[time-nuts] reference oscillator input circuit

[John Miles]

The Wenzel diff-amp circuit is OK but it will run out of steam before 100
MHz unless you use different transistors.  On the other hand you really have
to go out of your way to corrupt the signal at the -125 dBc/Hz level.  At
that level of play any decent comparator with the necessary slew rate will
be fine.

A 74AC gate looks good in this scenario:
http://www.ke5fx.com/ac.htm

... subject to what other people have said about it being sensitive to input
level.

-- john, KE5FX



> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of jimlux
> Sent: Wednesday, December 08, 2010 7:31 AM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] reference oscillator input circuit
>
>
> 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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