Design Integration Considerations
Frequency Control Analog
38 X72 Desiger’s Reference and User’s Guide 097-10603-01 Rev. A – November 2004
Frequency Control Analog
Greater Than ±1xE
–9
Internal or External Control
The external frequency control circuitry is designed so that with no voltage applied
at J-11, the voltage will self bias to mid-range, or 2.5V. This input can also be turned
off via the SSIP to eliminate any source of noise. In some versions of the X72, this
function can be turned off by customer request on power up. If it is to be used later it
must be enabled through the SSIP (refer to Appendix B, Symmetricom Serial
Interface Protocol).
Time Response of External Frequency Control
The external analog frequency control is a sampled input with a typical response
time constant of 154 ms.
Temperature Compensation of Frequency
One of the key specifications for an atomic frequency standard is the temperature
coefficient. The X72 is designed for a low temperature coefficient without the need
for temperature correction. However, if the application requires a tighter
temperature coefficient and a more automated temperature compensated unit was
not ordered, you can monitor the base plate temperature of the X72 and apply a
correction signal via the External frequency control pin, or through the SSIP. A
production calibration run is required to set the X72 temperature at several points
and determine the correction summary.
This method can be successfully used for moderate correction, for example to bring
the maximum frequency change over the full operating temperature range to less
than 1xE
–10
.
Compensation using this scheme is generally suitable only for steady state
conditions. Inherent mismatches between the thermal time constants of the
mechanisms that cause temperature coefficient errors, thermal time constant of the
monitoring circuitry, and transients from time constant mismatches will occur. These
transients are minimized if temperature ramp rates are limited. Changing less than
+2° C/minute base plate temperature should result in negligible transients from
mismatches.
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