VARIABLE
FREQUENCY
OSCILLATOR
MIC JACK
AUDIO
AMPLIFIER
AND FM MODULATOR
CIRCUIT BOARD
Fig.
3. The microphone amplifier
and
FM modulator
are shown
combined on
a
single circuit board
in a passive
variable-
frequency
oscillator
case.
power microphone
amplifier
and, of
course, the FM modulator
we've
been
talking about. A
simple one- or
two
-
transistor audio amplifier
of the inex-
pensive
imported
variety
can be used
as the
mike
amplifier. To
get the
needed
high- impedance
output,
simply discon-
nect the output transformer
leads
and
pick
up your audio from
the collector of
the final stage
through a
0.01
-; F capaci-
tor.
You may also consider
using
one of
the
new
transistorized mikes
to drive
the
FM modulator
instead
of
getting
a
separate amplifier. If you prefer
a
home
-
built job, try the circuit
shown in Fig.
1.
It
is a conventional three -transistor
au-
dio amplifier with
limiting
features. Lim-
iting helps prevent
excessive frequency
deviation on audio
peaks, and though not
essential,
generally
improves
signal qual-
ity.
Potentiometer
RI
allows you to
adjust
the microphone
output for the correct
frequency deviation
before the
audio
is
fed to the base of Ql
through capacitor
Cl.
Transistor Q2,
hooked
up as an
emitter follower,
drives output stage
Q3
through
C4.
Operating power is provid-
ed by
B1,
which also provides reverse
bias
for the modulator
Semicap diode
(see
Fig. 2) through R10.
The
FM
modulator consists
of capaci-
tors C6 and C7, resistor R12,
and
diode
February, 1967
LEVEL
CONTROL
Shown below is
the
fully
assembled VFO FM mod-
ulator with audio level
control
and
connecting
cable. A
transmitter is
converted from AM
to
FM by removing
transmit-
ter crystal and inserting
cable plug
in empty socket.
Dl.
Exhibiting
a
reasonably linear high -
Q characteristic
when
reverse
-biased,
Dl
functions
as a
voltage- sensitive de-
vice. However, it conducts when
forward
-
biased, causing both
Q
and
linearity to
deteriorate.
It
is important,
then, to
place a
high enough reverse bias on
Di
to prevent
high -level
audio
excursions
from
driving
it into conduction.
But
since the
applied
audio voltage has
a
greater effect on
Di's capacitance
at
low
bias
levels,
the
bias should not be
too
high
either.
A satisfactory average
volt-
age is that which
powers the
amplifier.
Since, in operation, the
FM
modulator
shunts the
VFO's tuning capacitor, the
transmitter's
r.f. must
be
kept
from driv-
ing
Dl
into forward conduction.
This
danger
is minimized somewhat
by the
modulator design since C7
is
effectively
in
series
with the VFO's tuning circuit
and
Dl,
thus dropping
the r.f. voltage
to a safe
level.
Note from Fig. 1 that
the modulator
75
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