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

Ulrich Bangert df6jb at ulrich-bangert.de
Fri Dec 15 07:47:01 EST 2006


i believe that Bruce as well as me is always referring to what the
receiver CAN do i.e. not the raw but always the sawtooth corrected
signal. That is indeed 2 ns (1 sigma).

> 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.

Thank you for clarifying this again! While i have been referring to the
measurement apparatus's noise floor for which my statements are correct,
one might indeed get into believing that every increase in resolution
leads to a increase in performance in a GPSDO. Clearly once that you are
below a certain point the GPS's jitter is the limiting number. 

I second Bruces's opinion about what is an overshot or not. When ps
reolution is ready available then why not use it? I attach a online
output from my DIY GPSDO from a few minutes ago that shows the M12+'s
signal properties when measured with abt. 110 ps resolution against a
FTS1200. The yellow line reperesents a prefiltered version of the
sawtooth corrected values (blue). The filter time constant is 1/3 of the
loop time constant as in a PRS-10. The yellow values are the ones to
feed the regulation loop.

What I wanted to explain is the Shera concept noise floor is a factor 20
above what a modern receiver can deliver (again inc. the sawtoth
correction). And yes, you are right: There were different numbers when
this concept was thought out! And exactly because different number were
there when this concept was thougt out I am going to ask why people
still built it today.

Best regards
Ulrich Bangert, DF6JB

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Van Baak
> Gesendet: Freitag, 15. Dezember 2006 08:23
> An: Discussion of precise time and frequency measurement
> Betreff: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS 
> locking circuit
> > Tom
> > 
> > A TIC with 0.5ns jitter at 1 second isn't actually too much 
> in the way
> > of overkill when the PPS signal has 2ns of jitter.
> Bruce,
> Can you clarify about the jitter, though. The TIC jitter
> that was quoted (500 ps) is the single-shot resolution
> for the 53131A. The "2 ns" M12+ jitter is an rms value,
> no? The short-term or "single-shot" M12+ jitter, if you
> could call it that, is more like +/- 20 ns. Averaging it,
> over many minutes, gets you below 10 ns. Also the
> sawtooth correction helps even further but that isn't
> being done with Shera's board.
> Maybe we're all agreeing even if with different words.
> /tvb
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