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FM LOOP ASSEMBLY
FM
LOOP
ASSEMBLY
19-0844
(Fig.
9)
Principles of
the
System
4.128
Frequency modulation
is
obtained by modulating
the
main
(240-540MHz)
oscillator
varactor
line
with a voltage derived from
the
audio
input.
This
type of
oscil lator
has,
however,
a non-linear voltage/frequency
characteristic,
there-
for
provision
is
made
for
monitoring
the
actual deviation
on
the oscillator
output.
4.129
Referring
to
the
block diagram.
Fig.
4.3 on
the
next page,
the
system operates
by converting
the
Range
2
(30-67.5
MHz)
main
oscillator signal
to
400kHz
in
a
frequency locked
loop..
The
deviation
on
this
400
kHz
is
demodulated by
a discriminator
and
the
audio
component
fed
to
the
meter
as
a display of deviation,
which can
be
manually
set
using the
FM COARSE
and
MODULATION
LEVEL
controls.
Circuit Description
:
400kHz
Loop
4V130
Referring
to
the circuit
diagram.
Fig.
9, the
frequency modulated
30-67
.5MHz
signal
derived from
the
main
oscillator
is
mixed
in
IC2 with a
signal
from
the
loop
oscillator
Q9/Q14.
The
difference frequency
400kHz when
the
loop
is
locked,
is
fed via emitter followers
Q37
and
Q40
to
Q3
and
Ql,
which
with
Q2,
form a
limiter
stage
and
also
drive a
pair of
Schmitt
triggers
ICla and IClb
in
anti-phase.
4.131
Each
Schmitt
trigger
feeds a
pump-type
frequency
discriminator,
Q6
and
Q8,
via driver stages
Q5
and
Q7.
The
stable
voltage supply,
essential
for
this
type of discriminator,
is
provided
via
Q4
which
is
temperature
compensated by D2,
D3
and D4.
The
output from
disciminators
Q6
and
Q8
is
fed via
C44
to
the
a.f. loop
and
via
TP3
to
the
400
kHz
loop.
4.132
Continuing
the description of the
400kHz
loop, the unfiltered discriminator
output
is
fed via
TP3
to
pin 2
of
comparator
IC3
.
This
comparator
attempts
to
maintain
identical levels
at
pins
2
and
3
and
provides
an
inverted output
level at pin
6
which
tunes the
oscillators
Q14
and
Q9
by
means
of varactor diodes
Dll and
D9
respectively.
4.133
Two
oscillators
(Q9
and Q14) cover
the
frequency range,
the
appropriate
oscillator
being switched
on
when
the control transistor
in
the emitter circuit
(Q10
or
Q13)
is
turned
on by
the half
octave
signal
from
the
ROM
in
the
Divider
and
Phase Detector
Circuit.
Transistor
Q12
acts as
a
logic inverter.
9081
&
9082
Vol
.
2
4-24
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