THEORY OF OPERATION
I
C23
couples the VCO signal to
the Synthesizer assembly by
a
Ihrough-the-plate
coaxial
connector P104,
The
other VCO signal is
connected to the Output
assembly A2A4 by a
plug-in
capacitor, A2C1 . This plug-in
capacitor allows
either
VCO or the Output PC
A to
be
removed independently
from the module A2
assembly
without the use
of a soldering
iron.
3-51.
Output
PGA, A2A4
The
Output PCA accepts RF
signals from the
Synthesizer
and the VCO
circuits
and
command signals from
the Controller. The
output circuit
provides
a 0.01-MHz to
1050-MHz RF
signal to the
Attenuator.
The Output
assembly
reduces harmonic
distortion
components
in the RF
signal,
controls
RF signal amplitude,
introduces AM, and
generates the low
(heterodyne)
frequency band O.OI
MHz
to 245 MHz though
mixing. It also
generates
a modulation
signal to provide
internal AM and FM, and
provides a digital
interconnect path
between
the
Controller
and Synthesizer.
3-52. RFPATH
The RF
path begins with the
two
RF signals from
the
VCO and the
Synthesizer
assemblies. The
SPOT
bandswitch circuit
selects between the
512-MHz
to 1050-MHz
signal at
PI06 and the 245-MHz
to
512-MHz signal
at
P107. The selected
signal is
applied to
buffer
amplifier QlOl
and
Q102.
The
245-to 512-MHz signal
directly generates the
245-to 512-MHz
mid-band
output
signal. The
512-MHz to 1050-MHz signal
generates the
512-MHz
to 1050-MHz
high-
band
output
signal directly and the
0.01-MHz to 245-MHz
low-band output
signal
by
mixing
with an 800-MHz LO
signal.
The buffer
amplifier
Q
101 and
Q102
is a
common-base,
common-emitter
cascade
circuit
with
7-dB gain. The
three cascaded filter
circuits
that follow the buffer
amplifier consist of
combinations
of discrete
components and printed filters that
suppress harmonics
in the
Generator RF
output signal.
The first
section
of the circuit is a
printed 1 100-MHz low-pass
filter.
The second section
is
switched into
the RF path via
PIN diodes CR106 through CRI 10 by
asserting
MIDL
when the
Generator
is operated in
the mid-band
(245
to 512 MHz).
PIN diodes CRI
14
through
CRI 16
select
capacitors Cl
19,
CI21,
and
C123
whenever
HAOCTH
is asserted
to change
the section
cutoff
frequency from 512 to 350
MHz. The third section
provides
harmonic
filtering
for the two
higher bands, 512 MHz to 730
MHz,
and 730 MHz to
1050
MHz. PIN diodes
CRI 1 1 through CRI 13
select
capacitors Cl 12 through
Cl
14 to change
the cutoff
frequency
from 1050
MHz to
730
MHz whenever
HAOCTH is not
asserted.
The
amplitude
modulator
consists of PIN diodes CRI
17 through CR120 and
associated
components
and follows the
switchable filters in the
signal
path. The
modulator
is a
voltage-controlled
variable
attenuator
that provides AM and
output level
control.
Modulator
control voltage is
determined
by the leveling-loop
circuitry.
The leveling
loop
is
described later
in this section.
Q209,
Q21
1,
Q213,
and associated components follow
the modulator in the
signal path
and
form
a
three-stage,
20-dB gain, 245-MHz to
1050-MHz amplifier.
This
amplifier
drives a
3-dB
power
splitter
that consists of resistors R253
through
R255 and
associated
printed
transmission
lines.
One
power splitter
output drives the
leveling-loop
detector diode CR202.
The other
output
goes
to the HET
band
switch that includes PIN
diodes
CR203 through
CR210
and
3-19
To Next Page
Loading...
