[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Tom Van Baak]

> But is it really an improvement that you get out of it? The answer is
> NO! He, why not? The answer is: Because you have to PAY the increase in
> precision with the increase in observation time. For every increase of
> 10 in precision you need to increase the observation time by 10! 

Ulrich,

Thanks for the long contribution. One minor correction:
you imply that increase in observation time is a bad or
undesirable thing. This is usually true. But not really in
the case of a GPSDO. Due to GPS receiver 1PPS noise
you must average over many minutes anyway so this
greatly relaxes the requirements on the TIC.

What you say later about the sigma-tau lines is all
correct. I just wanted to point out, for example, that
a picosecond accurate TIC is a complete waste for
a GPSDO when the 1PPS jitter is on the order of
several nanoseconds.

> Would the Shera design make use of a Agilent 51151
> as a phase comparator its noise floor would start at
> 5E-10 @ 1s which is a REAL improvement by a factor
> of almost 100!!! For any given precision the Shera TIC
> will need 100 X the time that the 51131 needs. 

Don't mislead yourself. At 1 s you are limited by GPS
1PPS noise. Having a better TIC doesn't fix this. If your
GPS noise is 2e-9 at 1 s you don't really need a TIC
that is good to 5e-10 at 1 s. So the gain isn't as useful
as you might think.

I should remind readers that the Shera design came
from an era of S/A and Oncore VP receivers where the
modern numbers you throw around do not apply.

/tvb