[time-nuts] HP5061B Versus HP5071 Cesium Line Frequencies

Tom Van Baak tvb at LeapSecond.com
Fri May 26 00:23:55 EDT 2017


You're familiar with the 9,192,631,770 Hz definition of the SI second; but that's only for an "unperturbed" atom. The bad news is that in order to make the cesium beam operate at the central resonance peak one actually has to violate the SI definition and perturb it -- by applying a magnetic field (the so-called C-field), as well as other factors. This cannot be avoided. The good news is that the shift can be calculated.

In other words, because a magnetic field must be applied the actual cesium resonance frequency is not 9192.631770 MHz. The synthesizer locks to the peak, but the peak is at a slightly higher frequency than the nominal book value. This detailed note from hp may help:


Different model beam tubes use different field strength / Zeeman frequency. Search the archives for lots of good postings about all these magic frequencies -- google: site:febo.com zeeman

If you want to see what the resonance peaks (all 7 of them) actually look after the C-field is applied see:

and (poster size):

See also John's version:


One final comment -- the perturbed vs. unperturbed issue is far more complex than a single correction. To get an idea of the math and physics complexity of a laboratory Cs beam standard read some of these:



----- Original Message ----- 
From: "Donald E. Pauly" <trojancowboy at gmail.com>
To: "time-nuts" <time-nuts at febo.com>; "Donald E. Pauly" <trojancowboy at gmail.com>
Sent: Thursday, May 25, 2017 7:55 PM
Subject: [time-nuts] HP5061B Versus HP5071 Cesium Line Frequencies


The synthesizer in the HP5061B generates a frequency of about
9,192,631,772.5 cps when the 5 mc oscillator is exactly on frequency.
First the 5 mc oscillator is multiplied by 18 to 90 mc on the A1
board.  That in turn is multiplied by 102 in the A4 board to give
9,180 mc.

The 5 mc is also divided by 4079 to produce 1,225.790635 cps.  That in
turn is multiplied by 10,305 to produce 12,631,772.5 cps.  This is
added to the 9180 mc in the A4 mixer to produce the final frequency of
9,192,631,772.5 cps approximately.  This is higher than the defined
frequency of 9,192,631,770 cps by about 2.5 cps or 271·10^-12.  If I
figured it right, the C field adjustment only has a range of
40·10^-12.  This seems to be insufficient to put the standard on

Can anyone explain these mysteries?  Does anyone know why this
frequency was chosen?  Does anyone know the choice for the frequency
of the HP5071 cesium?

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