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FM
LOOP
ASSEMBLY
4.134
Potentiometer
R50 on IC3/3
provides a reference voltage
for
calibrating the
loop frequency.
The procedure
is
described
in
Chapter
5.
If,
when
switching
on,
the
i.f.
is
outside the
capture range of
the
comparator
circuit
a
sweep
circuit
comes
into
operation,
as follows:
4.135
The
elements of
the
sweep
circuit are:-
(1)
The
sweep
transistors
Q15
and
Q16.
(2)
A
circuit
to
detect
when
the oscillators are
far
off
tune
and
generate
the
sweep
action
(Q21,
Q22,
Q23
with
bistable
IC4d/IC4d).
(3)
A
circuit
to
detect
when
the
i.f.
is
within
the
capture range
of
the
loop
(Q18,
Q19, Q20)
and
inhibit the
sweep
action.
(4)
A
means
of switching
the
sweep
action
on and
off (gates
IC4a/IC4b)
.
4.136
Sweep
Circuit
.
The sweep waveform
is
a
sawtooth
in
which
the oscillator
varactor tuning
is
swept
rapidly
upwards and
slowly
downwards,
with
the
object
of
achieving
lock
when
the oscillator
frequency
is
above
that of the input signal
.
Due
to
the inverting
action of IC3
the
sweep
direction
on
pins
2
and
3
is
opposite
to
the
direction
of
oscillator
sweep,
thus
Q15
with
R51
pulls
IC3/2
slowly
upwards
(oscillator
tuning
downwards) and
Q16
provides
an
opposite, but
faster,
sweep
in
IC3.
These
sweep
transistors
are controlled
as
follows:
4.137
The
conductivity of
Q15
and
Q16
is
determined
by
the bistable
IC4c/IC4d
and
the gates
IC4a/IC4b.
If
the
loop
frequency
is
within
the
capture
range
of IC3 then the
sweep
transistors
are turned
off.
This
is
done by
using
the
output
from
the
Schmitt
trigger
1C
1
b,
which
is
fed via
C38,
emitter-follower
Q18
and low
pass
filter
LI,
with associated capacitors,
to
a
diode detector
D14.
If
the
loop
frequency
is
within
the
capture range of
the
comparator,
the level of the
detected
signal will
be
sufficient
to
turn
on
Q19/Q20,
which
is
a long-tailed pair with positive
feedback
via
R66
giving a rapid Schmitt
trigger
action.
This
output from
Q19
inhibits
the gates
IC4a and IC4b
and
thus
prevents
the bistable
lC4c/IC4d
from driving
the
sweep
transistors
Q15
and
Q16.
If,
however,
the loop
frequency
is
outside the
capture
range,
there
will
be
little
or
no detected
signal
from
D14
and IC4a/IC4b
will
be enabled,
thus
allowing
the
sweep
to
operate.
4.138
The sweep
generator operates
by
applying
the
varactor
line
voltage
to
the
emitters
of
Q21
and
Q23
via
diodes
D16
and
D17
respectively.
If,
for
example,
the
varactor
line
is
sufficiently positive, the
p.n.p.
transistor
Q21
will
turn
on and
the
n.p.n.
Q23
will turn off, thus
causing
bistable
IC4c/lC4d
(via
gates
IC4a/IC4d)
to
turn
on
Q15
and sweep
the oscillator
slowly
downwards.
4.139
If
lock
is
not
achieved
the
varactor
line will
go negative,
thus
reversing the
conductivity of
Q21
and
Q23
and changing
the state
of
the bistable,
which
will
turn
off
Q15
and
(via
Q17)
turn
on
Q16
for
a rapid
upward sweep.
This
action
will
continue
until
an in-band
signal
is
detected
at
D14,
causing
Q19
to
inhibit the
gates
IC4a/IC4b
.
9081
&
9082 Vol
.
2
4-26
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