CSIII (MODEL 4310B) USER’S GUIDE
2021 Microchip Technology Inc.
DS50003057A-page 27
Chapter 5. Theory of Operation
This section provides the theory of operation of the CsIII Cesium beam frequency
standard. It is intended to supplement the functional description provided in earlier
chapters and provide a more complete understanding of instrument operation.
5.1
CESIUM BEAM FREQUENCY STANDARD CONCEPT
the SI unit of time is defined as
follows.
“The second is the duration of 9,192,631,770 periods of the radiation corresponding to
the transition between the two hyperfine levels of the ground state of the Cesium 133
atom.”
shows a conceptual diagram of the CsIII.
FIGURE 5-1:
Conceptual Diagram of the
CsIII
.
The fundamental accuracy and stability of the CsIII are derived from its physics
package or Cesium beam tube, whose function is to provide a near-ideal environment
for continuously measuring the Cesium hyperfine transition. All outputs of the CsIII are
derived from an ovenized low phase noise 5 MHz quartz oscillator, whose output
frequency is converted up to 9,192,631,770 Hz and applied to the Cesium beam tube
(CBT). The CBT produces an error signal, whose sign and amplitude reflect the
detuning of the quartz oscillator with respect to the definition of time. In normal
operation, the synthesizer, under microprocessor control, provides optimal
interrogation of the Cesium resonance, periodically hopping between interrogation
points located on either side of the principal Cesium resonance. The
microprocessor-based Loop Filter or Servo continuously demodulates and integrates
the error signal and adjusts the oscillator tuning voltage of the oscillator so as to null
the integrated error signal, thereby guaranteeing the long-term stability and accuracy
of the 5 MHz output frequency. For measurement intervals that are short compared to
the time constant of the loop filter, that is typically on the order of τ = 1 second, the
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