Monday, 15 January 2018

Polyphase harmonic rejection mixer: AirSpy HF+

Polyphase harmonic rejection mixer: AirSpy HF+


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

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 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; It is also subject to patent; One of the authors wrote the slide show.

Presumably the mixer is an analogue CMOS device, but I have not found one. And the RF cover on the HF+ is too hard to remove! The HF+ site cites ST Microelectronics as a collaborator, but I have searched their site with no success; It may still be in development or proprietary intellectual property.

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

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.


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.

Friday, 5 January 2018

Modifying cavity filters for DATV TX or for repeaters

Modifying cavity filters for DATV TX or for repeaters


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: 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

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.


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.

Sunday, 24 September 2017

LimeSDR running DATV Express DVB-S TX software

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; ( . 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.

Decontis dtvtools DVB-T/S measurement, analysis and monitoring software

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


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 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, but is network-based. I have managed to get it going and plan to use it for TX power amplifier modification and monitoring. My favorite element is a proper constellation chart.

Other software and hardware

The available DVB-T measurement, analysis and monitoring software is limited. CrazyScan2 for terrestrial/cable DVB-tuners uses PCTV USB tuner. The other alternative is to use a TV tuner, which gives MER and BER, but not constellation diagrams. A standalone TV installer instrument can be used, but good ones are expensive. Professional equipment is very expensive.

For DVB-S there is the excellent Tutioune;


Decontis dtvtools is an excellent DVB analysis and monitoring software package. It is available for DVB-T, DVB-S and others. It is complex, commercial grade software but relatively inexpensive.

The DVB-T bundle, comprising SAMalyzer, SAMcorder, SAMitor, SAMbuddy-RF, SAManalog, SAMager-Agent, and SAMrack, is shareware and 50 € to buy. It is amazing value.

There is virtually nothing on the web on how to use it. A non-English YouTube is helpful.

There are manuals for each module but none are particularly useful for setting the package up for first time use.

The sequence to use it is to install everything and connect a supported TV tuner. Start SAMcorder, use default settings and read the manual, scan your local TV stations and select one frequency. Use IP stream as output. Starting services will bring up another SAM window, select one frequency/stream, open to show channels. Clicking a channel will bring up SAMitor and the TV picture. Start SAMbuddy-RF and open both links to start analysis. Click constellation and start it to give constellation diagram. Then sit back in awe if it all works!

Use other components as desired, it helps to read the manual as it is complex software.


The software is limited by the tuner in terms of bandwidth. My interest is high quality DATV on 70cm and can use 7 MHz. I use a PCTV TripleStick 292e USB that can do 6, 7, and 8 MHz. Generally, dtvtools only supports tuners which support Microsoft DirectX BDA technology.

It is possible that the HiDes RX dongles might work as they are BDA. I quickly tried an old UT-100D with no success, but I am fairly sure it was the wrong driver. It unsuccessfully scans the device. I will investigate further later.

The software is available as a DVB-S bundle. I haven't tried it. It is hard to beat Tutioune hardware and software.

SAMcorder has an ASI input and a IP stream output which may be of interest to some.

TV Tuner

For compatible tuners see p6 of SAMbuddy-RF. Others may work. Generally, dtvtools only supports tuners which support Microsoft DirectX BDA technology.

I use a PCTV TripleStick 292e USB. It is a pretty amazing device, like most tuners, a SDR frontend.

Silicon Labs Si2157 tuner

MiniTiouner DVB-S receiver build

MiniTiouner DVB-S receiver build (draft)

Unique hardware:
All codecs:

Sunday, 9 April 2017

Japanese TRX use JIS not Phillips screws

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.

RF tap for panadaptor/second SDR after IC-7300 bandpass filters

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


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

TMP- Taiko Denki Connectors $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.


The RF tap can be used in many ways:

  1. 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.
  2. 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.
  3. 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.


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.