Operation
Model 8642A/B
Modulation, $M (cont'd)
If the HI and LO annunciators are not displayed then the external modulation signal is
between 0.95 Vpk and 1.05 Vpk. The HI, LO annunciators only apply to sinewave
inputs. The HI, LO detectors do not monitor dc voltage input levels.
External Coupling
Any one of tin. following fiaictions can be used to couple the external FM/'IM INPUT to
the 4>M circuitry:
AC-Coupled Source
This function selects the external, ac-coupled $M source (displaying the EXT and AC
annunciators).
• INT + (EXT AC) ( r5Him f EXT AC I )
This function selects the external, ac-coupled $M source and internal $M source
summed together (displaying the INT, EXT, and AC annunciators).
DC-Coupled Source
This function selects the external, dc-coupled <J>M source for dc to 15 kHz rates
(displaying the EXT and DC annunciators).
• INT+ (EXT DC) ( rgHTFTT fEXT DC I )
This function selects the external, dc-coupled $M source and internal 4>M source
summed together (displaying the INT, EXT, and DC annunciators).
Rate
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
*M (("SHIFT! (TfvQ ) and the desired value of *M deviation in radians ( [ RAD % I )
(per volt).
When using an external source, the $M deviation setting does not set the actual
deviation, it sets the input sensitivity per volt. A linear relationship exists between the
$M deviation setting and the external voltage applied (for voltages less than
approximately 1.2 Vpk).
NOTE
With simultaneous internal and external modulation, the sum of the
internal and external signals should not cause the $M deviation to
exceed the limits mentioned on page 3-119. If these limits are
exceeded, the Signal Generator could go out of lock which causes
queued up Hardware Error messages. Also, the external FM/$M
INPUT does not respond linearly to inputs greater than
approximately 1.2 Vpk.
3-122
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