3. Theory of Operation
Simplified Functional Description
128
Instrument Control Block
This block provides front panel and remote instrument management with a
keypad-LCD/menu interface or via SCPI (Standard Commands for
Programmable Instruments) commands transferred through the RS-232C
serial port.
The block consists of microprocessor, front panel, and interface assemblies,
and the I-bus that, together facilitate control of all internal housekeeping,
servo loop, diagnostic, and system status instrument functions. All instrument
functions are controlled and/or monitored by this block.
Reference Oscillator/RF chain Block
The reference oscillator/RF chain block generates a microwave probe signal
centered around 9.19263177138 GHz. It also provides a high-quality 10 MHz
sine wave signal (ovenized-quartz VCXO-generated) that is used as a basis
to synthesize the microwave probe signal. The synthesis is phase-coherent
so that the frequency of the probe signal
is arithmetically related to that of the reference oscillator.
Accordingly, the block also provides amplitude- and phase-coherent
frequency modulation applied to the microwave probe signal which is then
fed to the Cesium Beam Tube (CBT).
Cesium Beam Tube Block
The Cesium Beam tube block operates by matching the frequency of the
microwave probe signal with the hyperfine resonance of cesium atoms. The
comparison occurs in the CBT. This resonance constitutes the international
definition of the second, defining frequency and time.
In the CBT, the actual frequency of the resonance differs slightly from the
nominal value of 9.192631770 GHz because of the applied magnetic field
present, and the second-order Doppler shift resulting from the finite velocity
of the atoms in the beam. Both of these effects are calculable and have been
corrected for within the instrument. The correction yields an overall accuracy
of better than five parts in 10
-13
for the high-performance tube option.
The entire matching process creates a time-dependent CBT output error
signal. The error signal contains information used to determine frequency and
amplitude errors of the microwave probe signal. This error signal is routed to
the analog signal chain block.
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