[time-nuts] HP 1938 revisited

J. L. Trantham jltran at worldnet.att.net
Mon Apr 13 00:54:30 UTC 2009

I, too, snagged one of these since it has the reputation of being the
ultimate achievement of crystal oscillator technology with the goal of
building a GPS controlled reference using a Brooks Shera controller card and
a GPS receiver.

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 seem
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.

Thanks in advance and I would appreciate any 'links' to any programs that
would allow communication with the oscillator.


-----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 6:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] HP 1938 revisited

wje wrote:
> Fluke.l (China) was selling a number of 1938's on Ebay. I snagged one 
> just to have a piece of HP history.
> It works just fine, but I've noticed something a little strange.
> Comparing the 1938 to both my cesium and GPS standards, there's a 
> distinct periodic 1ns phase shift every second. Seems to smoothly 
> advance  1ns for 500 ms, then retard back to the original phase point 
> over the next 500ms. Question: is this to be expected? I'm assuming this 
> is from the AFC loop, but I would have expected it to be better damped.

The AFC loop (as opposed to EFC) is a purely analog loop in which there
is no mechanism for a 1 Hz oscillation.

Now if you are talking about the EFC, that is another story.  Depending
on how you are driving the EFC, you can pickup noise from any number
of sources.

I vaguely remember there was an LED that flashed at 1 Hz if everything
was working OK.  You might see if that is the source of what you are

> I've searched both this group and the HP group; there really doesn't 
> seem to be a great deal of info about these, other than schematics and 
> some nice variance plots on leapsecond. Just what is the serial port and 
> the PIC data lines useful for, if anything?

The serial port can be used to change the oven temperature set point and
IIRC monitor various oven parameters.  Probably nothing you want to
play with if you want a working oscillator.  If you want to experiment,
I have released the software to talk to the serial port.  It should
be archived somewhere.  In production, we ramped the temperature up
and down and found the exact turnover temperature for each individual
oscillator and set the oven to the turnover.  This required that we
make crystals that actually had a turnover.  Many 10811 crystals did
not have turnovers, only an inflection point.

Rick Karlquist N6RK
Designer of E1938A

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