OSMOCOM-fft

Sylvain Munaut 246tnt at gmail.com
Mon Oct 29 09:13:06 UTC 2018


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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