Oct 252019

In my last post, I mentioned that I was playing around with SDR a bit more, having bought a couple. Now, my experiments to date were low-hanging fruit: use some off-the-shelf software to receive an existing signal.

One of those off-the-shelf packages was CubicSDR, which gives me AM/FM/SSB/WFM reception, the other is qt-dab which receives DAB+. The long-term goal though is to be able to use GNURadio to make my own tools. Notably, I’d like to set up a Raspberry Pi 3 with a DRAWS board and a RTL-SDR, to control the FT-857D and implement dual-watch for emergency comms exercises, or use the RTL-SDR for DAB+ reception.

In the latter case, while I could use qt-dab, it’ll be rather cumbersome in that use case. So I’ll probably implement my own tool atop GNURadio that can talk to a small microcontroller to drive a keypad and display. As a first step, I thought I’d try a DIY FM stereo receiver. This is a mildly complex receiver that builds on what I learned at university many moons ago.

FM Stereo is actually surprisingly complex. Not DAB+ levels of complex, but still complex. The system is designed to be backward-compatible with mono FM sets. FM itself actually does not provide stereo on its own — a stereo FM station operates by multiplexing a “mono” signal, a “differential” signal, and a pilot signal. The pilot is just a plain 19kHz carrier. Both left and right channels are low-pass filtered to a band-width of 15kHz. The mono signal is generated from the summation of the left and right channels, whilst the differential is produced from the subtraction of the right from the left channel.

The pilot signal is then doubled and used as the carrier for a double-sideband suppressed carrier signal which is modulated by the differential signal. This is summed with the pilot and mono signal, and that is then frequency-modulated.

For reception, older mono sets just low-pass the raw FM discriminator output (or rely on the fact that most speakers won’t reproduce >18kHz well), whilst a stereo set performs the necessary signal processing to extract the left and right channels.

Below, is a flow-graph in GNURadio companion that shows this:

Flow graph for FM stereo reception

The signal comes in at the top-left via a RTL-SDR. We first low-pass filter it to receive just the station we want (in this case I’m receiving Triple M Brisbane at 104.5MHz). We then pass it through the WBFM de-modulator. At this point I pass a copy of this signal to a waterfall plot. A second copy gets low-passed at 15kHz and down-sampled to a 32kHz sample rate (my sound card doesn’t do 500kHz sample rates!).

A third copy is passed through a band-pass filter to isolate the differential signal, and a fourth, is filtered to isolate the pilot at 19kHz.

The pilot in a real receiver would ordinarily be full-wave-bridge-rectified, or passed through a PLL frequency synthesizer to generate a 38kHz carrier. Here, I used the abs math function, then band-passed it again to get a nice clean 38kHz carrier. This is then mixed with the differential signal I isolated before, then the result low-pass filtered to shift that differential signal to base band.

I now have the necessary signals to construct the two channels: M + D gives us (L+R) + (L-R) = 2L, and M – D = (L+R) – (L – R) = 2R. We have our stereo channels.

Below are the three waterfall diagrams showing (from top to bottom) the de-modulated differential signal, the 38kHz carrier for the differential signal and the raw output from the WBFM discriminator.

The constituent components of a FM stereo radio station.

Not decoded here is the RDS carrier which can be seen just above the differential signal in the third waterfall diagram.