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

[dan s]

Hi Sebastiaan,

Thanks for responding.  Given what you explained, I'm still not sure if my results are in the range they should be.  Do you have any guesses?

At 10 dB of gain set on the USRP,  I observed saturations at about +14,964 and -12,025 counts.  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).  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 Heunis <address@hidden> wrote:

From: Sebastiaan Heunis <address@hidden>
Subject: Re: [Discuss-gnuradio] USRP/LFRX ADC Saturation and Range
To: "dan s" <address@hidden>
Cc: "Gnuradio" <address@hidden>
Date: Saturday, May 30, 2009, 7:26 AM

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