Re: [Discuss-gnuradio] USRP/LFRX ADC Saturation and Range

[Sebastiaan Heunis]

Hi Dan.

I don't think 2^16 denotes the maximum sample value in the USRP.  We
have a 12bit ADC, so we start off with -2048->2047.  This then gets
bit shifted to the right by 3 places when it enters the USRP, so we
have x8.  The CORDIC causes our sample values to be divided by about
1.6.  Then we also have the CIC and HBF.  The CIC's gain varies
depending on the decimation and the HBF's gain is 1bit (6dB).

Looking at the ADC's datasheet, we need 2Vp-p to fully utilise its
dynamic range.  The input impedance of the ADC is around 220ohms so
this is equal to an input signal level of about 6dBm.  If you go above
this you will get saturation.

I want to do some simulations with input signals and the algorithms on
the USRP to determine the maximum sample value.  I'll post the results
of this when I'm done.

Sebastiaan