T801-4 Installation and Operation Manual
Operation
29
© Tait Electronics Limited September 2007
2.3
Oscillator Control
The T801-4 frequency generation system uses the UTC pulses (1PPS)
derived from the GPS as an accurate and stable reference source.
The master oscillator (VCO) uses the UTC pulses from the GPS whenever
it has a valid GPS fix. If the UTC pulses become invalid, and the previous
state of the correction process was that the accuracy was better than ±1 part
in 10
8
, the VCO will be “frozen”. It will remain in this state for 24 hours
(as long as the GPS fix remains invalid), after which time the frequency will
be left to “drift” at a rate determined by the ageing of the crystal. If a long-
term record of control voltage movement is available, this will be used to
track the VCO along the likely ageing path.
The UTC pulse (1PPS) source for the Satsync 7 module can be provided by
the following:
■
a Trimble internal GPS receiver
■
a Dalman external GPS receiver (via RS-485 signals on
PORT A
)
■
an RS-485 signal on
PORT B
■
a TTL input on
PORT A
.
You must program the Satsync 7 module according to which of the above
options is used. Refer to
“Timing Source” on page 40
for more details.
Refer also to
“System Configuration” on page 49
for more information on
how to configure the T801-4 for use in different systems.
2.4
Valid GPS Reference
When the 1PPS signal is considered to be valid, the following occurs:
■
The VCO is referenced to the 1PPS signal. Accuracy tends to ±1 part
in 3
×
10
9
.
■
The
GPS
LED goes green.
■
The GPS status relay (internal) is energized and the GPS alarm output is
inactive.
■
The
MASTER OSCILLATOR
LED goes green, and the master oscillator
alarm is inactive (internal relay energized) when the accuracy is better
than ±1 part in 10
8
.
Important
A green
MASTER OSCILLATOR
LED indicates that the
accuracy is better than ±1 part in 10
8
. It does not neces-
sarily mean that the oscillator has reached its specified max-
imum accuracy.
