Using External Reference Frequency with HPT Series:

1) Locate BNC on rear panel marked TTL Sync 2500 Hz.

2) You will need to provide a 2500 Hz TTL 0.5 V Square wave
into that BNC.

3) Open top cover. Locate FMO Module for transmitter section
(first on left side of top row - can also be identified by the presence
of the crystal inside). Inside such module, locate K1 Jumper, which
is close to crystal.

K1 Jumper needs to be moved to position opposite normal
position. This disengages crystal on transmitter section and
enables use of outside reference.