Ed,
Sounds like you are on the right track.
These things are fairly simple and fun to play with.
The Rb assembly on the 5065A can be opened at either end. One end has the
Rb lamp and is mounted by 3 screws, against the 'end' of the 'cavity'.
Inside that is a 'filter' assembly and beyond that, at the other end of the
assembly, is the 'detection' assembly. The 'detection' assembly is attached
with threads as the assembly 'screws' into the far end of the 'cavity'.
Opening the far (other) end of the assembly allows one to loosen a screw
that then loosens a 'wedge' that 'secures' the 'detection' assembly in
place. Then, the 'detection' assembly can be simply 'unscrewed' from the
'cavity'.
I have never seen anything about 'tuning' the cavity in any of the 5065A
manuals but, by the way this part of the assembly is constructed, I suspect
that, when tested on a bench, the 'detection' assembly can be moved in or
out to 'fine tune' the cavity, then 'secured' in place by screwing down the
'wedge'.
I suspect that there are others on the list that can speak to this much
better than I can.
I don't know how the Tracor Rb assembly is constructed but it might be
similar and allow a means of 'tuning' the cavity.
Good luck.
Joe
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Ed Palmer
Sent: Tuesday, April 24, 2012 8:30 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Antique Rubidium Standard Questions
Joe,
On 4/24/2012 6:50 AM, J. L. Trantham wrote:
Ed,
If it is, indeed, stable at exactly 5.000 000 000 MHz, it is probably
locked. That being said, I would wonder if there is an indicator
issue and perhaps a control logic issue.
On the HP units, the AC Amplifier, that receives the signal from the
Rb assembly, has been the source of some problems on my units. Also,
the AC Amplifier sends a signal that indicates both a 137 Hz
fundamental frequency being present and a 274 Hz 2nd harmonic signal
being present to the control logic assembly as two of the 'required'
signals to get a 'lock' indication. You might want to take the AC
Amplifier out, put it on the bench and feed it a low level fundamental
frequency, track that through the assembly then repeat with a 2nd
harmonic signal and look at it's output. Same thing for the logic
assembly to make sure that some of the 'required' signals are indeed
present rather than 'failed on'.
I did that. There were some bad solder joints in the amp / filter
circuit but otherwise, it was good. To test the entire signal chain, I
replaced the Rb lamp with an LED that was modulated at the fundamental
(155 Hz in this case) and then at the 2nd harmonic (310 Hz) and the
signals appeared& disappeared as expected.
I forgot to ask, did you find a manual?
That was one of the reasons I bought this thing. It included the
original manual. There's also a partial manual online so I was able to
familiarize myself with the unit before I made my bid.
Ed
Joe
-----Original Message-----
From: [email protected] [mailto:[email protected]]
On Behalf Of Ed Palmer
Sent: Tuesday, April 24, 2012 12:55 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Antique Rubidium Standard Questions
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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