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Re: [Discuss-gnuradio] A longish-term drift study of the USRP/DBS_RX com
Re: [Discuss-gnuradio] A longish-term drift study of the USRP/DBS_RX combination
Wed, 11 Jan 2006 21:26:35 -0500
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mike revnell wrote:
The astronomy club at the local college (New Mexico Tech) has a little
two element interferometer that I can get access to. I'm supposed to
be working on some software to point the antennas now. It has two 10
ft dishes with about a 200 foot baseline. I bought two dbsrx boards
with my USRP with the idea of maybe hooking it up one day and have
been giving a little thought about what is needed to get fringes.
Getting some kind of image out of it is another thing entirely.
You don't have enough u,v plane coverage to produces images, with a
fixed-baseline instrument--at least that's
what I understand, but you'd know better than me.
Using it as a transit instrument (point the antennas south at the
transit elevation of the source and letting the source drift through
the field) should be relatively straightforward. Getting fringes while
tracking the source across the sky is more involved.
The "getting fringes while tracking" is something I still don't fully
understand. And any time I think about
it and think I understand it, it slips away from me like a gossamer
thread floating on a summer breeze :-)
The receivers presently installed form an adding interferometer which
is nice and simple and don't require stable LOs and all that. The
signals from the antennas are combined at the receiver input and the
whole mess is downconverted together. Interpreting the data is simpler
as well. But a real correlator is more interesting.
I worked on the NRO amateur observatory, that never quite got to the
fringes stage. But it was
a (hardware) correlator design, specifically because of the drift
problems inherent in an adding design.
And it was simpler to build than something like a phase-switching system.
Matt demonstrated simple correlation interferometry with the USRP at
this summers SARA meeting. The PLLs
for the receiver boards are indeed driven by the same clock source,
and with a little tweakage, the intention
In looking at the schematics for the USRP and DBRX it looks like all
the LOs are derived from the same reference oscillator. This is a
requirement for correlating at baseband and would be a problem to
solve if using more than one USRP. Mine looks like it has a footprint
for an SMA to apply an external reference but it is occupied by a
piggyback that has a little crystal oscillator on it. It seems to be a
fix for a problem on the main USRP board.
was that the USRP could be slaved to a master oscillator of some sort.
On a strong source with a transit instrument it should be possible to
use short integration times and record AC fringes at the correlator
output. This is the simplest case. Just FFT both basebands multiply
one FFT output by the complex conjugate of the other and take the
magnitude. The inegration time is the length of the FFT input
sequence. Could average or filter by frequency bins or integrate over
the spectrum and work with that.
That sounds relatively simple.
Normal practice is to tune the LOs in the antennas to move the fringes
for a give piece of sky down to DC. This allows integrating for longer
times using hardware accumulators. This can be done, as in the VLBA
correlator, at the FFT input. The frequency translating FIR block
should work nicely but it may be necessary to work out some way of
controlling the relative phases of the chains in the two antennas. LOs
in radio interferometers need to be continuous phase. This means that
if you move from a source, to a calibrator say, and move back you need
to be able to put the LO phase back to where it would have been if you
had continued to track the source. This has a profound impact on the
design of the LO synthesizers.
Yikes, that sounds tricky!
Interestingly, the largest correlators that I'm aware of that are
being designed and built are XF architectures.
The correlator for the ALMA project being built at NRAO in
Charlottesville VA, and the WIDAR correlator for the EVLA project
being built in Penticton, BC Canada. A smaller version of the WIDAR
design is to be used in the EMERLIN project at Manchester, UK. The
reason for this is that FX correlators tend to be what people who
architect these things call copper dominated. It takes a lot of
interconnects to get the results from the frequency analyzers (usually
hardware FFT engines) to the core correlator. XF correlators tend to
be what's referred to as silicon dominated, there is a lot of
processing in the correlator. Right now silicon is cheaper than copper.
I'll have to re-read some of the WIDAR documents again--I thought it was
FX based, but it's been awhile.
And I admit to not fully understanding it all yet--but I do have the
NRAO Synthesis II book, which I'm
slowly working my way through. I have an acqaintance at DRAO in
Penticton, but he does't work
on the correlator stuff, he runs the 10cm solar observatory.