[time-nuts] Zeeman frequency oddness

Chuck Harris cfharris at erols.com
Sat Oct 27 19:13:31 EDT 2012


Two things come to mind:

1) is this the high performance tube?  Perhaps it has a different
    Zeeman frequency than the standard tube?
2) a non HP replacement tube?  FTS Cs beam tubes are way different
    Zeeman frequencies than HP.

-Chuck Harris

John Ackermann N8UR wrote:
> 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_
>
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