[time-nuts] How I got my FE-5680A to lock in Sydney, Australia

Charles Steinmetz csteinmetz at yandex.com
Mon Feb 10 19:57:27 EST 2014


Jim wrote:

>Sorry, but I didn't accidentally transpose the 'high' swing numbers. 
>That's why I was so confused about the results.

No need to apologize, the facts are the facts!  I take it that you 
recently received the GPS.  If so, it is probably still settling into 
its new natural frequency after being off for a while and jostled 
around in shipping.  It appears to have drifted far enough that it 
now scans through 10MHz in the normal position.  At some point, you 
may find that it no longer locks in the inverted position because it 
has drifted too far to scan through 10MHz in that position (according 
to your figures, it was right on the edge when you took the data you posted).

>The counter I'm using is one that I designed myself. As an 
>alternative to its internal 1MHz XCO timebase (with dividers down to 
>1Hz, 0.1Hz etc), it also allows you to use an external source of 1Hz 
>pulses like those from a GPS receiver or a GPSDO, or a Rb Freq 
>Reference. The dividers operate on these 1Hz/1pps pulses as well, to 
>provide gating times of 1s, 10s, 100s or 1000s. It seems to work pretty well.

For short gating times, I'd think that GPS PPS jitter might be a 
limitation.  (That's the trick of a GPSDO -- the quartz [or rubidium] 
oscillator controls the short-term stability, at time intervals where 
it is better than the PPS, and the PPS controls the long-term 
stability at time intervals where it is better than the 
oscillator.)  Specs for GPS engine PPS jitter seem to run from a low 
of around +/- 5nS to several times that, which would translate to a 1 
second stability of 10^-8 to 10^-9 at best.  A good quartz oscillator 
(or a well-designed GPSDO with a good quartz oscillator) should get 
you to 10^-11 or even into the neighborhood of 10^-12.  You'll add a 
little jitter dividing by 10M, but it shouldn't be anywhere near as 
much as the 3-4 decades by which the quartz oscillator beats the PPS at 1S.

Best regards,

Charles






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