Hi Sebastiaan,

Thanks for responding.Â Given what you explained, I’m still not sure if

my results are in the range they should be.Â Do you have any guesses?

At 10 dB of gain set on the USRP,Â I observed saturations at about

+14,964 and -12,025 counts.Â This was with a 32 bit decimation and

saving the recordings as 16 bit shorts

When I used a 50 ohm output waveform generator, I got a calibration of

about 37,900 counts/volt (with a 10 kHz sinewave input).Â I saw

saturations once I had an input voltage at around +/- .316 Volts (as

expected with 10 dB of gain), but I don’t recall what I got in terms of

counts for saturation.

Thanks,

Dan

— On Sat, 5/30/09, Sebastiaan H. [email protected] wrote:

From: Sebastiaan H. [email protected]

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

To: “dan s” [email protected]

Cc: “Gnuradio” [email protected]

Date: Saturday, May 30, 2009, 7:26 AM

Hi Dan.

I don’t think 2^16 denotes the maximum sample value in the USRP.Â We

have a 12bit ADC, so we start off with -2048->2047.Â This then gets

bit shifted to the right by 3 places when it enters the USRP, so we

have x8.Â The CORDIC causes our sample values to be divided by about

1.6.Â Then we also have the CIC and HBF.Â 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.Â The input impedance of the ADC is around 220ohms so

this is equal to an input signal level of about 6dBm.Â 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.Â I’ll post the results

of this when I’m done.

Sebastiaan