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CIRCUIT
ANALYSIS
L8r31072
amplifier provides approximately 20 dP of
IF gain.
The output of
Q501
is
coupled
through an impedance matching network
(L502) that
matches
the amplifier output
to
the input of IC
U501.
U501
and
associated circuitry
consists
of the 2nd oscillator,
mixer and 2nd
IF
amplifier.
The
crystal for the oscillator
is Y501, and the oscillator
operates aI
20
"945 lllHz
f
or lovr side
in
ject ion of
the
2nd IF (standard), or 21.855
MHz
for
high
side injection with (Structured Option
FT)
for those radios
determined
to be operating
on a tweet frequency.
This frequency is
mixed
with the 21.4
MHz
input.
The output
of the mixer is lirni.ted by
D501
and
D502.
L503 is tuned for the 455 kHz 2nd lF fre-
quency.
The
output of
U501
is coupled
through
ceramic filter
Z5O2
wlnich
provi.des the
455
kHz
selectivity,
and
applied to U502.
Test
Point
TP501
is used in
a
ligning the recei-
ver,
and can be used
to
check
the output of
u501.
U502
and associated
circuitry
consists
of a 455 kHz limiter,
a quadrature type
FM
detector and an audio
preampl
i
fier.
L504
is the quadrature detector coil.
Audio
Level potentiometer
R521
is
used
to set
the
audio output level to the audio amplifier.
AUDIO AND
SQUELCH
CIRCUITS
Audio
Audio is applieC to the
Channel
Guard
tone reject filter
through
P903-3 and
back
to the
de-emphasis
network through
P9O3-7.
The audio passes through the
de-emphasis
network (R633,
C608
and
C609
) to
Volume
Cont
rol
R634.
Audio arnplifier
IC U601 drives
the
speaker at the desired audio level (up to
three watts).
The feedback
loop containing
R637,
R638
and
C611
determines the ampli-
fiers closed loop plain.
R636
and
C613
pro-
vide the higb audio frequency rol 1-off
above
6
kHz.
The
audio amplifier can be
muted
by
a
DC voltage from the receiver mute
gate
(Q605)
which uses different
logic inputs.
These inputs are 8.5 V Tx,
MONITOR,
or
a
squelch signal.
The Rx
MUTE
function
from
the
Channel Guard
circuit
which is applied
through
P903-4.
Squel
ch
Tbe squelch circui t operates on
the
noise components contained in
the
FM
detector output.
The output of U502 is
applied
to
frequency selective
noise
arnplifier
Q601
that has a resonant circuit
(L601, R604 and
C6O2)
as the
col
lector
1oad.
The
output is noise in a
band around
7 k{z.
The
noi
se output
i
s coupled
through
Squelch control
R607
to
expander
ampllfier
Q602
which improves the level discrirnina-
tion characteristics
of the circuit.
The
output of Q602 is applied to a
passive
voltage doubler circuit
(D603 and
D604).
This circui t has a high source
impedance
and operates
as an
average
value rectifier.
Fol
lowing the
vol
tage doubler is
a
Schmidt trigger
(Q603-Q604
)
.
The
Schmidt
trigger provides the
necessa
ry hysteresis
and a
we1
1-defined output
signa
I for
Rx
IIUTE
gate
Q605
Wi
th
no RF
signa
1 present,
the
detected noise
a
t the vol tage doubler
output turns on Q603, turning off
Q604.
This
causes Q605
to turn on, applying
+1.
volts to pin 2 of audio amplifier
U601.
This voltage turns off
U601
and mutes
the
receiver.
When
an RF
signa
I is
received
,
the
noise at the output of
Q601 decreases
and
drive to e603 is removed. This turns off
Q603
and al1ows
Q604
to turn on. With
Q604
turned oD, Rx
IVIUTE
gate
Q605
turns
of
f
.
This turns on
U601
so that audio i-s
heard
at the
speaker.
The squelch
sensitivity
is adjusted
by
R607
in the
base
circuj.t of
expander ampli-
fier
Q602.
Pressing in the
MONITOB
pushbutton
on
the front of the radio
opens
the
Rx
MUTE
to
disable the
Channel
Guard. It also
grounds
the base of
Q601
and disables the
squelch
function.
CHANNEL
GUARD
Channel
Gua
rd is a continuous
tone
encoder/decoder
for operation on tone fre-
quencies in the 71.9 Hz to 210.7 Hz
range.
The encoder provides tone-coded modulation
to the transmitter.
The decoder operates
in conjunction with the receiver to inhlbit
all calls that are not tone
coded
wlth
the
proper
Channel Guard frequency.
The Channel Guard circui try consists
of discrete
components for
the
Encode
disable,
PTT switch, and receiver
mute
switch; four thick-film
integrated circuit
modules
consisting of
Decode Module
U1001,
Encode Module U1002, Frequency Swi.tchable
Selective Amplifier
(FSSA)
U1003,
plug-in
Versatone Network ZIOOI and monolytic
IC
U1004
in the tone reject filter.
For a
f
uncti-ona
I
di.agram
of the
Chan-
nel
Gua
rd
Encoder/Decoder
, refer
to
the
troubleshooting
procedures.
References
to
symbol
numbers mentioned
in the fol lowi-ng text are found on
the
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