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OUTPUT
SYSTEM ASSEMBLY
Range
1
Mixer and
Filtering
4.78
The Range
1
frequency band
is
obtained by mixing
a
120
MHz
reference with a
portion of the
Range 4
spectrum.
This restricted portion of the
range
is
determined by
the
ROM
in
the
Divider
and
Phase Detector Assembly,
which
limits
the
tuning
range on
this
band.
4.79
The
120MHz
reference
is
applied
to T1 in
the mixer, via R19,
R20
and
the
filter
L3/L4/
C
16/C
1
7/C
1
8
.
The Range 4
signal
from
Q10
collector
is
fed into
the opposite
side of the
mixer
at
T3.
The mixer
output
is
fed via
Q6
to
a
low
pass
filter
L5/L6 and
associated
components which removes unwanted mixer
frequencies.
The
output
is
enabled
when
Q7
is
turned
on by
a logical'O'on
line
RE,
applying
a positive supply
to
the
collectors of
Q6
and
Q8,
and
forward
biassing the
combiner diode D17.
RF
Output
Amplifier
4.80
The
r.f.
signal
from
the selected
range
is
fed via the
combiner diode D7,
or
D17, and
C74
to
the
modulator
preamplifier
Q13,
which
has
an
overall
gain of
approximately x4.
The
r.f.
signal
is
then buffered
in
Q14
and
fed via the
components
of the
modulator
stage
to
Q15,
which
is
the driver
for
the
balanced
output
power
amplifier
Q16/Q17.
Amplifier
bias
is
preset
by
R94.
The
output
is
fed via
C94
to
the
output attenuator.
The
diode
D36
with
R107 and
C97
forms a
peak
detector
on
the
output
waveform which
provides a
d.c.
level
for
automatic
level control
and
meter
information.
Output
Metering
4.81
The
r.f.
output
is
peak
detected by
D36 and
the information stored
on C97,
this
level
being applied
via the
filter
L22/C98 and
R128
to
the operational
amplifier
IC7
which
operates
in
a
conventional
circuit,
with the addition of diode
D38
for
temperature compensation.
Meter
calibration
is
by
potentiometers
R134 and
R136.
The
output
is
fed
to
the
meter
via pin 31 through the
Output
switch
SI
2
to
the
meter switch
SI
1
.
(see
Fig. 20).
Amplitude Modulator
Stage
4.82
The modulator
operates
on
the principle that the
impedance
of
the
p.i.n.
diodes
D33,
D34
and
D35
varies
according
to
the
applied
bias current.
If
this
bias
current
is
varied
at
an audio frequency
then amplitude
modulation
of the
r.f.
signal will
occur.
Similarly, the
r.f.
carrier level
can be
controlled automatically, or
manually, by a
d.c.
level
applied
to
the
p.i.n. diodes,
as
described
below.
The modulation
process
is
described
in
para.
4.87.
ALC
and
Carrier Level Setting
4.83
The automatic
level control
(ALC) comparator
is
IC9,
which compares
the
peak
detected
signal level
on
C97
with an adjustable reference
level
applied
to
the
other input (IC9a/2).
The
output
at
IC9/7
is
fed via L19,
R89 and R90
to
the p.i.n.
diodes
D33/D34
and D35.
The diode
attenuation,
and hence
the carrier level
is
thus
controlled
at a
4-13
9081
&
9082
Vol
.
2
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