FM Bug Transmitter with a range of 20 to 50 meters
- Introduction
The goal of this project is to learn a little
more about fm transmitters and fm bug making. The ideal outcome of this project
is a very small. In order to be able to build this, we'll have to learn a lot
about amplifiers, LC oscillators, mixers, antennas and FM. The success of this
FM BUG is the use of THREE transistors in the circuit. To create a good design
each transistor should be required to perform only one task. In any type of
transmitter, there is a minimum of two tasks. One is to amplify the signal from
the microphone and the other is to provide a high frequency oscillator. The
amplified microphone signal is injected into the oscillator to modify its
frequency and thus produce a FREQUENCY MODULATED oscillator. If an aerial is
connected to the output of the oscillator, some of the energy will be radiated
into the atmosphere. To increase the output of our design, an RF amplifier is
used which is used as the buffer stage.In simple terms, an RF amplifier becomes
a LINEAR amplifier. We have opted for
sensitivity and the first transistor is employed as a pre-amplifier. This will
enable you to pick up very low-level sounds and transmit them about 20 to 50
meters.
- Parts List
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For the most part, all of the parts used in this project are discrete and passive, with the exception of the transistors, which are the only real 'active' part of the circuit. The rest are just resistors, capacitors and inductors. Below I've described the most important parts for the project.
BC547 Transitors
These are tried and true HAM radio transistors, use them for this project. We would not substitute any other general use transistors like 2n2222 or 2n3904 when building this one, simply because the BC547's work. Further more, they're not even expensive. Use them!
AWG 18 Wire
This is standard copper wire, we'll be using it to make the RF coil in the oscillator as well as for the antenna. It is a critical component because the diameter does make a difference when building parts and making coils for this project.
These are tried and true HAM radio transistors, use them for this project. We would not substitute any other general use transistors like 2n2222 or 2n3904 when building this one, simply because the BC547's work. Further more, they're not even expensive. Use them!
AWG 18 Wire
This is standard copper wire, we'll be using it to make the RF coil in the oscillator as well as for the antenna. It is a critical component because the diameter does make a difference when building parts and making coils for this project.
6p8F Capacitor
Two of these capacitors are used in the oscillator circuit. Again these are mission critical values unless you have an LC oscillator calculator in front of you, and you know what you're designing. Connecting 22pF and 10pF in parallel will yield a capacitor equivalent to 6.8pF.
PC Board & Ferric Chloride
This circuit can be tried on a breadboard. And then you really need to use either Zero PCB or a Copper PCB layout board like I'm going to use in this project. Copper Clad PC Board is relatively cheap and widely available just like ferric chloride, so don't skimp out, it's cheap and available use it!
Here is the circuit for the FM Bug. In total there are 22-26 core components that are needed for the fm transmitter to work properly. The far left side has the electret microphone and then things move electrically to the right until they hit the antenna. (C7 = 1uF electrolytic capacitor)
Amplifier Circuit
The amplifier's job in this circuit is to take the audio input and beef up the voltage/current so that when it gets transmitted it travels further. More amplification means more higher fm transmitting power and longer range.
Oscillator/Mixer Circuit
The last portion of the circuit with the 2nd BC547 is where the oscillator for the FM frequency is generated and mixed with the audio signal in order to form the frequency modulation used for FM radio. Don't worry, if all this is described a bit too quickly, the theory will be described more in depth in the next section.
Creating FM Signals
What is so unique about this type of modulation is that, as long as we use a voltage controlled oscillator for creating the carrier frequency (i.e. 107.9 MHz), we just mix the amplified audio signal with that 107.9 MHz carrier and presto the newly created signal is frequency modulated, aka FM. If you look closely at the schematic below you'll see the exact point where this mixing/modulating occurs and the new FM signal is born.
Spice is used here to simulate the amplifier circuit seen in the schematic for the FM transmitter in this project. Above I've already drawn out the amplifier portion of the circuit using Spice. The two points of interest are [1] where the red probe is, which is the filtered audio input and [2] where the purple probe is, which is the amplified signal coming out of the transistor.
FM Bug Amplifier Simulation Results
The audio input, a simluated sin wave at 5000 Hz shows up at about 50 milivolts peak to peak (red line graph). This is tiny, but about how much voltage the electret microphone puts out with the design seen in the schematic.
The purple line graph shows the amplified signal coming out of the transistor. The signal is about 1.2 volts peak to peak which is over 50x larger than the input signal. Clearly the signal has been amplified to a much larger value than it was at before.
The LC Oscillator Is A Tank Circuit
It was discovered pretty early on in electronics that if you put an inductor and capacitor in parallel with each other, they will resonate at a specific frequency. Ideally charge flows back and forth between the 'top' and 'bottom' capacitor plates generating this resonant frequency (similar to how a pendulum swings back and forth). Because charge seems to get trapped in ths circuit it got the bizarre name of 'The Tank Circuit' which is very common throughout all electronics.
The LC Oscillator Formula
If we plug in the values seen above, the oscillator should create the correlating frequency and in an ideal world that is what happens. The big problem for us is we don't live in an ideal world and things like extra resistance and stray capacitance can effect the quality and frequency of the oscillator.
This means we need to keep parts close together to avoid large resistances and add a capacitor for feedback across the Common and Emitter pins of the transistor to keep the oscillator going. This type of oscillator is extremely sensitive and changes in other parts of a circuit can have an effect on the actual resonant frequency.
An LC Oscillator Simulation
Spice is usually the tool of choice when simulating any electronic circuits. For this example I drew up a quick and dirty LC oscillator. The schematic can be seen below. This circuit would not be good for practical use because there is very little current flowing into the oscillator, but it does a good job of illustrating how the oscillator works.
The green probe looks at the voltage at that specific point over a period of time. In this case, since the expected frequency is 101.9 MHz, the period of the frequency will be around 10nS. The time range we'll look at is set to 60nS and the results can be seen below.
If you look closely you can see the period is just about 10nS as expected. But to triple check, I did an FFT on the signal which gave me the frequeny domain graph, which more obviously points out that the peak frequency generated by the circuit is around 101.9 MHz.
Buffer Stage
Two of these capacitors are used in the oscillator circuit. Again these are mission critical values unless you have an LC oscillator calculator in front of you, and you know what you're designing. Connecting 22pF and 10pF in parallel will yield a capacitor equivalent to 6.8pF.
PC Board & Ferric Chloride
This circuit can be tried on a breadboard. And then you really need to use either Zero PCB or a Copper PCB layout board like I'm going to use in this project. Copper Clad PC Board is relatively cheap and widely available just like ferric chloride, so don't skimp out, it's cheap and available use it!
- Schematic Overview
Here is the circuit for the FM Bug. In total there are 22-26 core components that are needed for the fm transmitter to work properly. The far left side has the electret microphone and then things move electrically to the right until they hit the antenna. (C7 = 1uF electrolytic capacitor)
OR you can replace 10p & 22p capacitors with 6p8 capacitors.
Also you can replace three 47k resistors with one 150kohm resistor (as they are in series and it will be more precise and stable to use 150k ohm resistor).
Schematic Specifics
Electret Microphone
Don't let this microphone fool you, it's extremely sensitive albeit a bit clunky and big. You could shop around for a smaller microphone I'm sure but for this project, a standard electret microphone does the trick. The input is pulled high with a 22k resistor and passed to the amplifier circuit.Amplifier Circuit
The amplifier's job in this circuit is to take the audio input and beef up the voltage/current so that when it gets transmitted it travels further. More amplification means more higher fm transmitting power and longer range.
Oscillator/Mixer Circuit
The last portion of the circuit with the 2nd BC547 is where the oscillator for the FM frequency is generated and mixed with the audio signal in order to form the frequency modulation used for FM radio. Don't worry, if all this is described a bit too quickly, the theory will be described more in depth in the next section.
- Theory Of Operation
Frequency Modulation
Since the FM radio band (88 to 108 MHz) will be used for FM transmitting in this project, we must first understand what the FM means, how it works and how we use it. Frequency Modulation (FM) transmits at a certain frequency, 107.9 MHz for example, and depending upon the audio signal input, the transmitted signal will have a fast frequency (for large positive voltages) or a slower frequency (for large negative voltages). The animation below best illustrates how the signal transmitted by the FM Bug will look.
Since the FM radio band (88 to 108 MHz) will be used for FM transmitting in this project, we must first understand what the FM means, how it works and how we use it. Frequency Modulation (FM) transmits at a certain frequency, 107.9 MHz for example, and depending upon the audio signal input, the transmitted signal will have a fast frequency (for large positive voltages) or a slower frequency (for large negative voltages). The animation below best illustrates how the signal transmitted by the FM Bug will look.
You can think of the 'Signal' as what comes out of a microphone when you speak into it and the 'FM signal' as what goes out of the Antenna.
The Common Emitter Amplifier
Aside from acting as on/off switches, transistors can also be used as amplifiers. Whole ranges of different transistors exist for different types of amplification. The Common-Emitter Amplifier (CE Amp for short), is probably the most common type of amplifier seen in standard electronics. The basic idea at work for amplifier used in the FM Bug is that the 1 Megaohm resistor biases the transistor in a way that when the Audio input goes into the base pin of the BC547 it is multiplied to be many times larger by the time it hits the Collector point of the transistor. Try using different resistance values other than 1 Megaohm and you'll see differences in the amplification factor.
Simulating The Simple CE Amplifier
The purple line graph shows the amplified signal coming out of the transistor. The signal is about 1.2 volts peak to peak which is over 50x larger than the input signal. Clearly the signal has been amplified to a much larger value than it was at before.
The LC Oscillator Is A Tank Circuit
It was discovered pretty early on in electronics that if you put an inductor and capacitor in parallel with each other, they will resonate at a specific frequency. Ideally charge flows back and forth between the 'top' and 'bottom' capacitor plates generating this resonant frequency (similar to how a pendulum swings back and forth). Because charge seems to get trapped in ths circuit it got the bizarre name of 'The Tank Circuit' which is very common throughout all electronics.
The LC Oscillator Formula
If we plug in the values seen above, the oscillator should create the correlating frequency and in an ideal world that is what happens. The big problem for us is we don't live in an ideal world and things like extra resistance and stray capacitance can effect the quality and frequency of the oscillator.
This means we need to keep parts close together to avoid large resistances and add a capacitor for feedback across the Common and Emitter pins of the transistor to keep the oscillator going. This type of oscillator is extremely sensitive and changes in other parts of a circuit can have an effect on the actual resonant frequency.
An LC Oscillator Simulation
Spice is usually the tool of choice when simulating any electronic circuits. For this example I drew up a quick and dirty LC oscillator. The schematic can be seen below. This circuit would not be good for practical use because there is very little current flowing into the oscillator, but it does a good job of illustrating how the oscillator works.
The green probe looks at the voltage at that specific point over a period of time. In this case, since the expected frequency is 101.9 MHz, the period of the frequency will be around 10nS. The time range we'll look at is set to 60nS and the results can be seen below.
If you look closely you can see the period is just about 10nS as expected. But to triple check, I did an FFT on the signal which gave me the frequeny domain graph, which more obviously points out that the peak frequency generated by the circuit is around 101.9 MHz.
Frequency the buffer conerts a high impedance signal into a low
impedance signal as the antenna is commonly referred to as a 50 ohm load. The interposed
buffer amplifier prevents the second circuit from loading the first circuit
unacceptably and interfering with its desired operation. In the ideal voltage
buffer in the diagram, the input resistance is infinite, the output resistance
zero (impedance of an ideal voltage source is zero). Other properties of the
ideal buffer are: perfect linearity, regardless of signal amplitudes; and
instant output response, regardless of the speed of the input signal.
If the voltage is transferred unchanged (the voltage gain Av is 1), the amplifier is a unity gain buffer; also known
as a voltage follower because the output voltage follows or tracks the input voltage. Although
the voltage gain of a voltage buffer amplifier may be (approximately) unity, it
usually provides considerable current gain and thus power gain. However, it is
commonplace to say that it has a gain of 1 (or the equivalent 0 dB),
referring to the voltage gain.
·
Making The Oscillator Coil
The only critical component in the FM BUG is
the oscillator coil. Its critical nature only means it must not be touched when
the transmitter is in operation as this will detune the circuit completely. It
is the only component which needs to be adjusted or aligned and we will cover
its winding and formation in detail .The oscillator coil is made out of tinned
copper wire and does not need any insulation. This is not normal practice but
since the coil is small and rigid, the turns are unable to touch each other and
short-out.
THE DETAILS:
The coil has 5 turns and is wound on a 3.5mm
shaft. To be more specific, it has 5 loops of wire at the top and each end
terminates at the PC board. The coil must be wound in a clock-wise direction to
fit onto the board and if you make a mistake, rewind the coil in the opposite
direction.
- CONSTRUCTION
Construction is quite straight-forward as
everything is mounted on the printed circuit board. The only point to watch is
the height of some of the components.
Positioning of the parts is not as critical
as you think as the final frequency is adjusted by squeezing the coil together
or stretching it apart.
However it is important to keep the
component leads as short as possible and the soldering neat due to the high
frequencies involved. The components must be soldered firmly to the board so that
they do not move when the transmitter is being carried. The soldering may not
affect the resulting frequency but poor layout of the components certainly
will. All the resistors must be pressed firmly against the PC board before
soldering and the three transistors must be pushed so that they are as close as
possible to the board. All the small-value capacitors are ceramic as they are
not critical in value and do not need to be high stability.
HOW THE CIRCUIT
WORKS
The circuit consists of three separate
stages. The first is an audio pre-amplifier, the second is a 90MHz oscillator
and the third is the buffer amplifier.
The first stage is very simple to explain.
It is a self-biasing common-emitter amplifier capable of amplifying minute
signals picked up by the electret microphone. It delivers these to the
oscillator stage. The amplification of the first stage is about 70 and it only
operates at audio frequencies. The 22n capacitor isolates the microphone from
the base voltage of the transistor and allows only AC signals to pass through.
The transistor is automatically biased via the 1M resistor which is fed from
the voltage appearing at the collector. The output from the transistor passes
through a 2.2u electrolytic. This value is not critical as its sole purpose is
to couple the two stages. The 47k, 1n, 470R and 22n components are not critical
either.
The critical components are the coil and 6.8pF(made
by connecting 10pF & 22pF in series) capacitor. These determine the
frequency at which the bug will transmit. In addition, the effective
capacitance of the transistor plays a deciding factor in the resulting
frequency. This stage is basically a free-running 101MHz oscillator in which
the feedback path is the 6.8pF capacitor.
When the circuit is turned on, a pulse of
electricity passes through the collector-emitter circuit and this also includes
the parallel tuned circuit made up of the oscillator coil and the 6.8pF
capacitor. This pulse of electricity is due to the transistor being turned on
via the 47k resistor in the base
circuit. Whenever energy is injected into a
tuned circuit, the energy is firstly absorbed by the capacitor. The electricity
will then flow out to the coil where it is converted to magnetic flux. The
magnetic flux will cut the turns of wire in the coil and produce current and
voltage which will be passed to the capacitor.
In theory, this current will flow back and
forth indefinitely, however in practice, there are a number of losses which
will cause the oscillations to die down fairly quickly. If a feedback circuit
is provided for the stage, the natural RESONANT frequency of the coil/capacitor
combination will be maintained. The 6.8pF (parallel to transistor) provides this
feedback path and keeps the transistor oscillating.
The 6.8pF feeds a small sample of the
voltage appearing at the collector, to the emitter and modifies the emitter
voltage. The transistor sees its base-to-emitter voltage altering in harmony with
the resonant frequency of the tuned circuit and turns the collector on and off
at the same frequency. Thus there is a degree of stability in the oscillator
frequency.
The actual frequency of the stage is
dependent upon the total capacitance of the circuit and this includes all the
other components to a minor extent. Once the basic frequency of 101MHz is set,
the variations in frequency are produced by the changes in effective
capacitance of the transistor. This occurs when its base voltage is increased and
reduced. The electret microphone picks up the sound waves which are amplified
by the first transistor and the resulting frequency is passed to the base of Q2
via the 2.2u electrolytic. This alters the gain of the transistor and changes
its internal capacitance. This junction capacitance modifies the oscillator
with a frequency equal to the sound entering the microphone thus FREQUENCY
MODULATING the circuit. A short length of antenna wire is connected to the
collector of the oscillator via a coupling
capacitor and some of the energy of the circuit will be radiated to the
surroundings. Any FM receiver will pick up this energy and decode the audio
portion of the signal.
Posts
Hey Thanks it works Awesome !!!
ReplyDeleteNice work !!
ReplyDeletehow much did it cost around ??