Frequency Divider Ic; Phase Modulators, Amplifiers & Multipliers - GE Mastr II Maintenance Manual

LBI-4732
CIRCUIT ANALYSIS
OPERATIONAL AMP- LIMITER
--~~----.-----~~------------------+-------------~4
13r-----------+-~--~----+-_.
______ ._ __ _.
TX AUDIO LO
R20
40K
10 9
Rl9
40K
DECOUPLER NETWORK
TO Cl50
POST-LIMITER
FILTER
04
AMPLIFIER
Rl7
Rl5
REG +IOV
A-
RC-2668
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 out-
put at "Q" near A-). The first cycle of
the oscillator output causes it to switch
to the "1" stage (output at "Q" approximate-
ly 5 Volts). The second cycle will cause
the flip-flop to switch back to the "O"
state. Therefore, it requires two oscilla-
tor cycles to switch the left flip-flop
through one complete cycle from "O" to "1"
and back to "0".
When the left flip-flop switches from
"1" to "0", 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". There-
fore, four cycles of the oscillator output
are required for each cycle of output 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-
4
sents a tuned load to the IC, so that harmon-
ics 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 output of the fre-
quency 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 appeHrs
as a series-resonant circuit the RF output
of the oscillator. An audio signal applied
to the modulator circuit through blocking
capacitor Cll5 varies the bias of CVlOl, re-
sulting in a phase modulated output. A volt-
age divider network (Rl08 and Rl09) provides
the proper bias for varactors CVlOl, CV102
and CV103.
The output of the first modulator is
coupled through blocking capacitor Cl06 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 re-
move any amplitude modulation in the modula-
tor output.
Following Ql02 is another Class A Ampli-
fier, Ql03. The output of Ql03 is applied
to the second modulator stage. The second
modulator consists of two cascaded modulator
circuits consisting of CV102, Ll02, Ll03 and
CV103. Following the second modulator is a
Class A amplifier Ql04. The output of the
second modulator stage is metered through
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