HP 8642A Operation And Calibration Manual page 145

Model 8642A/B
Operation
Modulation, FM (cont'd)
DC-Coupled Suurce
When external dc-coupled FM (DC FM) is selected, the FM reference loop is no
longer phase locked. The applied signal is directly coupled to the FM VCO.
DC -coupling also minimizes phase shift in the modulating voltage, making
dc-coupled FM the preferred coupling path for maximum stereo separation.
• I EXT DC
I
This function selects the external, dc-coupled FM source for dc to 200 kHz rates
(displaying the EXT and DC annunciators).
e INT+ (EXT DC) ([SBIFT] ( EXT DC ) )
This function selects the external, dc-coupled FM source and internal FM source
summed together (displaying the INT, EXT, and DC annunciators).
$
AC-COUPLED DC FM (Special Function 118)
Rate
This function causes the external signal to be applied to the unlocked FM
veo
allowing maximum stereo separation characteristics (displaying the EXT and DC
annunciators). (Refer to page 3 -116 for more information about this special
function.)
Determined by the external modulation source frequency.
NOTE
External modulation frequency is not monitored by the Signal
Generator. The Signal Generator's modulation frequency display
refers to the internal modulation oscillator only.
Deviation
CE.MJ
and the desired value of FM deviation in MHz, kHz, or Hz (per volt).
When using an external source, the FM deviation setting does not set the actual
deviation, it sets the input sensitivity per volt. A linear relationship exists between the
FM deviation setting and the external voltage applied (for voltages less than or equal to
approximately 12 Vpk).
NOTE
With simultaneous internal and external modulation, the sum of the
internal and external signals should not cause the FM deviation to
exceed the limits mentioned in Figures 3-12 and 3-13 on page
3-106. If these limits are exceeded, the Signal Generator could go
out of lock which causes queued up Hardware Error messages. Also,
the external FM/il>M INPUT does not respond linearly to inputs
greater than approximately 1.2 Vpk.
3-109