dishes need full EL/AZ control. That might be possible with linear actuators and a small computer control, but I am not aware of one (but haven't looked either). It might be possible to use the EA4TX controller, although it uses a variable voltage for position; all rotators use this.

It may be possible to use a Yeasu EL/AZ rotator with the EA4TX controller. The dish would need to be mounted at ground level to reduce wind loads, but the weight of the dish itself is not high. That would simplify mounting and control.

There are designs for the 23cm feed horn and antenna around. They can be made from sheet copper, which I have. The feed horn is to reduce terrestrial noise.

Cables

Coax cable is the greatest loss in the RX/TC system. 50 Ohm RG142 is commonly used. So is RG814. The club has two 100m roles of RG214. It is thick and suited to long runs. We may need to get some more RG142, although I have some. The main advantage of these cables is the shielding to keep extraneous noise out; their losses are similar to common RG213.

Another cheap alternative is 75 Ohm satellite TV coax. It is quad shielded and low loss as it is designed for the first IF of satellite RX from 1 to 2 GHz. It could be used for TX up to 100W pulsed, provided appropriate matching is used.

The alternative is to use separate RX and TX feed to the antenna. This is almost necessary anyway as there is a low noise amplifier at the antenna for RX.

The impedance of coax is different between RX and TX. 75 Ohm is best for RX. About 38 Ohm is ideal for TX. 50 Ohm is a compromise for both RX and TX.

http://rfelektronik.se/manuals/Datasheets/Coaxial_Cable_Attenuation_Chart.pdf

Cavity filter

I haven't seen a cavity filter used in EME, but few know about them. It could be quite desirable to have a pass band cavity filter very close to the antenna and before the broadband low noise amplifier.

The LNAs are usually mounted at the antenna, but the loss of a couple of metres of coax (-0.3 db) to a cavity filter (-0.5 db) may be justified to give 20 db of out of band noise attenuation. This would greatly help in reducing overload of the LNA by such noise. With a filter it may/should be possible to use much more gain at the LNA. (and perhaps reducing the need for gain in the antenna?? but need antenna gain for TX!)

A high Q 70cm pass band filter is a little taller than a quarter wave length, 250mm, and up to 200mm diameter to give high selectivity. The same cavity can be tuned to its third harmonic to work on 23cm with high Q and selectivity.

It may be better to use a pass-reject filter as the sharpness is much higher than pass-band. The reject frequency is immaterial in this application.

The use of a cavity filter or two might be our contribution to EME practice. In I quick Google I found a discussion about using them but none on actually using them.

LNA (Low noise amplifier)

LNAs used to be a major problem and a significant cost, however with cell phones and satellite TV, the cost has reduced dramatically. LNAs with a noise figure of ).5 db are available for about $20.

RX/TX switching

The Black Art of Duplexers: Demystifying Cavity Filters

PDF of Power Point presentation:
https://www.dropbox.com/s/s6tgcddf53yz68t/The%20Black%20Art%20of%20Duplexers.pdf?dl=0

Rohde & Schwarz CMU200

Introduction

This post is a collection of information for the Rohde & Schwarz CMU200 Universal Radio Communication Tester that I have purchased. They can be bought on eBay and other places often for a very reasonable sum. In its day it was an expensive but capable instrument.

While the CMU200 is primarily designed for testing now obsolete mobile phone equipment, it can be used for working with analog radio. It has a spectrum analyser, RF generator, RF power measurement and with the option, an audio test set. While not directly having a tracking generator function, there are two PC programs that allow it to be used for testing filters. It can also be done with a noise source or an external tracking generator.

The CMU200 uses an embedded Celeron or similar AMD processor running MS-DOS. It has an internal IDE HDD that is wise to replace as the instrument can do tens of thousands of hours. An IDE SSD allows the instrument to boot much faster.

Documentation is available from R&S. The CMU200 is discussed often in the eevblog forum.

The CMU200 can be used with a PC via a GPIB USB adaptor. There is R&S and third party software that increases the instument's functionality.

Resources

Search CMU200
https://www.rohde-schwarz.com/us/home_48230.html
https://gloris.rohde-schwarz.com/anonymous/en/pages/toplevel/home.html?
https://www.eevblog.com/forum/index.php
https://www.mikrocontroller.net
cut and paste from forum in italics from forum

Replace HDD with SSD

"The HDD-Raw-Copy-Tool tool does a sector by sector copy of the entire drive. The resulting RAW image can be opened by a tool like PowerISO and this what I used to upgraded the CMU200 DOS software.

Both the CMU200 Celeron and CRTU-RU Pentium III boards have worked with various Fujitsu 20GB and IBM 30GB IDE drives I have connected. BIOS has autodetected all OK. I have replaced the CMU200 drive with Kingspec PATA IDE 2.5" 32GB SSD and again HDD-Raw-Copy-Tool was used to write the image to the drive before installation. Now the CMU200 boots like a rocket.

I note the Award BIOS FLASH tool and bios image can be found in \internal\install\bios folder. Run the batch file FLASH.BAT to reflash.

Oh, and something I haven't done for years, plugged the IDE cable incorrectly and burnt out a power wire. Thought the CRTU-RU had smoked it was just the wire. The SSD comes with same type of IDE cable - just longer. Put my glasses on this time when I reconnected it and everything ran fine.

Another tidbit - the Windows 2000 default password for user Administrator is blank on the CRTU-RU. Which is good as I haven't found a way to successfully boot from USB flash drive yet."

This is how I have done it:

1) remove the hard drive from the CMU, and attached is to a device like this: http://www.dx.com/p/unitek-y-3321-usb3-0-to-ide-sata-hard-disk-drive-hdd-docking-converter-black-230128

2) made an exact image of the drive to a file with this software: http://hddguru.com/software/HDD-Raw-Copy-Tool/ (I used the portable version to avoid installing the software)

3) removed an IDE harddrive from a back-up USB drive I had lying around (the HD in my CMU was a 20GB one, but I replaced it with a 40GB one without any issues

4) restored the image from the file to the "new" HD using the same software as in step 2

Manuals

http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/sr001Man?OpenDocument

manufacturer page:

https://www.rohde-schwarz.com/en/product/cmu200-productstartpage_63493-7830.html

and the brochure:
https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/CMU200_bro_en.pdf

Detailed specifications:
http://www.upc.edu/sct/documents_equipament/d_175_id-448.pdf

Service manual:
https://www.ntecusa.com/docs/RS_CMU200_CMU300_other.pdf

Drivers
https://www.rohde-schwarz.com/us/driver/cmu200/

Firmware

Only the most recent firmware seems to be available from R&S. Earlier versions seem to have expired. Probably a good idea to keep a copy of current firmware before updating. May be a good idea not to update if little extra functionalityis added. Se firmware text file for earlier versions.

5.21 firmware
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/SR001Z?OpenDocument

Base
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/SR001Z?OpenDocument
CMU200_GSM_5.22
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/SR003Z?OpenDocument
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/SR004Z?OpenDocument
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/SR006Z?OpenDocument
CDMA 2000 MS 5.20 package is here :
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/sr009z2?OpenDocument
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/sr015m?OpenDocument - manual
http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/sr015Z?OpenDocument - install

http://www3.rohde-schwarz.com/www/FileTranCS.nsf/alias/DATS?OpenDocument - It is PC part of CMU-K92 option, GPRS application testing package. Manual is also there.

CMU200 Software- CMUgo- Remote control software
https://www.rohde-schwarz.com/au/software/cmu200/
RSCommander
https://www.rohde-schwarz.com/de/applikationen/rscommander-flexibles-software-tool-fuer-instrumente-von-rohde-schwarz-application-note_56280-15410.html

All new firmware
http://gotroot.ca/cmu200/

Text file from eevblog?
https://www.mikrocontroller.net/attachment/337287/EEVBLOG_CMU200.txt

Third Party software

The free software FreRes from Rohde & Schwarz allows to make sweep and test for example filters or duplexers.

https://www.rohde-schwarz.com/us/applications/freres-program-for-frequency-response-measurements-application-note_56280-15551.html

https://www.keysight.com/upload/cmc_upload/All/readme_IOLibraries_18_1_22603_1.htm?&cc=US&lc=eng

http://vma-satellite.blogspot.pt/2018/03/vma-simple-spectrum-analyser-for-crtu.html
https://vma-satellite.blogspot.com/2018/06/remote-control-of-r-cmu200-and-crtu.html
https://vma-satellite.blogspot.com/2018/03/vma-simple-spectrum-analyser-for-crtu_24.html

My unit

Serial Number: 101330
Options
CMU-B11 (HW): Reference oscillator OXCO, aging 2 X 10E-7/year
CMU-B12 (HW): Reference oscillator OXCO, aging 3.5x10E-8/year
CMU-B21 (HW): Universal signalling unit CMU-B21V14 incl. CMU-B54
CMU-B41 (HW): Audio Generator and Analyzer
CMU-B83 (HW): CDMA2000® signaling unit (requires R&S®CMU-U65)
CMU-U61 (HW): PCMCIA
CMU-U65 (HW): Upgrade kit for CMU200: Measurement DSP module for measurement speed improvement
CMU-K29 (SW): Analog AMPS, for CMU-B21, CMU-B41 required V5.20
CMU-K84 (SW): CDMA2000 (cellular band) for CMU-B83 V5.20
CMU-K85 (SW): CDMA2000 (PCS band) for CMU-B83r V5.20
Hardware
Front Module: FMR5
CPU: AMD K6-III
Memory: 128 MB
Firmware: V8.50 02.05.06

450 MHz CDMA duplexer tear down and analysis- draft

Introduction

Why?

I am interested in how modern duplexers work. The club purchased a new 70cm duplexer, only 50 mm tall and not obvious how it worked, but they didn't want me opening it for a look!

I purchased a CDMA duplexer from Russia on the 450 MHz band on eBay. It was similar to the 70 cm one. I could get some idea how the UHF one works and some(?!) chance of re-tuning it for either a 70 cm DVB-T TV filter, 7 MHz bandpass, or as a 70 cm narrow pass band repeater duplexer (or both, as there are three chains of seven cavities in the device.

Unfortunately, I did not take photos of the duplexer's response before I opened it. However, it was a 6 MHz pass band, with steep skirts, and low pass in the 450 MHz band. I will do it when I put the top back on, but have probably disturbed the tuning. CDMA signals are 1.23 MHz wide, so it is unclear why the pass band is 6 MHz.

The outside, with a zillion screws out. The left three connectors are all SMA, the right are 7/16 DIN and N adaptors that I added.

The gizzards!

Click photo to see captions larger!!

It is a complex beast, requiring a very detailed examination to see all its features. Pore over the photo to see.

?? = I think!

The duplexer has three chains of cavity filters, RX (top), RX monitor (middle) and TX (bottom). RX has its own antenna. TX and RX monitor share an antenna, but are on different frequencies. Tx input is to left. The resonators use big capacitive hats to electrically shorten the resonator to one rack height, otherwise four rack high. The resonator adjustment screws are the larger screws.

The filters are pass band and low pass. The low pass comes from the capacitive tuning into the resonator??

The filter chains are pass-band, iris-coupled (port-tuned) filters that have a sharp cut off. The iris-coupling screws are the smaller screws. In addition to iris-coupling, both inductive (on lid) and capacitive inter-cavity coupling are used. I don't know why, presumably to get a sharper response (or impedence matching??). There are no notch filters, as are used in DVB-T filters, to get a very sharp cut at the edges of the pass band.

The input/output are either a separate smaller resonator with a gamma match?? coupling (left), or a conventional gamma match?? coupling direct to the main resonator (right).

Commercial DVB-T pass-band/notch filters: What we can learn

Introduction

Low power, UHF and VHF DVB-T pass-band/notch filters are commercially available at relatively low cost, ~US$750 that seem suitable for DATV. They seem a good off the shelf solution.

By examining such filters, it seems possible to see how they might work, giving some insight into possible home-brew.

The filters have two notch filters, one for each side of the signal, as per my earlier posts, to notch the TX skirts.

In addition, they have cavity pass-band filters to take out artefacts further out. A manufacturer indicates that the pass filter is a combline, but the mechanical construction suggests cavity filters with openings between cavities for coupling.

It seems possible to separate the notch and band-pass filters. For wideband UHF, two notch cavities and a pass-band filter. For 2m, two notch cavities and a single pass cavity may suffice.

Low-pass filters are still needed for odd harmonics in addition to a DVB-T filter.

Commercial filters DVB-T (UHF and VHF)

The UHF first commercial filter seems to have five band-pass cavities and two notch cavities, one at either end. The input, notch and first pass resonator seem to share the same cavity, similarly for output. Three of the resonators are in individual cavities. On the top, RHS of the filter are the resonator tuning knobs.

http://www.com-tech.it/products/cl-series-c/

From the response curves, the two sharp notches are evident to filter the skirts. This is similar to what I found in earlier posts on notch duplexers for DVB-T. However, one pair seems sufficient, something I have been working on, rather than 3 pairs in a duplexer.

How the notches work is not particularly evident. The connector has a loop coupling per the manufacturer's claim of DC to earth for lightning protection. There is a protrusion on the opposite side of the connector, the purpose of which is not evident. The notch and first pass-band resonator seem to be in the same cavity. Each resonator may be energised, one as a notch, the other as the first resonator of the pass-band filter.

The response shows the five minimums in SWR from the five cavities. The pass-band is shown without ripples, which seems a bit optimistic.

The response also shows the filter losses, less than 1 dB according to the specifications; quite remarkable!

The filter is meant to be a combline, presumably similar to the diagram from Piette 2010.

However, the cavities seem to have openings between them as the line of screws do not go all the way. It would seem to be similar to the band-pass filter from Piette 2010. It is not clear from the first drawing if there are screws to adjust coupling, but there seems to be another adjustment next to the resonator tuning.

From the mechanical design, it does not seem to be an inter-digital filter.

The filter is quite small,

Third harmonic?

Size Power

http://www.com-tech.it/products/cl-series-c/
http://www.com-tech.it/glossary/xline/
http://www.sira.mi.it/en/products/broadcasting/8/uhf-dtv-filters/287

http://www.microwavefilter.com/FCC_Repack.html
http://www.sira.mi.it http://nintermedia.com/pdfs/tv/CTV-V-DVB-025.pdf

Homebrew DVB-T TX filter?

http://www.ocicom.com/index.php/amateur-radio/products/440-mhz-bandpass-filter
Bernard Piette 2010 VHF/UHF Filters and Multicouplers: Applications of Air Resonators

HP AGILENT 8935 E6380A RF TEST SET

Service Monitor for HF and 2 way radio. They all generate AM, FM and have a calibrated output signal generator, have 2 separate audio tone generators, have 2uV sensitive "off the air receivers" with antenna input, encode/decode standard tone (PL) (CTCSS), have sinad, distortion, S/N meters, receive AM, FM and SSB, have modulation / deviation meter, frequency error meters.

Overview and screens for HP 8935 E6380A
http://www.amtronix.com/e6380a.htm

Comparison with other HP test sets
http://www.amtronix.com/diff.htm

All manuals available through Keysight, just search E6380.