WSJTX, MAP65 and the FunCube (pro +)

This is a very basic, non-detailed, overview of the FunCube pro + Dongle and it's use with WSJTX and MAP65 First some basic facts as I see them. I could be wring anywhere on this page so check facts for yourself!

Note - I'm talking about the Funcube Pro + Dongle because thats what I'm using and there is built in support for it (to some degree) in both MAP65 and WSJTX. In principle you could also use a much cheaper "generic" RTL-SDR dongle, but you would lose any direct WSJTX/MAP65 support. You would have to do everything via an SDR software interface program which would talk to the dongle and WSJTX or MAP65, which can complicate things somewhat. The FunCube pro + works well, but is around $165 at the moment and only obtainable from the UK. A decent RTL-SDR dongle is around $32. Their frequency coverage is different but both cover the 50, 144, 432 and 1296 EME bands directly, and of course the higher hands via downconverters. The funcube website says this about its frequency coverage: "...The receiver range is now 150kHz to 240MHz and 420MHz to 1.9GHz. In our first 20 units, all have covered 150kHz to 260MHz and 410MHz to 2.05GHz..."

If you consider using a FunCube pro + with WSJTX and MAP65 there are a few things you might want to do. Map65 has native support for the FunCube pro+. It's "plug and play", However it won't Doppler track. MAP65 is an old program, originally designed for operation where Doppler tracking was not required. This could include HF I guess, but it was (and is) mainly used on 144 and 432 MHz with polarization diversity. It will work on any ham band, but if you use it at, say, 10Ghz as a receiver with a downconverter, the 60Hz/min or more Doppler drift can cause problems! The FunCube pro + works fine with MAP65. The program has built in native support for the data the FunCube pro+ provides, but it can't control the frequency.

The second thing you might want to do is use a Funcube pro + with WSJTX and I'll get to that later.

FunCube pro+ basics

There's the original FunCube (no longer made) and the later (and current) FunCube Pro +. The pro + has some additional features, but they both operate in a similar way so I'll just call it the Funcube Pro + to be current. The FunCube Pro + is the hardware front end of a Software Defined Receiver (SDR). It's a direct conversion device, sometimes called a zero IF receiver, meaning that it downconverts a portion of the RF spectrum directly to audio frequencies. So if the local oscillator is at 1296MHz, it will convert the region 1296.000 to 1296.050 into an "audio" signal on the 0 to 50kHz range. However, since it's a simple heterodyne mixer, it will also convert the region 1295.950 to 1296.000 to the same 0 to 50Kz "audio" signal. So there is no image rejection. But things are slightly more completed than this simple picture. The incoming RF is actually split into two channels and so is the LO. One RF channel is directly mixed with the LO and the output is called the "I" output. The other RF channel is mixed with the LO signal which has been phase shifted by 90 degrees (i.e. is in Quadrature). This output is called the "Q" output. The the actual output is comprised of two "audio" channels, one called "I" and one called "Q" (usually written as I/Q). This can be sent over a standard stereo audio connection to sound card. Although called a "sound" card, it's actually just a D/A (and A/D) converter what can handle signals from the audio range up to some higher value. A typical basic sound card might be able to handle 192,000 samples/sec and I believe that's the data rate of the FunCube Pro+ I/Q output. Using the I/Q data it is then possible, via Digital Signal Processing (DSP) to use the amplitude and phase information in the I/Q signal to demodulate the data to receive USB or LSB (i.e. the image frequency is rejected), or indeed any form of amplitude or phase modulation including AM, FM and other modes.

The key point here is that the Funcube outputs I/Q data, not demodulated audio like SSB. If you just look at the I or the Q signal (the sum of the two) you get a Double Siide Band signal (DSB) with no image rejection.

WSJTX looks for an demodulated audio signal that is typically USB (but could be LSB). It cannot detect or process an I/Q signal. If you feed it an I/Q signal it will assume it's demodulated audio and will treat it as though it was SSB. If it's DSB (which it will be for processed I/Q) you will receive on the image frequency at the same time as the desired frequency. So you will see a desired signal+noise in one channel and noise (plus possibly another signal) in the other channel. At best this will double the noise, at worst it will mix two different signals as well as adding noise.

MAP65 on the other hand is designed to only accept an I/Q signal. The program itself then decides how to demodulate that signal and has the software required to turn it into single channel SSB audio with image rejection. MAP65 can also do other tricks with selective polarization but I won't get into that here.

The critical point is that WSJTX wants normal SSB audio, such as might come out of the speaker of your radio. That's an audio bandwidth from around 400Hz to at least 2500Hz, maybe as high as 4000Hz. On the other hand, MAP65 wants I/Q audio covering the range from 0Hz to around 85kHz, so it covers a much wider spectral range. If you feed it regular demodulated audio, it won't work.

Case 1 - using the FunCube Pro + as a receiver with WSJTX

When configuring WSJTX if you look in the radio section and scroll down the list of supported radios you will find an entry "AMSAT-UK FUNcude Dongle pro +". "Great", you might think. Problem solved. In fact you can enable this radio and with the rest of the configuration set right, it will work. It will track Dopper and receive signals just like a regular radio. The problem is that they the SNR you see will be at least 3dB weaker than you expect (and would see with a regular radio). This is because it's looking a Double Sideband I/Q audio and it's seeing whatever is on the image frequency (including noise) along with the desired signal and noise. SO you might think this is pretty much a useless steup, but you would be wrong as I'll get to shortly.

The way to get WSJTX working properly with the FunCube pro+ is considerably more complex and I won't go into details, just the principle. First you have to control the Funcube via an SDR program that understands I/Q audio and is configures to be able to control FunCube pro+ settings (frequency and gain). This can be something like HDSDR or SDR Console. The connection will be via a USB port. To Doppler track, you then have to configure the SDR program to accept frequency control from WSJTX. To do this the SDR software will emulate a radio (often the TS2000), so you tell WSJTX that it's talking to a TS2000 over a USB connection on the other end of which is the SDR program pretending to be a TS2000. In this way WSJTX Doppler frequency comments to to the SCR program and the SDR program then tells the FunCubeV where to tune. That's half the battle. Now you have to establish a virtual audio connection between the SDR program and WSJTX so WSJTX can get the SSB demodulated audio it's looking for. If and when you get all these connections working, WSJTX will be able to talk with the FunCube via the SDR program. I'm sure you can also use Linrad to do the same thing if you can ever figure out how to do it!

Case 2 - Doppler tracking the FunCube pro + with MAP65

The issue here is MAP65 doesn't Doppler track. It reads the FunCube just fine. You can (presumably) go though the steps outlined above for WSJTX, but this time instruct the SDR program to output I/Q audio NOT demodulated SSB audio. In that case MAP65 should be capable of understanding the I/Q, demodulating it internally and operating as normal. But there's an easier way.....

As I outlined in case 1 you can get WSJTX to Doppler track with the FunCube pro+ just fine. It's the audio that's the problem. But here in MAP65 the audio is fine, it's the tracking that's the problem. So what you do is startup WSJTX, configure it for the FunCube pro+, set your frequency and Doppler tracking mode and away it will do giving you decodes that are 3dB weaker then they should be. But now you start up MAP65 and configure it for the FunCube pro+ too. It seems that the FunCube pro+ is perfectly happy listening and talking to both programs at the same time. So WSJTX controls the frequency of the FunCube pro+ and MAP65 demodulated the audio and displays and decodes it as normal without the image frequency adding noise.

Life is not perfect though and MAP65 sets it's frequency marker scale manually. You type in the center frequency and the wide graph frequency scale appears (if you are lucky...). It won't change as the FunCube pro+ Doppler tracks. Your signal will stay in the same place on the waterfall as it should, but the frequency markers will also stay in the same place (which they shouldn't). Not a lot you can do about this. Yud'd have the same issue going through and intermediate SDR program.

Bottom line

If you need Doppler tracking on MAP65 with a FunCube pro+, you can do it by running both WSJTX and MAP65 at the same time. Compared to using an intermediate SDR program, this is very easy. If you need to run WSJTX with a FunCube pro+ it's easy if you don't mind a 3dB penalty because it has no image rejection. If you want that 3dB back, you have to go the complex route via and intermediate SDR program.

Acknowlwdgement: Thanks to Charie (DL3WDG) for first suggesting that the observed 3dB lower SNR with the Funcube pro+ feeding WSJTX could be due to noise from the image frequency.