Hi Joe,
On 4/23/2012 9:45 PM, J. L. Trantham wrote:
Ed,
I am not familiar with the Tracor units, only the 5061A and B as well as the
5065A. These units use the 2nd Harmonic as an integral part of the feedback
loop.
Without the 2nd Harmonic, is there another way to 'unambiguously determine
that it is locked', other than comparing it to a 'known', 'locked' signal?
Strictly speaking, the answer is probably 'No'. After all, why would
they include the 2nd harmonic circuitry if they didn't need it? There
should be 2nd harmonic and I hope to find some somewhere. Remember that
this unit is being brought back from the dead as a learning exercise so
a few 'minor' issues aren't a show-stopper. The unit has been running
for most of the day. I flipped the switch to open the loop. The
frequency went from 5 MHz to 5MHz +0.045 Hz while the error meter went
from 0 to -25 on a scale of 50. Close the loop and the frequency
returned to 5.000 000 000 MHz and the error meter went back to zero.
That certainly sounds like locking behaviour to me.
I guess another way to ask the question is do you think you happen to have a
particularly good OCXO?
It's a 40 year old AT-crystal that hasn't had nearly enough recent run
time to work the kinks out. I would be astonished to find that it's
that good. But I realized that I've never looked at the oscillator by
itself so I did a quick test. I measured an aging rate in the range of
0.2 ppm / day. If I cancel out all the aging, the results start to look
like the earlier attachment. But not when it's unlocked.
Ed
Joe
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Ed Palmer
Sent: Monday, April 23, 2012 10:17 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Antique Rubidium Standard Questions
Hi Joe,
On this unit (not sure about others), the 2nd harmonic is used to
unambiguously determine that it is locked. But the 2nd harmonic has no
part in the acquisition or maintenance of the lock. That is done by the
fundamental. I can tune through resonance and see the standard curve
like fig. 5-7 in the HP 5065A manual (see attachment) except mine
doesn't quite make it to the full-scale saturation level. I then tune
to the resonance point and flip a switch to close the loop. It doesn't
turn on the nice green light because that's done by the 2nd harmonic.
But it also doesn't drift like an OCXO. Take a look at the second
attachment for an ~10 hour data run. The relatively poor performance
below 1000 seconds is due to my measurement setup. I was looking for
high Tau performance, not low Tau.
Ed
On 4/23/2012 7:58 PM, J. L. Trantham wrote:
I don't understand how it can 'lock' without a 2nd Harmonic Signal.
Joe
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Ed Palmer
Sent: Monday, April 23, 2012 12:54 AM
To: Time-Nuts
Subject: [time-nuts] Antique Rubidium Standard Questions
I'm playing with a Tracor 304-B Rubidium Standard from 1969. I'm
using it as a learning exercise to find out more about the guts of a
Rubidium standard and how it works.
This thing is a beast! Rack-mount, 3U high, 39 pounds (~18 kilos), 9
plug in circuit boards. The OCXO is an oddball frequency that is
multiplied directly to 6.8 GHz. There's no synthesizer in that chain.
A synthesizer is used to convert the oddball frequency to a 5 MHz
output.
It's sort of working. The error signal isn't up to spec, but it's
strong enough to give a stable lock although there's no trace of a
second harmonic signal. Allan Deviation is in the Xe-12 range from 1K
to 10K seconds. The OCXO has a not-yet-resolved issue that is
probably degrading the results.
The lock frequency suggests that the Rubidium cell has drifted down by
~30ppt over the 40+ years since it was built. Is that reasonable?
That's much more drift than the specification states, but I doubt if
the spec was intended to be valid for 40 years!
Could the drift be at least partially responsible for the lack of
second harmonic? A message on the list (
http://www.febo.com/pipermail/time-nuts/2006-April/020562.html ) said
that you could peak the second harmonic by adjusting the cavity
tuning. If the cell and the cavity are out of sync would that kill the
second harmonic? How close to they have to be? If this thing has a
cavity tuning adjustment I haven't found it.
FYI, I checked my counter (Racal 1992 referenced to an Efratom FRK-H
Rubidium) against my Z3801A and Tbolt. Both measure 10.000 000 000
MHz. so I'm confident that my numbers are good.
Thanks,
Ed
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