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

Jim Lux jimlux at earthlink.net
Fri Dec 2 10:44:21 EST 2011

On 12/2/11 7:28 AM, Attila Kinali wrote:

>> 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?

no, you're not missing it.
The challenge is in getting a suitable comb into the LNA that hasn't 
been altered by the "stuff" between the junction that generates the comb 
and the LNA.  For the GPS kind of application, I suspect that you could 
set it up so that there's nothing between: the semiconductor device 
that's generating the comb just radiates directly.  Then you really only 
have the question of whether that radiator imposes any phase shifts.. 
but being careful about propagation paths and stuff should be enough to 
deal with it.

The radio telescope guys have the problem of sending the comb from the 
source to the multiple receivers.

>> 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.

When I get a chance, I'll ask the GPS guys here at JPL about what they 
do.  (my initial guess is that they do off-the-air tests of some sort or 

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