Re: [Discuss-gnuradio] USRP Dynamic Range and 8 Bit Problem

[Don Ward]

Hi Firas,

[several examples snipped]

> d) With Sinewave signal at frequency =250KHz and 10dBm input power to
> BasicRX we get ,
>
> Theoretical calculations :
> When +10dBm signal injected into 50 Ohm load, then signal RMS = 0.71Volt 
> and
> Signal P-P = 2.008183259Volt.
>
> USRP readings :
> The maximum counting obtained from USRP for this signal was 13570
> (Saturated).
>
> Discussion :
> From the above results we see that the BASIC-RX is saturated when the 
> input
> signal power was +10dBm. This is logical, since the maximum input signal 
> to
> the USRP ADC AD9862 is 2Volt P-P (according to its data sheets).
>
> But, what is not logical is the value obtained from USRP FPGA. Since the
> incoming data is 16bit signed number (for I and Q), we should get +/- 
> 32767
> readings for the maximum input signal of 2V P-P. Instead, we have maximum
> reading of 13570. I don't know how this number was generated, but it 
> should
> be scaled to make use from the full dynamic range offered by using 16 Bit
> data transfer.

The USRP takes the 12-bit sample (peak value of 2047), shifts it left 3 bits 
(peak value of 16376), then runs it through a CORDIC stage that multiplies 
it by approximately 1.647/2 (peak  value of  13485).  This is very close to 
what you see.  For decimation by values that are not powers of 2, there is a 
further decrease in signal strength associated with the CIC decimator.

I think we could quickly double the processing gain (and gain 6 dB S/N and 
dynamic range at higher decimation rates) by either shifting by 4 bits 
before the CORDIC stage or eliminating the divide by 2 at the end of the 
CORDIC stage.  The maximum processing gain would then be 8*1.647=13.176 
(currently it is half that).  We could also double the gain in the CIC 
decimator for decimations where the current gain is less than 1/1.647.

-- Don W.