THEORY
OF OPERATION
3-22
3-55.
MODULATION OSCILLATOR
The modulation oscillator generates a leveled sine
wave of 400 Hz or 1 kHz and is the
modulation source for the internal AM and
FM
functions. The oscillator is a level-
controlled Wien-Bridge type and
consists
of op-amps U405A, U405B. Frequency
is
determined by the series RC time
constant
of the components between pins 5 and 7 of
U405B
and
by
the parallel RC time
constant
of the components from U405 pin
5
to
ground.
The
modulation frequency
control line, MF400L, originating
at
the latch
U308,
selects either 400-Hz or 1-kHz operation, and is selected
by switching resistors with
JFETs
Q401
and
Q403.
The amplitude of oscillation is controlled by an
ALC
loop that varies the resistance on
U405B
pin
6
to ground. This resistance
comprised
of R412 and the drain resistance of
Q402,
is
nominally 2K ohms. The oscillator
signal
amplitude is sensed by rectifier CR401
.
The average current through
CR401 is made equal to the reference current in R416
by
integrator-amplifier U405A. Level adjustment is set
by
potentiometer
R419.
Temperature compensation is provided by R417,
R418, and CR402.
3-56.
FM DEVIATION CONTROL
The FM modulation signal
source
and
deviation
control circuits are on the Output
assembly. Analog switches U401A, U401B, and op-amp U402A .select the internal
or
external modulating signal, or no modulation. The selected and
buffered
modulating
signal at U402A pin? is applied to FM DAC U403. This DAC
provides fine
control of the
FM deviation. (The coarse control FM circuitry is part of the Synthesizer
assembly). The
output of the DAC, at U405C pin
8,
is the modulation signal multiplied
by a
factor
proportional to the 8-bit FM deviation control provided by the Controller,
3-57.
Attenuator/RPP
,
A2A5
The
Attenuator/
RPP Assembly, A2A5, consists of an Attenuator/ RPP PCA, A2A5A4,
in a metal
housing mounted on the top side of the A2 module section to form a
shielded
enclosure. The Relay
Driver
/
RPP PCA,
A2A5A5, is
included in this assembly.
The Attenuator assembly
controlled
by
the
microprocessor provides
coarse control of
the Signal Generator output level. The high-level signal from the Output PC assembly,
A2A4, is applied to the Attenuator
which provides 0 dB to 1 38 dB of
attenuation,
in 6-dB
steps,
to this signal before it goes to the Generator RF OUTPUT connector.
Compensation data for the attenuator in each Generator is stored in the Attenuator
calibration EPROM located
on
the
Controller PCA, A2A7. The instrument program
uses this data to correct for the combined deviations of the attenuator sections in
use.
For
more details on level correction, refer to paragraph 3-13, Amplitude Control.
The Attenuator/ RPP Assembly provides an attenuation range from 0 dB to 138 dB in
6-dB
steps and
consists of seven independently cascaded 50-ohm attenuation sections, a
6-dB, a 12-dB, and five 24-dB sections. Each section consists
of
a DPDT relay
and a
three-resistor attenuator pad.
One relay position (when power is applied to the relay provides a straight path for the RF
signal,
and
the other position (no power applied to the relay) inserts the attenuator pad
into the RF signal path. All seven
relays
are
inside
individual
shielded compartments in
the
Attenuator
housing.
The control of
the
Attenuator
relays is latched via U27, the open-collector drivers U30
and U31 on the Controller PCA A2A7
and
transistor
drivers on
the
A2A5A5
Relay
Driver/ RPP control PCA. For calibration and troubleshooting purposes, special
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