[time-nuts] reference oscillator input circuit

[Mark Spencer]

On a somewhat related note, does any one have any information as to the likely 
performance impact of using a single logic gate to convert a sine wave to a 
pseudo square wave ?    (I discovered one of my scopes doesn't consistently 
accept a sine wave time base input, thru trial and error and rummaging thru my 
junk box I ended up using a 74ALS00 Nand gate to convert a 10 mhz sine wave into 
a somewhat square wave (:  which the scope will reliably use as a reference 
input.)    Extreme accuracy is not really needed in this application but I am 
curious how much jitter I may have introduced.

I'll likely try one of the circuts in the attached link at some point.

Best regards
Mark



----- Original Message ----
From: John Ackermann N8UR <jra at febo.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Wed, December 8, 2010 7:46:12 AM
Subject: Re: [time-nuts] reference oscillator input circuit

Wenzel has some discussion and circuits at:
http://www.wenzel.com/documents/waveform.html.

The Shera GPSDO made clever use of the input circuit of a 74HCT4046 PLL 
chip for squaring.

John
----

On 12/8/2010 10:31 AM, jimlux wrote:
> 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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