Preface
xx Operating and Programming Manual
Modified Julian Day (MJD): The Modified Julian Day is a number which
is unique for each day. This numbering system is widely used by
astronomers and scientists to tag data and events. Within the 5071A the
MJD is used for two purposes:
• To tag events kept in the instrument‘s event log, and
• To program the insertion or deletion of a leap second at a future date
The MJD increments at 00:00:00 hours Coordinated Universal time (UTC).
To ensure proper synchronization of the MJD setting, the clock display
must be set to UTC. If the MJD is not set, the 5071A will start with 00000
in its MJD counter, and increment from there each time the clock display
passes 00:00:00 hours. See Chapter 2 for information on how to determine
and set the current MJD.
Steering (or offsetting) the 5071A output frequency: The 5071A output
frequency can be steered in a very precise and calibrated manner. When
steered, the stability of the output signals is identical to the un-steered
signal.
You can steer the 5071A over a range of ±1 part in 10
9
with a resolution of
6.331991 parts in 10
15
. These specifications are called the SETTABILITY
of the 5071A.
Steering is accomplished by changing the ratio between the VCXO output
frequency and the cesium transition frequency through a digital
synthesizer. The reference frequency of the cesium beam tube is not
disturbed. This system has the following advantages over previous cesium
beam frequency standards:
• No change in stability of steered output over un-steered output,
• Offset from un-steered frequency is precisely known,
• Fast change to the precise steered frequency, and
• Fast and precise return to the original frequency.
As a result of knowing the exact amount that the 5071A has been steered,
its value as an independent frequency source in an ensemble is maintained.
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