HP 83751A User Manual page 123

Modulation Keys
Pulse modulation is either internal or external. Internal pulse modulation is
either controlled by a built-in pulse generator, or preset to either a 1 kHz or
a 27 kHz square wave. When external pulse is selected, the instrument is
modulated by a signal present on the PULSE IN/OUT connector on the rear
panel.
When external modulation is enabled, the EXT MOD annunciator is turned on.
When internal modulation is enabled, the INT MOD annunciator is turned on.
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