[time-nuts] HP 1938 revisited

J. L. Trantham jltran at worldnet.att.net
Mon Apr 13 04:03:31 UTC 2009

Thanks for the info.

My plan is to develop a stable GPS disciplined reference suitable for use as
a reference for Microwave work in the 10 GHz range that can be used in
portable locations with relatively quick start up. 

Perhaps the 1938 would be better in the shop where it could be left on for
weeks/months at a time and an LPRO 101 or a 10811 could be used for the
portable application.  Reasonably high drift rates could be accommodated if
the GPS signal is reliable and the time constants (disciplining rate, drift
rate, etc.) are appropriate.



-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Richard (Rick) Karlquist
Sent: Sunday, April 12, 2009 7:57 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] HP 1938 revisited

J. L. Trantham wrote:
> I, too, snagged one of these since it has the reputation of being the
> ultimate achievement of crystal oscillator technology with the goal of

Thanks, we thought it was pretty good :-)

> Toward that end, since it takes a few minutes for the 1938 to 'lock', is
> there a signal that can be used to 'turn on' disciplining?  It doesn't
> logical to try to discipline the oscillator until it has stabilized and,
> therefore, if the device is to be used from time to time, not staying on
> continuously, it will need a way to optimize turn on performance.

There is nothing in the E1938A that "locks".  The closest to lock is the
oven current "cutting back".  After that, the oven will settle within
a few minutes to its set point.  However, the crystal will exhibit high
aging rates for hours.  It will not get back to the aging it had when
last turned off for something like a day.  During this recovery time,
you can discipline the oscillator if you like, but it won't give very
good holdover performance if you lose GPS.

No crystal oscillator will give the kind of performance you need for
this application unless it is continuously ovenized.  The SC cut is
a big improvement over the AT cut in terms of a cold start, but is
still not adequate for GPS work right after a cold start.

Hope that helps.

Rick Karlquist N6RK
Designer of E1938A

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