Model 8642A/B
Operation
Modulation, AM (cont'd)
The external modulation signal level should be adjusted to a 1 Vpk (0.707 Vrms)
sinewave input to the Signal Generator to get the displayed AM depth. With an
external 1 Vpk signal applied, the Signal Generator modulates its RF output to achieve
the modulation depth setting. For example, if a 1 Vpk external modulation source is
provided, with the Signal Generator's AM depth set to 50.0% per volt, the output has
50.0%
modulation depth. (If the source voltage is reduced to 0.5 Vpk, and no other
settings are changed, the output has
25%
modulation depth.)
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 the following functions can be used to couple the external AM/PULSE
INPUT to the AM circuitry:
AC-Coupled Source
• f EXT AC I
This function selects the external, ac-coupled AM source (displaying the EXT and
AC annunciators).
• INT + (EXT AC) (( SHIFT
1.
f EXT AC ) )
This function selects the external, ac-coupled AM source and internal AM source
summed together (displaying the INT, EXT, and AC annunciators).
DC-Coupled Source
•
1
EXT DC 1
This function selects the external, dc-coupled AM source (displaying the EXT and
DC annunciators).
• INT + (EXT DC) (I SHIFT I. I EXT DC
1
)
This function selects the external, dc-coupled AM source and internal AM 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.
Depth
I AM
1
and the desired modulation depth (in
%
per volt).
The modulation depth setting, when using an external source, sets the input sensitivity
per volt, not the actual depth. A linear relationship exists between the AM depth
setting and the external voltage applied (for voltages less than or equal to
approximately 1.2 Vpk).
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