David Carr wrote:
All this RA talk makes me curious about what a typical RA IF setup
Do RA samplers usually employ quadrature sampling (I and Q) or do they
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
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.
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