[time-nuts] GPS active antenna delay ?

Tom McDermott tom.n5eg at gmail.com
Sat Feb 7 13:07:44 EST 2015


I have a white-hockey-puck style active GPS antenna, probably about 1998
vintage with no markings except 'Made in Mexico'.

While compensating for cable delay is relatively straight forward by
measuring the length and compensating for
the velocity factor, a question is: how much amplifier / filter group delay
is to be expected within the antenna itself?

I'd assume that the amplifier in the antenna probably has pretty small
group delay, but the RF filter may be significant.

Looking through GPS SAW filter datasheets seems to show none with group
delay specifications.

googling leads to some research papers with delays of about:

L1 - 20 MHz wide SAW filter has about 15 nsec of group delay
L1 - 2 MHz wide SAW filter has about 65 nsec of group delay
L1 - LC filter - can't find anything, but suspect it's probably just a few
nanoseconds.

I'm not sure a consumer grade antenna even has a SAW filter, it may simply
be an LC filter.
If it does include a SAW filter, then just using cable delay alone would
seem to underestimate the actual antenna
delay compensation needed for GPSDO, perhaps significantly.

Has anyone on the list measured or otherwise estimated the active antenna
delay including the amp and filters?

-- Tom, N5EG


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