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8600 Operation Manual - V1.0
Theory of Operation
Circuit Description
At a high level, the 8600 consists of a number of separate blocks (input circuit, output circuit,
GPS Disciplined oscillator) which are all trained to a common 10 MHz signal generated by the GPS
Disciplined oscillator.
Without GPS signal the 8600 allows an internal oscillator to run in hold over (no correction for
short term,long term and temperature drift) to generate the internal (and external in 10 MHz output
mode) clocking source for each element of the circuit to use.
Traceability
When a GPS lock has been achieved a phase comparator adjusts the output of the oscillator to
match the timing signal received from the GPS satellites.
As the oscillator is self disciplining the effects of short term, long term and temperature drift
are compensated for through the lock to the GPS system. During normal operation the 8600 will not
operate in holdover mode unless an antenna fault occurs so the unit can be assumed to be drift free
and therefore not require external re-calibration.
However, for traceability purposes some accreditation bodies recommend an initial calibration
by inter-comparison to a National Metrology Institute (NMI) or accredited calibration laboratory
prior to installation to confirm that the GPS receiver is disciplining the oscillator correctly as well to
confirm the stability of the output over time.
The long term performance of the 8600 is dependant upon the local environment (including
antenna installation and temperature fluctuations). To evaluate these effects it is advised to confirm
correct operation of the 8600 after installation by comparison against another frequency standard. A
portable frequency standard or counter can be used for this operation.
Although the 8600 does not require periodic calibrations to maintain its specification, some
accreditation bodies may wish to see proof of external traceability through periodic calibrations either
by an NMI or another accredited laboratory. This can be performed either by sending the 8600 to
the laboratory to have the 10 MHz output verified, or alternatively an external oscillator calibrated
and compared against the output of the 8600. As the measurement circuitry is trained to the internal
10 MHz oscillator it is possible to compare the measurement of the external oscillator against the
certified output to confirm correct operation of the 8600.
Further information on the traceability of GPS disciplined oscillators (GPSDO’s) can be found
by contacting Transmille at [email protected]
