●●●●
USE SURFACE-MOUNT DEVICES
TO BUILD AN FM
TRANSMITTER
BY JIM STEWART
In this article, we will construct a
low-power FM transmitter that can
be received with a standard FM
radio. Along the way, we will look
at how varactor diodes work and
how to use an inductor that's in
the circuit board.
Though I've built many projects over the years, I have
to confess that I've never used surface-mount devices
(SMD). Those little parts just had me intimidated. I thought
soldering them would be a nightmare. It seems like now
all the really neat new parts coming out are SMD only,
so the time had come for me to face my fear. This FM
transmitter is my first attempt at using SMD and, as it
turned out, soldering wasn't that bad once I figured out
the trick. More on that later.
There are many designs for small FM transmitters on
the Internet. Most of what you find look something like
Figure 1. Q2 is a common-base oscillator with frequency
set by L, C3, and the base-collector junction capacitance.
RF on the base is grounded through C2. Feedback is via
C4. Modulation is achieved by applying audio to the base
of Q2 which varies its base-collector capacitance.
There are problems with such a design. First, you
need an audio amplifier (Q1) to get enough modulation.
Second, the antenna (the load) is attached directly to the
collector (a high impedance source). Third, L usually must
be wound by hand and adjusted by stretching. Fourth, C3
is often a "gimmick" capacitor: two pieces of insulated
wire twisted together. It all adds up to a weak signal that
tends to drift in frequency. And if things go really wrong,
you get "motor-boating" (or squegging) as Q2 goes in and
out of oscillation at an audio rate.
In contrast, the design in this article uses a common-collector oscillator which eliminates the possibility of
motor-boating. It uses a varactor diode for tuning and
modulation, giving great sensitivity without an audio
amplifier. It uses an emitter-follower RF power amplifier
to drive the antenna. It uses an inductor etched into the
printed circut board (PCB). It puts out a strong signal with
little drift.
How It Works
The Circuit
Figure 2 shows the schematic of the transmitter. It
consists of two stages: an oscillator and an output
amplifier. Modulation is from an electret microphone.
■ FIGURE 1
Oscillator Stage
Transistor Q1 is configured as a Colpitts oscillator.
The frequency of oscillation is determined by the parallel
resonant circuit (tank circuit) formed by inductor L,
40
September 2008
