comments on synchronous AM

ka9q

Thank you for adding this a few months ago! I've wanted it for some time.

Some comments/questions. In the "SAS" (stereo) mode you put the LSB and USB on opposite channels, which is really ISB (Independent sideband) with AFC. That's one way to do it; the other is just like I/Q mode except you phase lock the carrier to the +I axis. Since there's no real AM analog stereo anymore I suppose ISB is more useful.

I assume that in SAM (synchronous mono) mode that you do put the carrier on the +I axis and then put the I axis signal on both channels?

Is the carrier loop bandwidth set by the PLL bandwidth setting in the "I/Q display extension" window? I sometimes have trouble tracking carriers, e.g., Voice of (North) Korea's H3E transmissions on 6070 kHz. H3E is upper sideband + carrier, e.g., as on CHU. I can't tell if VoK's carrier is reduced amplitude, which could be part of the problem.

jks

I'm simply using SAM code from Warren Pratt's excellent wdsp package as pulled from the Teensy-ConvolutionSDR project. So I don't know too much about how it works. The code can be found on your Kiwi in the file /root/Beagle_SDR_GPS/rx/wdsp/SAM_demod.cpp

It has its own internal PLL code and does not use Christoph's C++ PLL code that is shared by the IQ and timecode extensions. Comments in the SAM code mention that the PLL zeta and bandwidth should probably be adjustable for varying conditions (e.g. dx, "medium", "fast"). Right now they default to a zeta of 0.65 and b/w of 200 Hz.

With the recent reorganization of the noise blanker/filter controls there is now some empty space on the audio tab of the main control panel. Perhaps some SAM PLL adjustment controls could be added there.

ka9q

Thanks much! A loop BW of 200 Hz does seem rather wide for AM on HF. If the loop gain or damping factor depend on signal strength, as they often do, it can also be upset by low frequency modulation components.