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L8I31072
CIRCUIT
ANALYSIS
standard electret
microphone wi.thout
a
built-in
preamplifier.
The preamplifier
circuit
is located on the
IOC
board.
With this
microphone, MIC HI is
coupled througb J911-5 to the preamplifier
stage.
The
ampl
i
fied output is
coupled
through
R908
and C905 to the audio pro-
cessor.
For optiona 1 microphones with
a
bui
I
t-in
preampl
i fier,
audio is coupled
through J977-4, bypassing
MIC
PRE AMp
e901.
AUDIO
PROCESSOR
U1O1
The audio n"o".=.o.
provides audio
pre-emphasis with amplitude limiting
and
post limiter
filtering.
A total gain of
approximateTy 24 dB
is realized through
the
audio processor.
20 dB is provided
by
U101B and,
4
dB
by
U101A.
The 8.5
Vol
t regula tor
powers the
audio processor
and
applies regulated
+8.5V
through p9O3-2 to a voltage divider
con-
sisting of
R101, R111,
R110
and R109.
The
+4.25V output from the voltage divider
establishes the operating reference point
for both operational
amplifiers.
C1O6
provides an
AC ground
at the
summing
input
of both operational amplifiers.
Resistors R109,
R110, R111 and diodes
D101
and
D102
provide limiting
for
U101B.
Diodes D101 and D102
are reverse biased at
+7.7 VDC. Voltage divider network
R109,
R110 and R111 provides +b.9
VDC
at
the
cathode of
D101
and +2.6
VDC
at the
anode
of D102. The voltage at the junction of
D101
and
D102
is 4.25 VDC.
C104
and
C108
permit a DC level change between
U101B-z
and the voltage divider network for
diode
biasing.
When
the input signal to
U101B-6
is of
a
magnitude such
that the amplifier
output
at
U1018-7 does
not
exceed
4 volts p-p,
the
amplifier
provides a nominal 20 dB gain.
Itfhen the audio
signa
1 level at
U101B-7
exceeds 4 volts PP, diodes D101 and
D102
conduct on the positive and negative half
cycles providing
100%
negative feedback to
reduce the amplifier
gain to 1.
This
limits the audio amplitude at
U1018-Z
to
b
volts
PP.
Resistors
R105,
R106
and
R10?
and
ClOz
comprise the audio pre-emphasis network
that
enhances
the signal to noise ratio.
R107 and C1O7 control
the pre-emphasis
curve below limiting.
R106
and
CLOT
con-
trol
the cut-off
point for high
frequency
pre-emphasis.
As high frequencies are
attenuated, the gain of
U101
is
increased.
Audio
from
the
preamplifier
or
microphone is coupled to the input of
operational
amplifier
U101B-6.
The
amplified
output
of U1018 is
coupled
through R114,
R7I2
, R104 and R11Z to
a
second
operational amplifier
U101A.
The Channel Guard (CG) tone input is
applied to U101A-2. The
CG
tone is
then
combined
with the
microphone audio.
A post limiter
filter
consisting of
U101A, R112-R114,
C10B
and
C110
provides
12
dB
per octave roll-off.
R104
and
C102
pro-
vide an additional 6
dB
per octave roll-off
for a total of
18
dB.
The
output of the post limiter
filter
is coupled through
C110
to the
temperature
compensated
transmitter
oscillators
Q151
and
Q152.
TRANSMIT
OSCILLATOR
A
tempera
ture
compensa
ting network
consi,sting of
R151, R152, R1b3, R154,
D1b1
and
C151
maintains oscillator
frequency
over a temperature range of -30o to
+60.C.
The temperature compensating
DC
voltage
and
audio is applied to FM modulator
DIS2
through
MOD
ADJ control R155 and
R1b9.
Modulator varactor
D151
varies the transmit
frequency at the audio rate applied
from
the audio
processor.
Q151, Y1b1 and associated circuitry
comprise a Colpitts oscillator
which g.ne:
rates the ninth
subharmonic (50
MHz)
oi
the
RF carrier
frequency.
The transmit
osci
I
lator
is adjusted to the assigned
operating frequency by L151.
The oscil-
lator output is applied to buffer e153,
and
is then coupled through
p1b1
to the exciter
circuitry
transmitter/receiver
(Tx/Rx)
boa
rd
.
The second oscillator
stage
is
identical
to the F1 oscillator.
The
desired frequency is selected through
Tx
frequency
swi
tch S1b1 and channel
Common
switch
Q154.
Q154
is normally turned on so that
the
emitter
of the selected oscillator
is
grounded through
S151
and Q1b4,
tqrning
the
selected oscillator
on.
SERVlCE
R112-R114
are
7%
resistors.
This
tolerance must be maintained to
a
ssure proper
opera
tion of
the
post limiter
filter.
Use exact
replacements.
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