Table of Contents
Advertisement
Amplitude Modulation and Automatic
Level Control
(ALC)
3.6.
The
carrier level at the
output amplifier
is
peak
detected
and compared
with
a
d.c. reference voltage
which
is
determined by
the setting
of
the front
panel
VERNIER
control
.
The
resulting a
.
I
.c
.
voltage automatically maintains
a constant
carrier
amplitude
in
CW
mode.
Amplitude modulation
is
obtained by varying
this
a.I.c. feedback voltage
at
an audio frequency
supplied
by
the
modulating
tone.
3.7.
The modulation
oscillator
provides three fixed audio frequencies
for
amplitude
(a.m.) frequency
(f.m.)
and
phase modulation
(0M).
External
modulation
frequencies
can be
used
up
to
100
kHz
on
a.m.
and
f
.m.
,
and
5kHz
on
Frequency Modulation
3.8.
Frequency modulation
is
obtained by modulating
the
main
VCO
(240-540MHz)
varactor tuning with
an a.c.
voltage
related
to
the
audio modulating
tone.
Because
of the
non-linear voltage-frequency
characteristic of the
varactor-tuned
VCO,
provision
is
made
for
monitoring
the
actual deviation
on
the carrier
and
displaying
it
on
the
meter.
The
user
can
then
set
any
desired
level
of deviation
by
use of the
FM COARSE
and
MODULATION
LEVEL
controls.
3.9.
The f.m.
monitoring
referred
to
above
is
carried
out by feeding
the
Range
2
carrier signal into a
mixer operating
in
a
frequency locked
loop, giving
an
i.f.
output
of
400 kHz.
The
deviation
on
this
400
kHz
is
demodulated,
filtered,
and
the
audio
component
fed
to
the
meter
as
a display of actual
deviation obtained.
In
models
with
serial
numbers up
to
1409
the
f.m.
peak
deviation
level
is
controlled automatically.
That
system
is
described
in
Issue
1
of the
Manual
.
Phase
Modulation
(0M)
3.10
Phase
modulation
uses the
same
circuitry
as for
f.m., except
that the basic
concept
of
phase
mpdulation'
is
that
deviation
increases linearly
with increase
of
modulation frequency.
This
is
achieved by
providing amplifier
roll-off
which
is
compensated by
increased
modulation
levels.
Frequency Counter and
Display
3.11
The frequency
counter monitors
the
r.f.
output
of
the signal
generator.
Frequencies
up
to
32
MHz
are
counted
directly.
For the
remaining
frequency coverage
the
same
input
range
is
used but
the
counter time base frequency
is
automatically adjusted (by
the
Range
logic)
in
accordance
with
the
division
ratio
in
use
for
each range,
thus
giving a
frequency readout
from 5
MHz
to
540
MHz
(1.5MHz
to
540MHz
for 9.082).
9081
&
9082 Vol.
2
3-3
Advertisement
Table of Contents
