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Power Sweep
In this mode, the CPU has the ALC step the RF out
-
put through a range of levels specified by the user.
This feature can be used in conjunction with the
sweep mode to produce a set of identical frequency
sweeps, each with a different RF power output level.
Amplitude Modulation
Amplitude modulation (AM) of the RF output signal
is accomplished by summing an external or internal
modulating signal into the ALC loop. External
modulating signals come from the front panel or
rear panel AM IN inputs; the internal modulating
signal comes from the A8 Function Generator PCB.
The AM Input Sensitivity DAC and the AM Calibra
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tion DAC, under the control of the CPU, adjust the
modulating signal for the proper amount of AM in
both the linear (log amp in) and the log (log amp by-
passed) modes of operation. The adjusted modulat-
ing signal is summed with the level reference, slope,
and detector inputs into the ALC loop. This pro-
duces an ALC control signal that varies with the
modulating signal. The action of the ALC loop then
causes the envelope of the RF output signal to track
the modulation signal.
Pulse Modulation Operation
During pulse modulation, the ALC level amplifier
(A10 ALC PCB) is operated as a sample/hold ampli
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fier. The level amplifier is synchronized with the
modulating pulses from the A6 Pulse Generator
PCB so that the ALC loop effectively operates only
during the ON portion of the pulsed modulated RF
output.
Pulse
Generator
Operation
The A6 Pulse Generator PCB provides the internal
pulse generating function for the 682XXB/683XXB.
It also interfaces external pulse inputs from the
front or rear panel connectors to the pulse modula
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tor driver in the external mode.
The pulse generator produces a pulse modulation
waveform consisting of single, doublet, triplet, or
quadruplet pulse trains at variable pulse rates,
widths, and delays. It operates at two selectable
clock rates—10 MHz and 40 MHz. In addition, the
pulse generator produces a sync pulse and video
pulse output that goes to the rear panel and a sam
-
ple/hold signal that goes to the A10 ALC PCB. The
sync pulse output is for synchronizing auxiliary in
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2-18 682XXB/683XXB MM
FUNCTIONAL ALC/AM/PULSE
DESCRIPTION MODULATION