[time-nuts] reference oscillator input circuit

Bob Camp lists at rtty.us
Thu Dec 9 16:38:03 EST 2010


...and sometimes they leave the Schottkys out.

The original request was not a really low jitter application. I think Jim
can get away with an active circuit. 


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Miles
Sent: Thursday, December 09, 2010 4:00 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] reference oscillator input circuit

> No, actually to feed a bunch of synthesizer chains (for which the sine
> wave will work fine) and to drive sampling clocks on ADC/DAC (for which
> one wants a low jitter square wave).
> A digital radio...

There are some nice residual plots for AC and CMOS chips at
http://www.xs4all.nl/~martein/pa3ake/hmode/dds_ad9910_pmnoise.html .  Seems
like the AD9515 family in PECL mode is about as good as anything is likely
to get at VHF.

However, why not use the sine wave directly, converted to differential with
a transformer and clipped by back-to-back Schottkys?  At VHF clock
frequencies any active sine-to-square conversion circuit I'm aware of will
contribute more jitter than the ADC's own tJ spec.  (Put another way, if I'm
a semiconductor house designing a high-end ADC or DAC, I am probably going
to put all the secret sauce I have into the on-chip clock conditioning,
leaving little or no room for improvement outside the chip.)

The eval boards from the various ADC manufacturers bear this out.  No one
puts anything but a 50-ohm SMA jack, transformer and Schottkys on their
clock inputs.

-- john, KE5FX

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