Question 1: There are three multiplexed ADC channels in the Airspy, is there a simple way to access the other two if I wanted to implement a custom DDC firmware?
Answer 1: .By default channel 0 is used for the R820T2, but channel 1 and channel 2
are unconnected. You could hack the firmware source code and modify
this line "#define DEFAULT_ADCHS_CHAN (0)" in
"firmware/airspy_m4/airspy_m4.c" to use channel 1 or channel 2 instead
of channel 0 which is connected to the output of the R820T2 tuner chip.
Of course the R820T2 would still be powered on so you should disable it,
since you are not using it (use disable_r820t_power() anywhere where
enable_r820t_power() is called) because it may add unwanted noise. Of course disabling the R820T2 chip, also means that you should
delete/disable any calls to set registers in it, since you are not be
using it. You should also delete/disable the frequency tuning from the firmware since the tuner is not used. And by the time you have done that it might be easier just to
write your own custom firmware.
Extra info 1: Since the spyverter is now available it might be easier just to use it unless you actually need an ADC, that in theory could run at 80MSPS. The problem is that the USB bus will limit the data transfer to about 20MSPS. And there is not enough RAM in the LPC4370 to buffer more than about 2x16KiB (double buffering) or 32KiB of data, and there is not enough processing power within the CPU to reduce this data. So unless 32KiB of data sampled at 80MSPS is enough for your function this is probably a dead end for your idea. If you are still going to use the two ADC ports the good news is that there is ESD protection in the R2 board, the R0 board had no ESD protection on the ADC inputs.