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Al's picture

Thanks for replying with your findings.

- I'd expect the level at the dongle input to remain constant with your input, the various gain blocks are found after this in both the tuner and the RTL2832U (which contains the ADC).

- The RBW of a digital spectrum analyser differs from an analogue one; in analogue ones it relates to the IF filter width whereas in the digital world it is based on the FFT bin size, National instruments have a brief white paper on this subject.

- I haven't been able to find and specs on linearity but I'd expect it to be quite reasonable, any errors introduced by the FFT should be very small.

- I'm a bit surprised clipping occurred at -5dB, what gain was this tested at?

It sounds like I better add a calibration setting to the software, I think I'll add it to the 'Radio Devices' dialog in the 'Edit' menu.  Stay tuned (pun intended)!

The software calculates the power spectral density (using Welch’s average periodogram method) of the input signal rather than the raw FFT, this results in an average level of the signal being displayed. Reducing the dwell will reduce this but also increases the chance that a non-continuous signal will be missed, I consider this an acceptable trade-off.

The 2-3dB drop could also be attributed to the PSD but I've also got a feeling that the driver doesn't turn off all the AGCs in the signal path with R820T tuners, but I could do with having a proper look at the code to be sure.