Re: [Discuss-gnuradio] USRP2 rx copy handlers

[Eric Blossom]

On Sun, Feb 28, 2010 at 02:57:45PM +0000, Tim Pearce wrote:
> Hi,
> 
> I've been having a look at usrp2/host/lib/copiers.cc and the schematics to
> try and work out if the units mean anything.
> 
> As far as I can tell the LTC2284 ADC is set up to use the +-1V input range
> and output 14bits as a twos complement.
> 
> From the datasheet/the maths I think the ADC is thus giving possible outputs
> of -8192 to 8191 (i.e ~ +- 2^13) where -8192 represents -1V and 8191
> represents 1V-0.000122V.
> 
> Where I'm confused is with copy_u2_16sc_to_host_32fc the integers are scaled
> to produce floats where -32768 and 32768 (i.e using the full 16bits of the
> storage variable) are mapped to -1.0 and 1.0 respectively. Is there a reason
> for this? I haven't tried to look at what the FPGA is doing with this data
> yet..

The output from the ADC is run through a digital downconverter in the
FPGA.  In general, the output of that process is between -32768 to
32767.

If you're looking for "volts", you'll need to calibrate the entire
signal path.  That will depend on the daughterboard, it's gain,
temperature, etc.

A bit of searching of the list archives will turn up quite a bit of
discussion on this topic.

Eric