Introduction
I have had a few inquiries about a low power FM transmitter, and this article
should satisfy those who might want to build one. It is designed to use an
input from another sound source (such as a guitar or microphone), and transmits
on the commercial FM band - it is actually quite powerful, so make sure that you
select an unused position on the dial!
The FM band is 88 to 108MHz, but is getting fairly crowded nearly everywhere,
but you should be able to find a blank spot on the dial somewhere.
NOTE: A few people have had trouble with this
circuit. The biggest problem is not knowing if it is even oscillating,
since the frequency is outside the range of most simple oscilloscopes. See
Project 74 for a simple RF probe that will (or should) tell you that you have a
useful signal at the antenna. If so, then you know it oscillates, and just
have to find out at what frequency. This may require the use of an RF
frequency counter if you just cannot locate the FM band.
Description
The circuit of the transmitter is shown in Figure 1, and as you can see it is
quite simple. The first stage is the oscillator, and is tuned with the
variable capacitor. Select an unused frequency, and carefully adjust C3
until the background noise stops (you have to disable the FM receiver's mute
circuit to hear this).
Figure 1 - Low Power FM Transmitter
Because the trimmer cap is very sensitive, make the final frequency
adjustment on the receiver. When assembling the circuit, make sure the
rotor of C3 is connected to the +9V supply. This ensures that there will
be minimal frequency disturbance when the screwdriver touches the adjustment
shaft. You can use a small piece of non copper-clad circuit board to make
a screwdriver - this will not alter the frequency.
Note: A reader has suggested that the
frequency stability is improved considerably by adding a capacitor from the
base of Q1 to ground. This ensures that the transistor operates in true
common base at RF. A value of 1nF (ceramic) as shown is suitable, and
will also limit the HF response to 15 kHz - this is a benefit for a simple
circuit like this.
Capacitors All capacitors must
be ceramic (with the exception of C1, see below), with C2 and C6 preferably
being N750 (Negative temperature coefficient, 750 parts per million per degree
Celcius). The others should be NPO types, since temperature correction is
not needed (nor is it desirable). If you cannot get N750 caps, don't worry
too much, the frequency stability of the circuit is not that good anyway.
How It Works
Q1 is the oscillator, and is a conventional Colpitts
design. L1 and C3 (in parallel with C2) tunes the circuit to the desired
frequency, and the output (from the emitter of Q1) is fed to the buffer and
amplifier Q2. This isolates the antenna from the oscillator giving much
better frequency stability, as well as providing considerable extra gain. L2 and
C6 form a tuned collector load, and C7 helps to further isolate the circuit from
the antenna, as well as preventing any possibility of short circuits should the
antenna contact the grounded metal case that would normally be used for the
complete transmitter.
The audio signal applied to the base of Q1 causes the frequency to change, as
the transistor's collector current is modulated by the audio. This
provides the frequency modulation (FM) that can be received on any standard FM
band receiver. The audio input must be kept to a maximum of about 100mV,
although this will vary somewhat from one unit to the next.
With the value shown for C1, this limits the lower frequency response to
about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go
lower than this, then use a 1uF cap instead, which will allow a response down to
at least 15Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be
used, either bipolar or polarised. If polarised, the positive terminal
must connect to the 10k resistor.
Inductors
The inductors are nominally 10 turns (actually 9.5) of
1mm diameter enamelled copper wire. They are close wound on a 3mm diameter
former, which is removed after the coils are wound. Carefully scrape away
the enamel where the coil ends will go through the board - all the enamel must
be removed to ensure good contact. Figure 2 shows a detail drawing of a
coil. The coils should be mounted about 2mm above the board.
For those still stuck in the dark ages with imperial measurements (grin), 1mm
is about 0.04" (0.0394") or 5/127 inch (chuckle) - you will have to work out
what gauge that is, depending on which wire gauge system you use (there are
several). You can see the benefits of metric already, can't you? To
work out the other measurements, 1" = 25.4mm
NOTE: The inductors are critical, and must be wound exactly
as described, or the frequency will be wrong.
Figure 2 - Detail Of L1 And L2
The nominal (and very approximate) inductance for the coils is about
130nH. This is calculated according to the formula ...
L = N2 *
r2 / (228r + 254l)
... where L = inductance in
microhenries (uH), N = number of turns, r = average coil radius, and l = coil
length. All dimensions are in millimetres.
Pre-Emphasis
It is normal with FM transmission that "pre-emphasis" is used, and there is a
corresponding amount of de-emphasis at the receiver. There are two
standards (of course) - most of the world uses a 50us time constant, and the US
uses 75us. These time constants represent a frequency of 3183Hz and 2122Hz
respectively. This is the 3dB point of a simple filter that boosts the
high frequencies on transmission and cuts the same highs again on reception,
restoring the frequency response to normal, and reducing noise.
The simple transmitter above does not have this built in, so it can be added
to the microphone preamp or line stage buffer circuit. These are both
shown in Figure 3, and are of much higher quality than the standard offerings in
most other designs.
Figure 3 - Mic And Line Preamps
Rather than a simple single transistor amp, using a TL071 opamp gives much
better distortion figures, and a more predictable output impedance to the
transmitter. If you want to use a dynamic microphone, leave out R1 (5.6k)
since this is only needed to power an electret mic insert. The gain
control (for either circuit) can be an internal preset, or a normal pot to allow
adjustment to the maximum level without distortion with different signal
sources. The 100nF bypass capacitors must be ceramic types, because of the
frequency.
The mic preamp has a maximum gain of 22, giving a microphone sensitivity of
around 5mV. The line preamp has a gain of unity, so maximum input
sensitivity is 100mV.
Select the appropriate capacitor value for pre-emphasis as shown in Figure 3
depending on where you live. The pre-emphasis is not especially accurate,
but will be quite good enough for the sorts of uses that a low power FM
transmitter will be put to. Needless to say, this does not include
"bugging" of rooms, as this is illegal almost everywhere.
I would advise that the preamp be in its own small sub-enclosure to prevent
RF from entering the opamp input. This does not need to be anything fancy,
and you could even just wrap some insulation around the preamp then just wrap
the entire preamp unit in aluminium foil. Remember to make a good earth
connection to the foil, or the shielding will serve no purpose.
