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Re: [Discuss-gnuradio] A longish-term drift study of the USRP/DBS_RX com

From: Marcus Leech
Subject: Re: [Discuss-gnuradio] A longish-term drift study of the USRP/DBS_RX combination
Date: 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.

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.

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
 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.

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