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

[Dan Halperin]

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The maximum (at 16 bits complex samples) data rate across USB on the
USRP is 16 MHz; hence the minimum decimation rate of 4. My understanding
is that this decimation gives more bits -- since you're combining 4
samples together, you can now extrapolate between 12-bit ADC samples for
a higher resolution. So the min/max data range for complex data from the
USRP is -32768/+32767, the full range of the 16-bit short coefficients.

- -Dan

Bahn William L Civ USAFA/DFCS wrote:
> 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?
> 
> Thanks.
> 
> 
> 
> 
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