Re: [Discuss-gnuradio] Maximum value of samples

[Don Ward]

Sebastiaan Heunis wrote:

> Can anyone please suggest a setup that I can use to test the maximum
> value of samples?  I know that we have 16bit signed values.  Looking
> at the ADC interface on the USRP, we sign extend each 12-bit sample
> and add 3 zeros to the right.  This should give us a maximum value of
> 16376 for samples?
>
> I tried using a BasicRX daughterboard.  I have a function generator,
> inputting a 20MHz, 10dBm signal into the db.  In my app I mix this
> down to 100kHz using the cordic.  I also decimate by 16.  The PGA gain
> is set to 20dB.  The FPGA build is the standard 2rx/2tx with hbf.  The
> maximum sample value that I get with this is 8635, so I'm using about
> half of the ADC's dynamic range.

You also need to consider the inherent gain of the CORDIC algorithm (approx. 
1.647/2 in the USRP) and the effect of filtering out the negative frequency 
components of your signal.  Because of these, you can use the full dynamic 
range of the ADC and never see sample values close to 16376.  In fact, at 
10dBm input and 20dB PGA gain I expect your ADC is clipping.  Have you 
looked at a plot of your recieved waveform?

-- Don W.