Dear all, I am confused about some parameters of osmocom fft spectrum analyzer.
I am wonder what exactly the vertical axis shows? It is written Power(dB); if so, what is the reference power? Is it 1dBW? Moreover, by changing the PGA Gain, I will have different values of power (dB). What is the relation between PGA gain and the received power in dBm?
In general, does anyone know how should I find the received power in dBm, having the PGA gain(dB) and the value on vertical axis. (Picture in attach)
Looking forward to your reply. Bests,
Hi,
I am wonder what exactly the vertical axis shows? It is written Power(dB); if so, what is the reference power? Is it 1dBW?
Generally it's dBFS ... i.e. relative to the full scale value you get from the SDR. Whatever that is is hardware dependent.
Moreover, by changing the PGA Gain, I will have different values of power (dB). What is the relation between PGA gain and the received power in dBm?
If the SDR gain are correct, increasing the gain by 1 dB should raise the value by 1 dB. But most SDR gains are not calibrated and really not that precise on only bear a vague correlation between their nominal value and their actual values.
In general, does anyone know how should I find the received power in dBm, having the PGA gain(dB) and the value on vertical axis. (Picture in attach)
You can't. The only way to do that would be for you to do a bunch of calibration measurement with your SDR at every gain level and across the temperature range and frequency range with known power sources. This is the exact reason why your Keysight/Tek/R&S/... spectrum analyzer costs a few order of magnitude more than your SDR ... they can do theses absolute measurement and the entire signal chain is done for that.
Cheers,
Sylvain
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Babak Mafakheri -
Sylvain Munaut