[time-nuts] Zeeman frequency oddness

Sat Oct 27 23:28:32 UTC 2012

John,

Did you do the Zeeman frequency adjustment before doing the 'degauss'?  In
other words, did you 'magnetize' rather than 'de-magnetize'?

Have you checked the A15 board to make sure it has the correct resistor and
current?  I have a 5061B Service Manual if you need some sections or
schematics.

Good luck.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Ackermann N8UR
Sent: Saturday, October 27, 2012 2:16 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Zeeman frequency oddness

Since I recently got an HP Cs degausser (thanks, Stijn!), I though I'd 
go through the whole setup routine for my 5061B/004 and see how close 
the C-field-via-Zeeman setting would bring me to GPS-derived frequency. 
  It turned out to be an interesting and puzzling exercise.

This 5061 seems to be in perfect working order -- quick lock, good meter 
readings, and measurements indicate frequency within parts in e12.  I 
don't have any reason to believe that it's not tuned or working 
properly, except for the Zeeman-setting results.

The problem is that when I tune the audio source around 53.53 kHz, per 
both the manual and the sticker on the door, I don't see any change in 
Beam I at all.  Nor do I see anything at the alternate frequency of 
42.82 kHz.

Instead, I see the expected three peaks -- primary with a smaller 
secondary on either side -- at about 48.21 kHz, which doesn't show up 
anywhere in the literature I've found.

Below, I've cut and pasted a years-old message from TVB and Corby that 
explains the Zeeman frequencies.  I've measured the synthesizer output 
and it's nominally 12.6317725 MHz, which per that message should 
correspond to a 53.53 Zeeman.  Where 48.21 kHz comes from, I have no idea.

I'm using a Rigol arbitrary function generator locked to an external 
reference as the audio source, in sine wave mode.  I know that's not the 
cleanest device in the world, but the wave doesn't look too bad on my 
scope and a counter indicates the frequency is what the dial says.  As I 
adjust the audio amplitude, the beam current responds, and I see a peak 
at around 500mV, which the 5061B manual says is correct.

Any ideas why I might be seeing this very off-the-wall result?  Could 
distortion in the audio source cause something like this?  I'm more 
inclined to blame technique or gremlins than the 5061B -- again, 
external measurements indicate that the thing is tuned correctly and 
operating properly, just having this goofy Zeeman response.

Thanks!

John

[time-nuts] Zeeman frequency and cesium tube interchange
Tom Van Baak
Fri Apr 22 13:32:35 EDT 2005

Hi Brian,

The SI second is defined for mean sea level and no
external fields.

If there were no magnetic field, in theory, a cesium
tube would show a resonance when its synthesizer
generated exactly 9192.631770 MHz; the definition
of the second.

But in practice, a weak uniform magnetic field is
necessary for the beam apparatus to operate; to
isolate the center peak from the other peaks. This
DC field also has the side-effect of slightly shifting
the frequency of the center line.

Fortunately the frequency shift is a calculatable
amount (a function of magnetic field strength) so
the trick is that the synthesizer must be designed
to generate a slightly higher frequency to exactly
compensate for the shift that will be induced by
the field.

Thus the synthesizer for a 5061A does not actually
generate 9192 631 770 Hz as one might expect,
but because of the nominal 61 milligauss C-field,
the synthesizer must generate 9192 631 771.6 Hz
in order to lock onto the Cs peak precisely. The
Zeeman frequency for 61 mG is 42.82 kHz (info
from an old 5061A manual).

Other 5061A/B use a 76 mG field, corresponding
to a 53.53 kHz Zeeman frequency, and require the
synthesizer to generate 9192 631 772.5 Hz (info
from a new 5061B manual).

The short HP 5062C runs at 9192 631 774.3 Hz
with a Zeeman of 70.40 kHz.

Below is a great reply from Corby Dawson about
the problems this can cause when mixing FTS
4050, 4060, HP 5060A, 5061A, 5061B parts.

/tvb

----- Original Message -----
 > Tom,
 >
 > The zeeman frequency required depends on two things, the magnitude of the
 > C-field current and the synthesizer frequency.
 >
 > HP 5061A and B units that have a synthesizer freq. of 12.6317725 Mhz are
 > configured for a lower value of C-field current by selecting a higher
 > value series resistor on the A15 board. In this case any tube installed
 > (5061A/B 5060 4050 4060) will operate at the 53.53Khz zeeman frequency.
 >
 > HP 5061A and B units and 5060A units that have a synthesizer frequency of
 > 12.6317716 Mhz are configured for a higher C-field current due to a lower
 > value series resistor in the C-field circuit. In this case any installed
 > tube will operate with a 42.82Khz zeeman frequency.
 >
 > Problems arise when synthesizer and or A15 modules are swapped around
 > indiscriminantly leaving a unit with modules that do not match!
 >
 > Since the HP and FTS tubes C-field windings are designed to provide the
 > same field for a given input they are interchangeable. You can operate
 > any of these tubes at a 42.82 or 53.53Khz zeeman. I usually just stick
 > with how the mainframe came configured.
 >
 >
 > I don't remember what I ended up with as far as the zeeman freq. was
 > concerned when I installed a Frequency Electronics tube into a 5061A, but
 > do remember the line width was quite broad in keeping with the reduced
 > accuracy spec. of the FE tubes. (Same spec. as the 5062C tube)
 >
 > I have installed FTS tubes into the 5062C giving it the +-7Z10-12th
 > accuracy spec. and did have to modify the synthesizer for 42.82Khz and
 > set the C-field accordingly.
 >
 > Hope this helps!
 >
 > Corby_

_______________________________________________
time-nuts mailing list -- time-nuts at febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.