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Ltsr31072
Schema
tic
Diagram,
Parts List.
Outline
Diagram
and
FSSA
Frequency
Switchable Selective
Ampli-
fier
(FSSA) U1003
is a highly stable active
bandpass
filter
for the 71.9
Hz
to 2I4.7
Hz
frequency range.
The selectivity
of
the
filter
is shifted
across the
bandpass
frequency range by
swi
tching Versatone
Networks
in the fi
I
ter
ci rcui t
.
The
gain of the
FSSA
is a function of
the tone frequercy.
The Tone Frequency
is
determjned
by the
Tone Network connected
in
the
FSSA
circuit.
Versatone Network
21001
is a precision resistor
network.
ENCODE
V/hen
PTT
swi
t
ch
i.
s
opera
t
ed
, the
Cha
n*
ne
I
Gua
rd encode
t
one
i
s
genera
ted
by
coupling the output of
FSSA
bandpass
filter
U1003
back to its
input through a
phase
inverting amplifier circuit
and a limiter
circuit.
The
output of the
FSSA
is
coupled
from U1003 to the input of the
phase
inverting amplifier at
U1002-9.
An
amplifier provides
180o phase
shift
of the tone frequency at the output.
The
output of the phase inverting
amplifier
circuit
is coupled from U1002-6 to
the
input of the limiter circuit at
U1002-5.
A limiting network sets the tone out-
put coupled from
U7402-4
to the input of
the
FSSA
(U1003-12)
at 53 millivolts
peak
to
peak.
The
limiter circuit is also
used
as
an
encode switch.
Keying the transmitter
applies +5.4 Volts to UlO02-2. This starts
the
ci rcui
t
osci
I
1a
t i
ng.
Tbe
tone
frequen-
cy is determined by the tone network
con-
nected in the
FSSA
circuit.
The tone
output of the
encoder
circuit
is taken from
U1002-7
and coupled
through
tone output
ampl
i
fier
Q1002
and
modula
t
ion
adjustment 81010 to the audio processor
on
the
transmi
tter/receiver
board.
DECODE
Aud
i
o from
Vol ume/Squel
ch high
tha
t
contains the correct frequency is
coupled
to pin 1 of
Decode
Module
U1001. Pin 1 of
U1001 is the input of an
act
ive, three
stage, low pass filter.
The low pass fil-
ter attenuates frequencies over
21O.7
Hz.
The output of the 1ow pass fi lter
at
U1001-15
is applied to U1001-14.
U1001-14
is the input of a limiter circui t, limiting
CIRCUIT
ANALYSIS
the output
at
peak
to
peak.
U1001-13 Lo 55 millivolts
The
output from the limiter is
coupled
to Pin
12
of
FSSA
U1003. Since the tone is
the proper frequency, the
FSSA
will
a1low
it
to pass.
The output of the
FSSA
is
coupled to U1001-3.
U1001-3
is the input
to an amplifier circuit.
The
output of
the
amplifier at
U1001-4
is coupled to the in-
put of a threshold detector at
U1001-6.
In the
mute mode, when
the tone
decod-
er in
U1001
detects the
Channel Guard
fre-
quency,
Q1005
turns
Q1006
off.
This
un-
mutes the receiver audio.
In the
squelch
mode,
Q1006
is operating, grounding the
Rx
MUTE
lead
and muting
the receiver
audio.
Audio from
VOL/SQ
HI is applied to
the
tone reject filter.
The
tone reject filter
is an active filter
consisting of
U1004 and
associated circuitry.
A11
frequencies
from
7O
to 210.7 Hz are rejected by the fj.Iter,
while passing all
other audio frequencies
back to the recei'rer audio circuits
(fil-
tered
VOL/SQ HI
).
Encode
Disable
The Encode Di.sabIe
circuit consists of
Q1003
and Q1004. To disabte the
encode
circuit,
a positive voltage (+8.b to
14
VDC)
is applj.ed to Molex connector
p910-b
at the rear of the radio.
This is
accom-
plt"!gO by temporarily jumperins pgto-9
(CG
DISABLE)
to p910-11 (A+).
Thls positlve
voltage is applied to the base of
e1003,
turning
on
both
Q1003 and
e1004.
STANDARD
TONE
FREQUENCIES
(Hz)
77.9
7
4.4
77
.O
79
.7
82.5
85.4
88
.5
91
.5
94.8
97.4
100
.0
103.5
707.2
110.9
114.8
118.8
123.0
t27 .3
131.8
136.5
141
.3
146.2
151
.4
756.7
203.5
270.7
162
I67
t73
179
186
192
2
o
8
o
2
8
SERVICE
The
Encode
Disable circuit
has
been incorporated as a
maj-nten-
a
nce
a
id for
the servi
ceman
.
This circuit disables the
Channel
Guard encode cireuit
and allows
the
serviceman
to
make
transrnit-
ter distortion
and modulation
checks
without
removing
the
cover
from the radio.
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