HP 3326A Operating And Reference Manual page 59

Modulation
2 Phase, 2 Tone, or Pulse Mode
Both channels are synchronously phase modulated by an external source through the
rear panel A-PHASE MOD INISYNC PM IN connector when the (AB) SYNC PM indica-
tor is illuminated. Channel B is also phase modulated by an external source through
the rear panel B-PHASE MOD IN connector when the channel B EXT PM indicator is
illuminated.
HP-IB Example:
"AEPI
"
or
"AEP ON"
"BEPI
"
or
"BEP ON"
"SPEI
"
or
"SPE ON"
Disabling Modulation
Modulation is disabled by pressing the MODULATION selection keys corresponding to
an illuminated indicator until all the indicators are extinguished. Modulation is also dis-
abled by changing the mode with the MODE key or presetting the HP 3326A.
HP-IB Example:
"NOM"