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

From: dan s
Subject: [Discuss-gnuradio] USRP/LFRX ADC Saturation and Range
Date: Fri, 29 May 2009 14:57:39 -0700 (PDT)

Hi I've done some tests over the last couple of months with a USRP 
and a LFRX daughtercard.

I have been using the usrp_cfile_rx.py script with decimation
of 32 (2 MHz sampling rate).

I did some calibrations using a waveform generator that has a 50 ohm
impedance output. I set the waveform generator to a low amplitude of
50 mV peak to peak.

I recorded some files using the usrp_cfile_rx.py script and saved
the output as 16 bit integer shorts. I tried 0 and 10 dB USRP gain settings.

Using a 50mV peak to peak sine wave at 10kHz:
At 0 dB of gain I computed a calibration of 12,383 counts/volt
At 10 dB of gain I computed a calibration of 37,679 counts/volt

I used the USRP/LFRX with a hydrophone (underwater microphone)
and obtained some recordings of sound events. Running the same
script and 10dB gain, I observed saturations
from loud events at about +14,964 and -12,025 counts.

I'm confused on a couple of issues.
1. Its strange that at 0dB I don't measure something like 32,767 counts/volt
the full range of the 16 bit short integer data type.

2. Its also strange (assuming my calibrations are valid) that I see saturations
at +14,964 and -12,025 with 10dB of gain. I would have expected to see
this closer to +11,923 and -11,923 (my counts/volt calibration divided by 3.16 the 10 dB gain)

Does anyone have any ideas on the two issues of confusion? I've
read many posts and still can't find answers.


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