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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 120
MHz
reference
is
applied
to
T1
in
the
mixer, via R19, R20
and
the
filter
L3/L4/C16/C17/C18.
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
'0
1
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.
The output
is
fed
to
the
meter
via pin 31
through
the
Output
switch
S
1
2
to
the
Meter
switch Sll (see Fig. 20).
Meter
ranging
is
by
a
bank
of
resistors
on
the
1
dB
Step Attenuator switch
and
selected
by
it.
Meter
calibration
is
by
resistors
R134 and R136
for
zero
and
full
scale deflections respectively.
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
1C9,
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
D 33/D
34
and D35.
The diode
attenuation,
and hen
ce
the carrier level
is
thus
control led at a
point
which
produces
a
balance between
the
two
inputs of
comparator
IC9.
9082G
Vol.2
4-13
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