HP 3326A Operating And Reference Manual page 56

Modulation
STEP 1
STEP 2
internal PMlPM DEV
Internal phase modulation is enabled in the 2 CHANNEL mode when the channel A INT
PM (internal Phase Modulation) indicator is illuminated (see Figure 1-26). Internal phase
modulation uses the channel B synthesizer as the modulation source for channel A.
Thus, only the channel A output is active. For internal phase modulation, 5 kHz is the
' ' u o
a
maximum channel B frequency.
When internal modulation is selected, the channel B high voltage option is disabled.
If internal modulation cannot be selected (i.e. the INT PM indicator does not illuminate),
check that the 2 CHANNEL mode is selected, COMBINED operation is disabled, and
the channel B frequency is below 5 kHz.
HP-IS Example:
"AIPI
"
or
"AIP ON"