[time-nuts] Compensating phase differnces in dual frequency GPS receviers?

Attila Kinali attila at kinali.ch
Fri Dec 2 10:28:09 EST 2011


On Fri, 02 Dec 2011 07:11:38 -0800
Jim Lux <jimlux at earthlink.net> wrote:

> On 12/2/11 6:28 AM, Attila Kinali wrote:
> > On Fri, 02 Dec 2011 05:57:35 -0800
> > Jim Lux<jimlux at earthlink.net>  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)

Thanks, I got that one paper. I'll read it on my way home.

> 
> 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.

IEEE papers are not a problem, i have access to them.
 
> >> 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.

Hmm? Wouldnt exactly that dispersion be the thing i want to measure.
Maybe i should describe what i mean. Using a receiver, which has
one common LNA for the antenna input, then different filters and down
mixers for each frequency band until it reaches the ADC. Ie all bands
have their own path trough the system (worst case scenario in terms of
phase difference).

If i then feed a comp into the first LNA, i should be able to measure
the phase difference/dispersion of the different paths and get exactly
the value i'm looking for. Or am i missing something?

 
> 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)

Ok, i'll try to google and read a bit over the weekend.

Thanks a lot!

			Attila Kinali

-- 
The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin




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