FM Band Monaural Transmitter

FM Band Monaural Transmitter
FM Band Monaural Transmitter
IN
OUT
LM78L12
or similar
+
+
>15 VDC
22 pF trimmer
Use a multi-turn
type for easiest
frequency adjust.
+
10 uF
0.01 uF
near coil
1 uF
-
5T, 1/4 inch dia.
or 0.1uh molded
choke.
5k
100k
1k pot
Audio Input
(low-Z)
22 pF trimmer
1 pF to 2.2 pF
22k
KV2201
1 uF
33Ω
Delete 33 ohm
for high-Z
+
U310
2N4401
10k
100Ω
The dot indicates the "hot" end of the trimmer capacitor.
This end is usually the stator since the rotor will touch
the trimmer tool.
Many varactor diodes
will work fine. Adjust
the trimmer for good
modulation.
Most other FETs will work,
especially RF types like the
2N4416 or switches such as
the 2N4858.
If a 2.2 pF capacitor is used
10 pF then use a short antenna to
prevent excessive loading
(under 1 foot).
33 uH
150 Ω
A high quality monaura FM transmitter is quite useful in a variety of applications. The
audio from a TV or entertainment system may be transmitted for remote or portable
listening and for wireless earphone listening. The audio from an AM receiver placed near
a window can be retransmitted within a metal building where AM reception is not
possible. Two transmitters will convert two portable FM receivers into cordless stereo
speakers for outdoor events. A keyboard may be played through the stereo without wires
simply by tuning in the transmitter. A child wil love an operational, miniature FM station
complete with transmit "on the air" light. Simply connect the stereo's monaural output to
the transmitter and transmit CDs, tapes, microphones, and other audio sources. Connect
the transmitter to the computer's sound card and have big speaker sound. Even silence
may be transmitted. Tune the transmitter to your favorite station and when the annoying
commercial starts, simply flip on the power and... silence (or the audio of your choice).
Ideas:
1) Use ground plane construction. Solder the ground connections directly to the copper circuit board material and keep the RF
leads short.
2) Add a 680 ohm resistor in series with an LED to make an "on the air" light.
3) Leave the transmitter on when not in use to eliminate warm-up drift. Use a jack for the antenna so that it may be unplugged
when not transmitting.
4) A lower voltage regulator may be used to regulate a lower voltage molded supply.
5) Bypass the 100 ohm resistor in the 2N4401's emitter with a 470 uF capacitor for greatly increased audio gain. Also remove
the 33 ohm resistor and use a 100k pot for increased sensitivity. The entire audio stage may be replaced by an op-amp biased for
V/2 volts on the output.
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