I’ve been prototyping parts of SmartMic (more on that later), for example the interface between the micro-controller and the radio microphone input. I have been using FreeDV as a test tool to evaluate the quality of the signals passing through my prototype circuits. As I worked I recalled a conversation with Mel Whitten. He said that many people over-drive transmitters when using FreeDV, and even how he wasn’t sure what Scatter Plots were used for – after years of DV work!
So here is a video that explains some of the mystery behind Scatter Plots, and how they can be used to measure the quality of a FreeDV signal. For example as you change the Tx drive, AF gain, and the effect of noise and Tx pass-band filtering. As well as FreeDV it applies to any QPSK modem (they are used everywhere, from HF radio to Wifi to Sat-com).
Also from Mel on this topic:
“Gerry and I have tested the effects of overdrive both audio and RF showing the same results as you demonstrated. Its very easy to lose 10-15db and 20dB in the shoulders of the flat top signal with the “skirts” flying out spewing IMD into the adjacent channel. Nasty. I guess we need a spectrum mask for FreeDV like commerical DRM and DV broadcasters use. How may dB should the shoulders be down with FreeDV? 40+ dB is possible. All this is very easy to see with a Flex radio spectrum display. Spectrum re-growth from IMD is easily seen in the Flex waterfall also. I think very few hams understand all of this and as a result can be a very bad “neighbor” on the adjacent channel due to their over-driven rigs (especially amplifiers). There are times when EasyPal and its 2.4kHz of OFDM will “spread” 6-8kHz wide because of non linear amplification.”
I feel there is a lot of tuning we can do to match FreeDV to radios and optimise performance, perhaps by developing some simple tools so operators can test their FreeDV signal and make sure it is optimal. In the video above I think the last carrier is attenuated by the FT817, which has a -6dB point at 2600Hz. In low SNR channels, a weaker carrier will be wiped out first as the noise floor climbs, leading to a bit error rate of a least 1/16.