On 12/2/11 6:28 AM, Attila Kinali wrote:
On Fri, 02 Dec 2011 05:57:35 -0800
Jim Lux<[email protected]> wrote:
Combs are used all the time for this kind of thing (e.g. calibrating
Deep Space Network). There's an old paper about calibrating a
interferometer radio telescope at Stanford using this kind of thing (by
Bracewell, as I recall)
My google skills fail me on locating this paper. Could you give
me a bit more information? Like the title or the journal it was published in?
I'll see if I can find it. It's in my files at work
there's a Bracewell paper out there called "the Stanford Five Element
Radio Telescope" from Sep 1973 IEEE Proceedings but that doesn't have
the description I'm thinking of (it was an earlier paper.. I'm thinking
late 50s or early 60s)
"The local oscillators at the antennas are phase locked to a reference
signal which is distributed from a centrally located oscillator. A
system for monitoring variations in the electrical lengths of the
reference-signal cables is incorporated using modulated reflectors at
the five antennas. "
There are a fair number of papers out there describing how we do
calibrations at deep space network, and they're not behind a paywall.
I'll look for those links too.
It's easy to distinguish the comb from the GPS signal... the GPS is PN
coded, the comb is not. If you pick your levels right, depending on
your digitizing strategy, it might not even jam the GPS, so you could
leave it on.
Hmm.. right...
However, even that's not a panacea, because generating and distributing
that idealized comb is non trivial without destroying the phase
relationship between the comb "teeth". I guess it really depends on how
nutty you want to be. 1 nS is pretty easy, I would think. 1pS is a lot
harder. 1 fS is very hard.
I don't think that sub 100ps is even necessary, as the rest of the
system will be hardly able to get to that resolution (The available ADCs
have<100Msps and too long integration times do not work well in hardware).
But<1ns should be feasible and desirable.
But, what's the problem in distribution of the signal? I would have
(probably naively) feed that into the input LNA.
dispersion in the components after the comb generator. You may generate
them all in phase at the diode, but by the time they've propagated
through the buffer amplifier, filter, coax, they're no longer aligned.
Again, if you're just looking for nanoseconds, that's like whole cycles
at 1GHz, so these factors are irrelevant. It's when you're looking at
fractions of a wavelength at 30 GHz it gets tough.
What people who obsess about such things do is look at the reflected
signal coming back from where you inject it into the receiver, and
calculate the round trip time.
There's some papers about calibrating the VLA that also describe the
techniques. Radio Astronomers have been doing comb calibrations for
decades, so it's pretty well known (and all the myriad sources of error
have been identified, as well)
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