Re: [Discuss-gnuradio] bandwidth for USRP

[Marcus Leech]

David Carr wrote:
> All this RA talk makes me curious about what a typical RA IF setup 
> looks like.
>
> Do RA samplers usually employ quadrature sampling (I and Q) or do they 
> sample
> a "real" signal at twice the desired bandwidth? Also, if the big boys 
> use 1GHz sample rates, what do the little boys use? 10s or 100s of MHz?
They typically use quadrature sampling, with dual polarization, giving 
you a total of 4 digital channels you need to
 deal with.

Amateur RA types have typically, up until now, done sampling *after* a 
hardware square-law detector and
 analog integrator.  That requires only very-modest sampling 
bandwidth.  But the pre-detection bandwidth
 is usually on the order of a few Mhz, and sometimes only a few dozen 
Khz, depending on observing
 frequency, local RFI environment, etc.


But that is changing quickly with hardware like the SDR-14, and now the 
USRP/Gnu Radio.   I've been able to
 compute continuum and spectral data at 8Msps on a 3.4Ghz 
relatively-recent server-class box.

> If you only have one or two bits of dynamic range there must be a 
> pretty big AGC in the loop.
> -DC
Gain control is manual.  RFI is mitigated, and because of the tremendous 
bandwidths in use, you get
 to "synthesize" extra bits of dynamic range.   In Radio Astronomy, 
you're looking at random noise
 buried below the noise floor of your equipment.  A few bits is all you 
need for this type of thing.
 The system noise floor is integrated out, leaving fluctuations arising 
from sources coming in and
 out of the beam of the antenna.  At least, ideally :-)