THEORY
OF
OPERATION
Modulator
attenuation is thus
approximately
proportional to the
modulator control
voltage at the
emitter
of
Q
103. Proportionality is
required to maintain
constant
leveling
loop bandwidth as
modulator attenuation varies.
Minimum
attenuation is
obtained
with
a
modulator control
voltage of
lOV, while maximum
attenuation
is obtained with
OV.
Comparator
U3
lOA and
associated
components form an
unleveled
indicator
circuit. The
comparator
senses
the modulator
control voltage at the emitter
of
Q103.
This
voltage is
normally
less than +1
IV, and the comparator
output is high. If the
modulator
control
voltage
exceeds +1 IV, the
modulator attenuation is
at
a
minimum, and
the leveling loop
becomes
inoperative (unleveled).
This
condition could be
due
to a fault or some
abnormal
operation such as
over-modulation. In
this
case, the comparator
output (UNLVLL)
goes
low.
The Controller
senses
this low and causes the
front panel ‘UNCAU
indicator to
flash
and displays
an unleveled status
if interrogated.
3-54.
LEVEL
CONTROL
The instrument
output
level is set by the
level-control
circuit. Inputs to this
audio signal
processing circuit are
the internal and external
modulation signals, a dc
reference voltage,
and
the
digital control
commands.
The circuit output is
the
leveling loop
control
voltage
that provides
vernier
level control of the
Generator output.
Digitally
encoded
level,
modulation
depth, and
temperature-compensation
information
are provided
by
the
Controller,
Selection of the
internal or external modulating signal,
or no
modulation, is made by
analog switches
U401C,
U401D, and Op-amp U402B,
The
selected, buffered
modulation
signal at
U402B
pin 1 is applied to pin4 of U30I, adual
8-bit DAC U301, with
U302D,
acts as a
digitally
programmed variable attenuator and is
labeled AM DAC.
Binary
AM depth control
information from the Controller is
applied to DAC U301.
The
output at U302D
pin 14 is the modulation signal
scaled to the
programmed
AM depth.
This ac
signal is summed by
op-amp U302B
with a dc reference
voltage provided by
CR403. The
output
at
U302B
pin 7 is called the
1+AM signal. This signal
provides the
desired
AM depth when
scaled
by
the LVL DAC and applied
to
the leveling loop,
AM
depth
adjustment is
provided by
potentiometer
R421.
The instrument
RF output amplitude is temperature
compensated
in a
frequency-
dependent
manner
as
follows. The H-AM signal is applied to pin 1 8
of dual 8-bit DAC
U301, the DAC B
reference input. The DAC output, at U405D pin
14,
is
the H-AM signal
attenuated by an RF
frequency-dependent factor provided by the
Controller using
constants stored in
the Generator firmware. This voltage is
applied to a
resistor/thermistor
network that
includes R303, R305, R306, and RT301.
The network output is the 1+AM
signal attenuated by an RFfrequency
and
temperature-
dependent
factor, and is applied to summing
op-amp U302C.
The 1+AM
signal is also
applied to this
summing
amplifier. Thus, the voltage at U302C
pin
8
is the
temperature-
compensated and scaled
1+AM
signal
This
signal is applied to the reference
input
of Level DAC U303. This 12-bit DAC,
with
op-amp
U302A,
latches U304, U305, controls the Output
assembly
RF output
amplitude.
The DAC
output
voltage, at U302A pin
1,
is the
temperature-compensated
1+AM
signal
multiplied by a
factor
proportional to the 12-bit level control
number
provided by the
Controller. This voltage
is
the leveling loop control voltage. The
Generator
RF output
level
adjustment is provided by
potentiometer
R3
1
1 ,
and DAC offset voltage
adjustment
is
provided
by potentiometer R309.
3-21
To Next Page
Loading...