Frequency Divider; Phase Modulators, Amplifiers And Multipliers - GE MASTR II Maintenance Manual

LBI4898
CIRCUIT ANALYSIS
OPERATIONAL AMP- LIMITER
4
MIC A+ Rl8
REG +IOV
POST- LIMITER
FILTER
.RIO
Rll
..
Q4
C4 C5
C3
11~--------~~~-4----~~~----._--~
AMPLIFIER
MIC LO
Rl7
R20 Rlt
401(
40K
Rl5
10 9
OECOUPLER NETWORK
A-
RC-2445A
Figure 4 - Simplified Audio IC
FREQUENCY DIVIDER IC
The output at pin 3 of the selected
ICOM is coupled through buffer amplifier
QlOl to frequency divider UlOl, which di-
vides the oscillator frequency by 4. The
divider consists of two J-K flip-flops con-
nected as a binary counter.
When the·transmitter is not keyed (no
ICOMs on), QlOl is saturated (turned on)
with its collector voltage near zero. Key-
ing the transmitter starts one of the ICOMs,
and its output cuts QlOl on and off once
each cycle. As QlOl turns off during each
cycle, the drop in collector voltage causes
the left flip-flop to change state. Assume
the flip-flop was in the "O" state (the
output at
''Q"
near A-). The first cycle
of the oscillator output causes it to switch
to the "1" stage (output at
''Q"
approxi-
mately 5 Volts}. The second cycle will
cause the flip-flop to switch back to the
"O" state. Therefore, it requires two os-
cillator cycles to switch the left flip-
flop through one complete cycle from "0"
4
to "1" and back to "0"•
When the left flip-flop switches from
"1" to "O", it causes the right flip-flop
to change state. It requires two cycles of
the left flip-flop to switch the right
flip-flop from "O" to "1" and back to "0".
Therefore, four cycles of the oscillator
output are required for each cycle of out-
put from pin 9 of UlOl.
If UlOl was operating into a pure re-
sistive load, its output would be a square
wave. However, the modulator circuit pre-
sents a tuned load to the IC, so that har-
monics are filtered out and the waveform at
the junction of Cl02 and Cl03 (modulator
input) is essentially a sine wave at one-
fourth the oscillator frequency. The out-
put of the frequency divider is coupled
through DC blocking capacitor Cl02 to the
first modulator stage.
PHASE MODULATORS, AMPLIFIER
&
MULTIPLIERS
The first phase modulator is varactor
(voltage-variable capacitor) CVlOl in series
with tunable coil LlOl. This network appears
as a series-resonant circuit to the RF output
of the oscillator. An audio signal applied
to the modulator circuit tbrougb blocking
capacitor Cll5 varies the bias of CVlOl,
resulting in a phase modulated output. A
voltage divider network (Rl08 and Rl09} pro-
vides the proper bias for varactors CVlOl,
CV102 and CV103.
The output of the first modulator is
coupled through blocking capacitor ClOG to
the base of Class A amplifier Ql02. The
first modulator stage is metered through a
metering network consisting of Rll5, Rl50,
Cl07 and CRlOl. Diodes CR102 and CR103
remove any amplitude modulation in the modu-
lator output.
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