Ramsey Electronics COM 3 - Modulation and Receiver Functions

5.7 FM MODULATOR
FM modulation takes place in the fine loop
VCO circuit ( 5 - 7 MHz). Analog switches U 15
route the external modulation signal, or the internal
modulation signal to either the AM or FM
modulator. In the FM modulator, the modulation is
applied directly to the control voltage of the VCO,
along with the phase detector output. The phase
detector loop filter in the fine loop has a very long
time constant, therefore-any modulation applied to
the VCO is effectively not seen within the loop. If
the loop had a faster response, then the phase
locked loop would attempt to take the modulation
out, however this is not the case. Therefore the
FM is a true FM signal which is generated directly
on the fine loop signal. Modulation sensitivity
control, R70, is also contained in this circuit which
is used to calibrate the proper amount of FM
deviation. ›
5.8 AM MODULATOR
The AM modulator consists of a double
balanced
modulator/mixer
constructed
on
8
monolythic
silicon
chip
contained
within
IC
U3,
MC1496.
The
5
-7
Mhz
output
frequency
from
the
fine
loop
is
applied
to
one
input
of
the
double
balanced
mixer.
Applied
to
the
other
input
is
the
output
of
IC
U7
which
is
a
DC
voltage.
The
double
balanced
mixer
does
nothing
more
than
a
multiplication
process
.
The
DC
voltage
which
is
a
constant,
is
multiplied
by
the
5-7
Mhz
from
the
fine
loop.
The
constant
can
be
1,
1.5,
2.5
etc.,
therefore
you
can
see
how
it
controls
the
level
by
multiplying
it
up
or
down.
The
output
level
is
controlled
by
the
front
panel
potentiometer
called
OUTPUT
LEVEL,
and
is
also
controlled
by
the
AC
coupled
input
signal
through
capacitor
C75.
_
The
output
of
the
double-balanced
mixer
U3
is
coupled
through
capacitor
C20
to
emitter
follower
transitor
Q2.
The
output
impedance
of
the
double-
balanced
mixer
is
fairly
high,
and
must
go
through
the
emitter
follower
in
order
የዕ
match
the
500
7.
|
Mhz low pass filter that follows.
The
7
MHz
low
pass
filter
is
composed
of
L9
-
២11,
and
capacitors
C14
-
C17.
The
7
MHz
low
pass
filter
is
used
to
filter
out
any
higher
order
harmonics
which
come
from
the
5
-
7
MHz
loop.
Any
higher
order
harmonics
would
be
mixed
in
the
output
and
would
produce
an
output
spectrum
with
many
undesirable
spurrious
signals.
After
the
lowpass
filter,
the
signal
is
run
through
a
small
pad
composed
of
resistors
R32
-
R34
where
it
continues
to
mixer
X1.
The
output
level
at
this
point
is
approximately
-20
to
-40
dBm,
variable,
controlled
by
the
front
panel
RF
output
control.
5.9 1 KHz SOURCE
The 1 KHz oscillator is formed by a section
of
U7,
itis
a
standard
Wein-bridge
oscillator
which
uses
an
incandescent
bulb
for
feed
back
bias
stabilization.
The
output
is
a
very
low
distortion
1
KHz
tone
which
can
be
used
to
modulate
the
internal
oscillator
in
the
AM
and
FM
signal
generation
modes,
or
switched
to
the
output
jack
to be used for other purposes within the shop.
5.310 RECEIVER
The input to the receiver portion of the
Com
3
is
coupled
to
the
RF
input
jack
to
mixer
X2.
Mixer
X2
takes
the
coarse
loop
output
frequency
which
has
two
ranges
0.1-505
MHz
or
505-1018
‘Mhz
in
2
MHz
steps,
mixes
it
wth
the
input
from
the
front
panel
jack.
The
output
frequency
is
in
the
range
of
14.5
to
16.5
MHz.
The
output
of
mixer
X2
is
applied
to
a
bandpass
filter
and
then
to
a
low
noise
microwave
transistor
which
is
used
to
set
the
noise
figure
of
the
entire
receiver.
The
output
of
that
amplifier
is
applied
to
a
resistive
pad
to
provide
proper
impedence
matching
to
mixer
X3.
Mixer
X3
is
driven
by
the
5-7
MHz
loop,
which
has
500Hz
steps.
The
output
of
mixer
X3
is
the
final
9.5
MHz
final
IF
frequency.
The
9.5
MHz
IF
frequency
is
amplified
by
transistor
Q12
and
applied
to
a
double
pole
tuned
filter
and
into
AGC
amplifier
U
19.
The
AM/FM
receiver
portion
consists
of
two
receivers,
an
AM
receiver
and
a
FM
receiver.
The
FM
receiver
uses
a
standard
MC3359
FM
IF
IC.
Inside
this
IC
is
another
oscillator
which
operates
at
9.045
MHz,
which
converts
the
9.5
MHz
down
to
the
455
KHz
IF.
The
455
KHz
IF
goes
through
a
ceramic
IF
filter
which
is
approximately
+8
KHz
wide,
then
into
a
transistor
gain
stage
which
drives
another
gain
stage
as
well
as
the
AM
detector.
The