Hello Mic, > I have done a bit of experience with RTL-SDR, which I am trying to use it > as a spectrum scanner. That should be possible although you can not expect miracles;-)
> 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. ??????? Gain response. Do you mean how the real gain changes for different settings of hardware gain? > 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). OK. > 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 am afraid 3.2 MHz will cause loss of data points in an unpredictable fashion. You should not go above 2.4 MHz. When you loose data randomly you might even get I and Q mixed up and you might get signal energy from near the Nyquist frequency into the baseband below 1 MHz. > 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) But it is NOT zero mean. You should compute the average value, then compute the standard deviation from the average value. > 3) if needed, adjusts manually the gain in order to excercise the optimal > number of bits of RTL2832U and goes back to 1) ???????? It is unclear to me what you do here. > 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". What you say is very confusing to me. It seems a lot of information is missing. > Of course I tried using > http://eartoearoak.com/software/rtlsdr-scanner > for a comparison. Something seems wrong with this link. The noise floor is very different above and below the signal. I have not studied in detail, but at a first glance it does not look right. > 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. They look at the center and the aliases will be pretty well attenuated. I guess they want a small safety margin vs the maximum at 2.4 MHz. > 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. Again, I am confused. You say the signal is 8 MHz of random noise. What do you mean by "skirts appear?" Maybe you are looking at nonlinearities in the stages preceeding the filter. You say inject -65 dBm. It that the total power in the full bandwidth of 8 MHz or is it what you read in some instrument bandwidth of your spectrum analyzer? The 820T is more sensitive to out-of-band signals than E4000 and FC0013. Look here: https://www.youtube.com/watch?v=14CD1ERdRn0 In the E4000 tuner I know how to set the gain distribution and Linrad uses that knowledge. You can see what it means here: https://www.youtube.com/watch?v=cVU5X1d2XYU The problems with 820T seems to be filtering and aliasing. Front end linearity seems good: https://www.youtube.com/watch?v=e8UBxlBlM6k My experiments indicate that all the dongles are reasonably good and use the 8 bit range of the RTL2832 chip reasonably well. The 820T does not have zero IF so it suffers from mirror images. It seems to suffer also from aliases on the signal itself as well as on the images. Maybe that is what you see? I think the FC0013 would be a better choice for a spectrum scanner. > 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? I do not think your observation indicates that the ADC is not properly driven. I think the 820T suffers from other weaknesses. Regards Leif / SM5BSZ
