alantlk wrote:After mulling it all over on a train journey today I'm pretty sure the disparities in readings are being caused at least in part by parasitic or unbalanced capacitance to ground, and associated voltage nodes.
I've run several tests again using a signal generator and an RSP2, the indicated levels between SAS and SDRuno are usually within 1 or 2db of each other. I don't have a transmitter available so I'm not able to perform any tests under the conditions you've mentioned. My test setup consists of a signal generator connected directly into the RSP2. All equipment was bonded to a solid ground consisting of the usual ground rods. The connection from the ground rods to my equipment is around 2 metres, they are just outside my office window. The same test was carried out with my main ground disconnected, the results were the same. My USB cables are nothing special, and are not fitted with ferrite beads.
alantlk wrote:I don't understand why SAS and Uno measure differently in each setup? I assume those measurements are done a different way in each software?
The SDRplay API reports the current system gain and this is applied to the output from the FFT after the power calculation. I have to assume that I'm doing the calculations correctly as the correlation between my calculated dBm levels and those observed with SDRuno is very close when using a signal generator.
Pseudo code for the calculation is:
for each FFT bin:
real = real * 1/32678 // ADC normalisation
imag = imag * 1/32768
V = sqrt (real * real) + (imag * imag)
V = V/num_FFT bins
V = V * 1.5 // ADC FSD
P = (V * V)/50
dBm = dB + 30 // scale to dBm
dBm = dBm - API_gain_value
The output level from my generator appears to be accurate as tests made by SDRplay with their own generator correlate very closely to the results I obtained. Perhaps somebody may be able to shed some light on this ?