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Hi Al,
I played with dongle calibration, using Siggen (modulated with UMTS downlink, BW=3.85MHz) and Agilent SA (N1966A).
I have couple of comments:
- Gain setting of the dongle affects the displayed level accordingly, which means the displayed result is for the level at ADC input, and not the level at dongle input.
- FFT size setting affects the RBW value displayed on "properties", but it doesn't change the displayed level, which means it is not the same thing as RBW of the spectrum analyzer (it is more like VBW).
- The change of input level by 10dB changes the displayed level by the same amount, which means the displayed result is proportional to power level at ADC input.
- Saturation of displayed signal occurs at displayed level a bit below -5dB.
Taken all above into account, my RTL dongle has correction factor -16.28dB (at 920MHz).
So, when displayed peak level value is corrected for bandwidth and previously mentioned correction factor (-16.28dB), I am getting pretty accurate power level reading for WCMA signal (or WB noise).
However, CW signals scale doesn't seem to be treated the same way (FFT issue?), so calibration factor for those will be different.
Also, I noticed a 2-3 dB drop of noise floor close to signal (AGC-like?) but otherwise the displayed level value is consistent with attenuator setting... which means RTL dongle and your app is very useful as simple spectrum analyser.
Cheers, Bojan