Technical data

Installation & Operation Manual GM-TTT
Page 59
4.3 Genlock to a 1pps Signal
• Connect a 1pps signal to pin 1 of DSUB9M REF. DATA IN.
• At menu SET - ... - LOCK select SEC and press enter button.
• Press genlock button.
This genlocking mode should only be used in the time operating mode. It must be provided
that a real time reference delivers serial data (time and date) as well as a 1pps signal. The
LTC output gets a phase adjustment to the reference edge of the 1pps signal, this may be the
rising or the falling edge, selectable at menu REFER - MODE 1.
The LED’s genlock and intern indicate the following status:
LED genlock LED intern
Adjustment procedure, coarse trim Flashing Lighting
Adjustment procedure, fine trim Flashing Off
Phase-lock On Off
After switching on this genlocking mode the LED genlock starts to flash. The internal reference
is selected, but this frequency will be slightly shifted until frame 0 of the time code starts with
the reference edge of the 1pps signal. This adjustment procedure starts again after a fail of
the genlock caused by a jitter or a disturbance of the 1pps signal. This procedure varies the
output frequency of the LTC a little bit but avoids a break in frames.
Using a stable 1pps signal with a jitter < 1.2ms the LED genlock will light up after a while, this
means a real phase-lock has been reached. During the adjustment procedure the LED intern
indicates the fine trim (LED off) and the coarse trim (LED lights up).
Fine trim means: frame 0 more than 1.2ms but less than 16ms apart from the 1pps signal.
Coarse trim means: frame 0 is more or equal than 16ms apart from the 1pps signal.
Reaching a phase-lock is important for a correct timing of the real time data transfer. The
serial interface inputs time and date, and the 1pps signal is used as the timing reference to
write these data into internal registers. There is a definite moment at each time code frame
where the generator reads these registers to prepare the next frame. Having an unstable
timing due to an unstable 1pps signal writing into the register may happen one time before,
next time past this moment. This will result in generating a discontinuity in frames. Having
reached a phase-lock the optimal timing is assured, i.e. a maximal jitter of ±16ms will be
allowed. As long as LED intern lights up the data transfer is at a critical timing. If for example
the data transfer is selected to take place each second (at menu REFER - MODE1) it is not
guaranteed to generate a continuous time code. The timing is not critical if the LED intern is
flashing or off.
GM-TTT permanently supervises the 1pps signal. An error counter increments in case of any
faults, this counter is displayed at menu STATUS - ... - 4. Another error counter increments in
case of a failure, this counter is displayed at menu STATUS - ... - 5. Another error counter
increments in case of a disturbance, this counter is displayed at menu STATUS - ... - 6. The
error counters get an update each second. Each time the genlock button is pressed the error
counters get a reset to zero. At STATUS - ... - 7 the delay between frame 0 and the 1pps signal
is indicated. The current programming of the internal reference is displayed at menu STATUS -
... - 3.