>> * plot the RF signal FFT and the FFT of a WBFM demodulator >> block as found in GNURadio (always same settings: 30 dB IF >> gain and 30 dB RF gain) >Hmmm, Dongles are different. I suggest that you compare them >at maximum gain which is different for different dongles.
this is an issue I have been wondering about and maybe you or someone on the list can comment: what are the gain ranges for the various front ends ? I have not been able to identify neither in the source code of osmosdr & rtl-sdr nor on the datasheets what the RF and IF gain ranges are. Are they different for the E4K, FC0013 and R820T ? If so indeed, I'll have to adjust the emitted power to observe a constant received power. > * assess the receiver quality by the signal to noise ratio of the FM > output. This is a very good method - but it is necessary to place the > frequency right. I can see from your graphs that the signal is at > the passband centre where you have a spur and 1/F noise. this also explains what I observed when I was trying to reproduce the measuements with AM: the signal would be lost if locking exactly on the carrier frequency due to the band pass filter after the mixer in the zero-IF scheme. I was not able to identify the reproducible setting condition you mention below for the experiement: I will do so. > Repeat and place the FM modulated signal signal halfway between the > passband center and the passband edge. Your current results are not > valid because of the center spur which is within your passband. Thank you for these informations. I am away from the lab next week but will update the measurement results asap. JM
