So, as a general rule of thumb the noise floor seen on the panadater with minimum amplification
and a 50 Ohm dummy load as antenna (or disconnected) should be given by the quantization noise:
NF = -6.02*8-1.76-10log(FFT bin size / 2)
but I think this formula is missing something in the case of the rtl-sdr since in the case
of r820t tuner a single IF on the I branch is sampled at 28.8 MSPS and then this is digital down
converted and decimated. So I tried to figure out the gain process as:
10log(28.8/2/3.2) ~= 6.5 dB as process gain. This seems to fit with what I see on the panadapter
of gqrx. I get about -92 dBFS with 2.4 MSPS, 0 LNA, and FFT of 8192.
Now the question for this group: there is a correction value, default offset or formula used in
gqrx code that take account of hardware implementation when displaying value on the dBFS scale?
and what are your opinion on my calculation?
Luigi,
to answer your question one really needs the system diagram that you're attempting to quantify, and details of how any adjustments are made. In regards to SDR, then there is an issue of processing gain (the digital equivalent of Q), when signals are passed through an FFT or an FIR filter, or some other DSP process. Generally, signal processing needs more bits than any front-end A/D converter provides to allow for this.
As for quantization noise, a well-designed system will generally perform better with dithering, either added or from noise at the input (or both).
A further observation is that an A/D is also characterised by the
ENOB (effective number of bits).
HTH, 73,
Robin, G8DQX
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There is one piece you are missing: the signal meter you are looking at does not refer to the entire input bandwidth from the hardware, but rather after the filter of the chosen demodulator. In other words, there is a second downconvert-filter-decimate step performed within gqrx, and you must account for that as well.
If I remember correctly (and I did look at the code for this once) there is no particular correction value in the meter.
Just remember that it is not dBm or any such real-world power level but dBFS — I think you've kept that straight but I'm not certain. Calibrating digital signal levels to a reference in mW or such is a whole other problem and must be done empirically rather than by calculation.
Generally, signal processing needs more bits than any front-end A/D converter provides to allow for this.
As for quantization noise, a well-designed system will generally perform better with dithering, either added or from noise at the input (or both).
A further observation is that an A/D is also characterised by the ENOB (effective number of bits).
HTH, 73,
Robin, G8DQX