[time-nuts] reference oscillator input circuit

jimlux jimlux at earthlink.net
Wed Dec 8 15:31:08 UTC 2010


I'm looking for suggestions on a general circuit that can be used to 
receive an external frequency reference (nominally a real clean sine 
wave at, say, 10 MHz, although up to 100 MHz is possible) and turn it 
into a "real clean" square wave.  Galvanic isolation is a plus (a 
transformer or capacitor would probably do that).

I was thinking about rummaging through the schematics for test equipment 
reference inputs (since they've already "solved" the problem, eh?), but 
any other ideas would be welcome.

I've scanned the archives of time-nuts, and while we have a fair amount 
of discussion on how to square up the 1Hz (or 100Hz) in a phase 
noise/ADEV setup, not so much on what to do with the 10 MHz.  Rick has 
commented that you don't want to use a comparator. I have the papers by 
Dick, et al, and Collins, as well as all the others.. they tend to be 
looking at the low frequency problem, although the analysis is certainly 
applicable.

I don't know that I'm looking for the whole multiple limiting stages 
scheme in any case.

Oh, as far as performance.. Say the need is to not horribly degrade a 
good quality crystal oscillator... here's a typical set of specs:
76 MHz
1Hz <-90dBc
10Hz <-110dBc
100Hz <-120dBc
1k-100k <-125dBc

Adevs of the oscillator run from 5E-12 at 0.1 sec, down to 1E-12 at 10 
sec, and back up to 2 E-12 at 1000sec.



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