Dear list, I have done a bit of experience with RTL-SDR, which I am trying to use it as a spectrum scanner.
The first part of my plan involved connecting the RTL-SDR to a source of filtered white noise and measure the gain response. I do that with a Bladerf broadcasting a random signal in a 8MHz band around 626MHz. The output of the bladerf is connected to a splitter and goes to a spectrum analyser and a RTLSDR, so that I can always monitor the power level I inject into the dongle (around -65dBm). I set a sampling frequency of 3.2MHz and discard frequency components outside +/- 1MHz in order to minimise aliasing in band. Packet losses are not important for a spectrum scanner so I don't consider that possibility a problem. I designed a simple software starting from rtl_sdr which 1) collects a packet 2) computes the standard deviation of the signal (which is assumed zero-mean) 3) if needed, adjusts manually the gain in order to excercise the optimal number of bits of RTL2832U and goes back to 1) 4) saves the packet to the disk and the parameters of its acquisition into a header later processed with Octave Well it does not work very well. Essentially what I see is that when the number of bits exercised in the ADC is greater than 2-3, the channel already contains alias images of out-of-band signals. So my 8-MHz channel around 626MHz has long "skirts". Of course I tried using http://eartoearoak.com/software/rtlsdr-scanner for a comparison. I don't understand why a sampling of 2MHz is used there, when it is the bare minimum since the filter 3dB bandpass seems to be precisely that. I see that all AGCs are disabled there, as I also did of course. But with high gain set (>30dB), skirts appear in that program too. Now what I cannot see is the ADC dynamic corresponding to those data captures. What the above seems to suggest is that the ADC is not driven optimally in the R820T? Ideally, one would expect to be able to exercise many bits without saturation of the tuner chain.. but there is little information on the IF VGA. Any speculations on this topic? Cheers, Mic
