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[Discuss-gnuradio] What is the range of USRP complex sample values

From: Bahn William L Civ USAFA/DFCS
Subject: [Discuss-gnuradio] What is the range of USRP complex sample values
Date: Sun, 9 Sep 2007 18:19:03 -0600

For complex IQ data values, what is the min/max range both for data coming from 
the USRP and data sent to the USRP?

There are three obvious guesses:

1) Normalized to +/-1.0.
2) Direct match to the ADC/DAC ranges.
3) Normalized to a nominal ADC/DAC range.

The first is right out since the values coming back are typically much, much 
larger than that.

The second would result in the RX range (which uses a 12-bit ADC) being either 
0 to 4095 or -2048 to +2047 while the TX range (which uses a 14-bit DAC) would 
be either 0 to 16383 or -8192 to +8191. However, while I see both positive and 
negative values in the RX data stream, I also see values that are well above 
even the unsigned range.

This leads me to suspect that the data is being normalized to signed 16-bit 
values in both directions yielding a range of -32,768 to +32,767 by the simple 
expedient of left shifting the ADC output by four places and right shifting the 
DAC output by two places (with sign extension, of course). This seems 
reasonable as it would make migrating to higher (or lower) resolution ADCs and 
DACs transparent, at least until devices with greater than 16-bits of 
resolution are used.

Is that correct, at least where the useful range of values for complex IQ data 
is concerned?


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