FM TRANSMITTERS

The first group of circuits we will discuss are FM TRANSMITTERS. They can be called SPY TRANSMITTERS, FM BUGS, or a number of other interesting names. They all do the same thing. They transmit on the FM band in the range 88MHz to 108MHz. Most of them can be adjusted to transmit above or below the band and you need a radio that will pick up these frequencies, to detect them. Since the FM band is almost entirely filled with radio stations, we will be providing details on how to adjust a radio so it will scan EITHER above the band or below the band. Most radios can only be adjusted 10MHz above or below but this will be enough to provide a "blank" space for your transmitter. FM transmission provides perfect quality and when one of these transmitters is used in a house and received on a good quality radio, you cannot tell if the person is actually talking in the next room or via an FM link, through a radio. This means all the FM bugs have the same perfect audio, but some circuits will detect fainter sounds and others will transmit further. Some circuits can be handled without drifting off-frequency and others are designed to be very small or fit on top of a 9v battery. By building these circuits you will learn an enormous amount about high frequency, audio and getting the maximum output with the least current. This is the main aim of this article. It will add a number of "building blocks" to your understanding of electronics. Before I start, there are two things that particularly annoy me. The first is a circuit diagram with C1, R1 etc and a parts list identifying the values. Circuit diagrams like this are obviously drawn by a non-electronics person. The whole concept of looking at a circuit diagram and seeing the values gives the reader an indication of how each section will work. A section may be operating very lightly with high value components or it may be working very hard with low value components. The whole idea of providing a circuit diagram with marked components is to give the reader an immediate understanding of how the circuit is operating. The second thing that annoys me is the labelling of parts on a PC board as R1, C1 etc. Again, the board has been designed by a non-technical person. Why design a board without component values? Do they think the values will change? How can you assemble a board without referring to a circuit diagram? The whole purpose of well-designed PC board is to build it without referring to any other data. Keep this in mind when designed your own boards. Also, don't name your boards "A51/834-2." Give then a name you can remember or one that refers to the application it will perform. Let's start:

1 TRANSISTOR CIRCUITS There are a number of 1 transistor FM transistors on the market in kit-form and already assembled. These circuits are interesting to look at but do not really perform very well. 1. They do not have a good transmitting range. 2. They do not detect low-level sound, and 3. They do not operate very well on 1.5v. No transmitter can be expected to operate very well on 1.5v. If you want to use a single cell, use a lithium cell as it produces 3v. 4. Some have a coil etched on the PC board. No FM transmitter will perform very well with a coil etched on the board.

Why use 1.5v?????

Transistors do not operate very well below 0.9v and the collector load resistor needs a small voltage so it can perform its task (the same applies to an emitter resistor) and thus the lowest voltage for a circuit is 1.5v. If only a single cells is used, there is not allowance for a voltage-drop as the cell becomes depleted. Always use 3v as the lowest supply voltage.

THE SIMPLEST CIRCUIT

The following circuit is the simplest FM circuit you can get. It has no microphone but the coil is so MICROPHONIC that it will pick up noises in the room via vibrations on a table. The circuit does not have any section that determines the frequency. In the next circuit and all those that follow, the section that determines the frequency of operation is called the TUNED CIRCUIT or

TANK CIRCUIT and consists of a coil and capacitor. The transistor and components surrounding the tuned circuit simply keep the tuned circuit operating at its RESONANT FREQUENCY. This circuit does not have this feature. The transistor turns on via the 47k and this puts a pulse through the 15 turn winding. The magnetic flux from this winding passes through the 6 turn winding and into the base of the transistor via the 22n capacitor. This pulse is amplified by the transistor and the circuit is kept active. The frequency is determined by the 6 turn coil. By moving the turns together, the frequency will decrease. The circuit transmits at 90MHz. It has a very poor range and consumes 16mA.

THE SIMPLEST BUG

The components soldered to the 2 cells

Rear view of the simplest bug

After making a transmitter, you will want to know if it is transmitting. In the case above, the circuit will only produce a carrier and this will be heard on the radio as a "quiet spot." Rather than chasing up and down the dial, Talking Electronics has produced a piece of test equipment to let you know the bug is transmitting and the approx frequency of transmission. It is called FIELD STRENGTH METER MkII. The photo below shows the Field Strength Meter near the bug. The plastic knob on the trimmer allows adjustment without affecting the detecting circuit. Simply turn the knob (with the two antennas near each other) and the 3 LEDs on the project will illuminate.

Field Strength Meter and Bug

So far we have seen an unstable circuit in action. Placing a finger near the bug will change the frequency. This is totally unsuitable.

A GOOD ONE TRANSISTOR CIRCUIT

The next circuit uses a TUNED CIRCUIT or TANK CIRCUIT to create the operating frequency. This is clearly shown in the diagram. For best performance the circuit should be built on a PC board with all components fitted close to each other. The photo below shows the circuit using a coil etched on the board. This type of coil is totally unsuitable. It does not have a high "Q" and the range is very poor. The board cannot be touched as the capacitance of your body causes the circuit to drift. A wound coil will improve the stability considerably. See photos below for the details of a wound coil.

A good one transistor circuit

Do not use an etched coil

A PNP DESIGN

Before we go to an improved design, here is an unusual circuit using a PNP (BC 557) transistor. Firstly, PNP transistors do not work as well as NPN transistors. I would reverse the 4k7 and electret mic as the voltage between base and 0v rail is very small and the 4k7 is not biasing the transistor. The range will be 50 to 100 metres and the current is about 3mA.

Simple 1 transistor FM Transmitter

The 22n is not shown. This is a later addition.

AN IMPROVED DESIGN This design uses a "slug tuned coil" to set the frequency. This means the slug can be screwed in and out of the coil. This type of circuit does not offer any improvement in stability over the previous circuit. (In later circuits we will show how to improve stability. The main way to improve stability is to add a "buffer" stage. This separates the oscillator stage from the output.) The antenna is connected to the collector of the transistor and this "loads" the circuit and will cause drift if the bug is touched. The range of this circuit is about 200 metres and current consumption is about 7mA. The microphone has been separated from the oscillator and this allows the gain of the microphone to be set via the 22k resistor. Lowering the resistor will make the microphone more sensitive. This circuit is the best you can get with one transistor.

MORE STABILITY

If you want more stability, the antenna can be tapped off the top of the tank circuit. This actually does two things. It keeps the antenna away from the highly active collector and turns the coil into an auto-transformer where the energy from the 8 turns is passed to a single turn. This effectively increases the current into the antenna. And that is exactly what we want. The range is not as far but the stability is better. The frequency will not drift as much when the bug is held. As the tap is taken towards the collector, the output increase but the stability deceases.

LOW-VALUE EMITTER RESISTOR

The next circuit has been picked out for its low emitter resistor on the oscillator. This resistor does not have to be a very low value as the transistor is working at its maximum potential, due to the high frequency and a low emitter resistor will simply consume more current without improving the output.

The emitter resistor is too low

Two photos of the bug

STEREO TO MONO To combine two channels to a mono output, the following circuit can be used:

2 TRANSISTOR CIRCUITS The next progressive step is to add a transistor to give the electret microphone more sensitivity. The electret microphone contains a Field Effect Transistor and you can consider it to be a stage of amplification. That's why the electret microphone has a very good output. A further stage of amplification will give the bug extremely good sensitivity and you will be able to pick up the sound of a pin dropping on a wooden floor. Many of the 1 transistor circuits over-drive the microphone and this will create a noise like bacon and eggs frying. The microphone's used by Talking Electronics require a load resistor of 47k for a 6v supply and 22k for a 3v supply. The voltage across the microphone is about 300mV to 600mV. Only a very simple self-biasing common-emitter stage is needed. This will give a gain of approx 70 for a 3v supply. The next circuit shows this audio amplifier, added to the previous transmitter circuit. This circuit is the best design using 2 transistors on a 3v supply. The circuit takes about 7mA and produces a range of about 200 - 400metres.

2 Transistor FM Transmitter

Five points to note in the circuit above: 1. The tank circuit has a fixed 39p and is adjusted by a 2-10p trimmer. The coil is stretched to get the desired position on the band and the trimmer fine tunes the location. 2. The microphone coupling is a 22n ceramic. This value is sufficient for the location as its capacitive reactance at 3-4kHz is about 4k and the input to the audio stage is fairly high, as noted by the 1M on the base. 3. The 1u between the audio stage and oscillator is needed as the base has a lower impedance as noted by the 47k base-bias resistor. 4. The 22n across the power rails is needed to keep the rails "tight." Its impedance at 100MHz is much less than one ohm and it improves the performance of the oscillator enormously. 5. The coil in the tank circuit is 5 turns of enameled wire with air core. This is much better than a coil made on a PC board and is cheaper than a bought inductor. The secret to long range is high activity in the oscillator stage. The tank circuit (made up of the coil and capacitors across it) will produce a voltage higher than the supply voltage due to the effect known as "collapsing magnetic field" and this occurs when the coil collapses and passes its reverse voltage to the capacitor. The antenna is also connected to this point and it receives this high waveform and passes the energy to the atmosphere as electromagnetic radiation.

When the circuit is tightly constructed on a PC board, the frequency will not drift very much if the antenna is touched. This is due to the circuit design and layout as well as the use of large-value capacitors in the oscillator. If low value capacitors are used, the effect of your body has a greater effect on changing the frequency.

THE VOYAGER The only way to get a higher output from two transistors is to increase the supply voltage. The following circuit is available from Talking Electronics as a surface-mount kit, with some components through-hole. The project is called THE VOYAGER.

Voyager on a 9v Battery

All the elements of good design have been achieved in this project. The circuit has a slightly higher output than the 3v circuit above, but most of the voltage is lost across the emitter resistor and not converted to RF. The main advantage of this design is being able to connect to a 9v battery. In a technical sense, about half the energy is wasted as the stages actually require about 4v - 5v for maximum output. The Voyager has been copied by many kit-makers but none has surpassed its performance. Here is a "knock-off" of our older design. It is mounted flat on a 9v battery:

Here are 2 more two-transistor circuits using a 9v supply. We have also included the technical limitations of the circuits:

DAVID'S DESIGN:

Faults with this circuit: 1. Very low microphone load resistor. 2. 4u7 not needed from microphone. 22n is sufficient. 3. 3p3 is very low for BC 547. May need to be higher. - 10p preferred. 4. Bridge biasing of audio stage is not needed. Simple biasing is adequate. 5. Base biasing of oscillator is very wasteful in current. 6. 22n is very high for base bias of oscillator - restricts incoming audio.

A BIRD'S NEST DESIGN

Faults with this circuit: 1. Load resistor for microphone is very low - should be 47k 2.10u on output of microphone is not needed - 22n is sufficient. 3. Current through audio state is very high. Load resistor should be 47k 4. Base biasing of oscillator very wasteful. 5. Load resistor for oscillator (emitter resistor) is very low 6. Feedback capacitor for oscillator should be 10p. 7. No ceramic in tank circuit. Adding a ceramic makes it easier to adjust the trimmer capacitor. 8. No capacitor across the battery.

See the layout below:

Birds-nest of above circuit showing how tight the circuit can be made.

There is nothing wrong with a bird's nest. It is very easy to experiment with components and wiring that can be seen and changed without having to

work on a PC board. The only problem with the bird's nest above is the lack of an earth plane. When you have an earth plane, the signal can push against the large mass of an earth rail (or battery) so that it

can push the signal out the antenna.

The circuit on proto-typing board - a quick way to build a project.

The oscillator components must be kept near each other, otherwise the circuit will not oscillate!

The circuits we have discussed so far demonstrate the maximum output that can be achieved from a 3v to 9v supply and the maximum sensitivity from the microphone. The next stage in the development of a better circuit involves a BUFFER STAGE so the oscillator is not driving the antenna. This will give the circuit more stability and more output. The simplest buffer is shown in the following HAND-HELD MICROPHONE CIRCUIT:

HAND-HELD MICROPHONE

The following circuit is suitable for a hand-held microphone. It does not have an audio stage but that makes it ideal as a microphone, to prevent feedback. The output has a buffer stage to keep the oscillator away from the antenna. This gives the project the greatest amount of stability.

To get good audio amplification, and a stable oscillator and the ability to handle the circuit without it drifting, we need 3 transistors. These circuits are on the following page.

P1 P2 P3

3-TRANSISTOR CIRCUITS

Three transistors will give a wide range of designs. Here are 6 circuits showing how to connect a buffer stage to an oscillator. But first we need to show the buffer can be connected to the oscillator stage via point A or point B. Point A has a higher amplitude but since this point is a high-impedance point, any energy taken from this point will affect the amplitude of the oscillator. Point B a low-impedance point, but has a much lower amplitude

Connecting a buffer to point A or point B

Thus we have a decision to make. I prefer the collector take-off point as it has a larger signal and this signal can be passed to the buffer stage via a small capacitor to fully drive the buffer transistor. The capacitor will actually convert a large signal with low current into a smaller signal with higher current. This is one of amazing things a capacitor will do.

You may think point A is a low impedance point as it is just a fraction of an ohm away from the positive rail. But the inductor (coil) is creating a voltage and waveform at point A and if any load is applied at this point, the waveform will decrease, because the inductor does not have much "strength" to produce the waveform. To understand this more clearly, you need to know how the stage works so you can see how delicate the circuit is. When the power is applied, the circuit start to operate due to the 47k bias resistor on the base. The next point to note is the base is held rigid by the 1n on the base. This capacitor has an impedance of less than one ohm at 100MHz and you can consider the 1n as a 2v battery wit an impedance of 1 ohm or a one ohm resistor sitting on top of a 2v battery. In any case the base is held very rigid by the 1n. Now we come to understanding how an NPN transistor "turns on."

It can be turned on in two ways. The emitter can be held rigid and the base can be raised to 0.6v and if the voltage is raised slightly more and the base is fed with current, the transistor will conduct and current will flow in the collector-emitter circuit. This is the case with the transistor in the audio stage. The emitter is held rigid and the base is fed with current, once the base is above 0.65v. The other way to turn on an NPN transistor is to hold the base firm and lower the emitter. Once the emitter is lower than the base by 0.65v, the transistor turns on and if the base is lowered slightly more, the transistor turns on more. This may be difficult to visualise, but it is occurring in the oscillator stage. Let's advance a few cycles and see what is happening. The transistor turns on and the collector pushed the top of the 10p capacitor towards the emitter. The energy in the capacitor gets converted to a small voltage and larger current. We said before, that a capacitor can do this. The small voltage pushes the emitted lower and this turns on the transistor. More current flows though the 470R and this has the effect of turning off the transistor. The two actions fight against each other and the capacitor wins. The end result is the transistor is turned on and a little bit of energy is pumped into the capacitor in the tuned circuit. At this instant, the coil does not get energy from the transistor because a coil resists any quick flow of current into it and it acts like a very large resistor. The energy in the 10p is now spent and the transistor turns off slightly. The energy from the capacitor passes to the coil and when the capacitor can no-longer keep the flux in the coil increasing, there comes a point where the flux starts to collapse. This flux produces a voltage that is larger than before and is opposite to the previous voltage. This is one of the amazing things of a coil and capacitor in parallel. The voltage at the lower end of the coil-capacitor combination is higher than the supply and this raises the top plate of the 10p capacitor. This rises the voltage on the emitter and turns the transistor off completely. This allows the tank circuit to produce its amazing HIGH VOLTAGE without the transistor loading the circuit. But he coil/capacitor is a very delicate arrangement and it is producing the high voltage at very low current as it is converting the original low voltage small current into high voltage very low current. If we put a load on the circuit at point "A" we will reduce the voltage and may freeze the circuit. The following 9 circuits show different ways to "tap-off" the waveform and amplify it in a stage called a BUFFER. The BUFFER converts a HIGH IMPEDANCE signal into a LOW IMPEDANCE signal as the antenna is commonly referred to as a 50 OHM LOAD.

The simplest buffer is shown the following circuit. It is a common emitter stage with a resistive load.

1. STABILITY

BC547 is not suitable as a buffer

A BC 547 transistor is not very good at amplifying at 100MHz. The circuit above does not give a greater range than the 2-transistor version, but the stability is much greater. The antenna can be touched without the bug drifting off frequency.

2. INCREASED RANGE To increase the range, the output must be increased. This can be done by using an RF transistor and adding an inductor. This effectively converts more of the current taken by the circuit into RF output. The output is classified as an untuned circuit.

Use an RF transistor for the Buffer

3. MORE RANGE More output can be obtained by increasing the voltage and adding a capacitor to the output to tune the buffer stage. The 5-30p must be adjusted each time the frequency of the bug is changed. This is best done with a field strength meter. See Talking Electronics Field Strength Meter project.

A tuned output stage delivers more output

The 2N3563 is capable of passing 15mA in the buffer stage and about 30% is delivered as RF. This makes the transmitter capable of delivering about 22mW.

4. DIFFERENT COUPLING We have already mentioned the fact that a capacitor can convert a large waveform with low current into a small waveform with large current. The following circuit taps off the inductor at a point of low amplitude to put the least load on the tank circuit. The coupling capacitor has been increased to transfer enough energy at low amplitude. This coupling has exactly the same result as shown in circuit 3. Circuit 3 is preferred as it is easier to connect to the collector than tap the inductor.

Tapping the oscillator tuned circuit

5. A PNP BUFFER A PNP transistor can be used in the buffer stage, but as we said before the BC557 is not as good as an NPN transistor, when operating at high frequencies.

A PNP in the buffer is not a good performer

6. WASTED POWER The following circuit (from the web) takes 30mA. This is wasted current. As we said before, any voltage above 4.5v is excess and any current above 12mA for this type of circuit is excess. A BC 557 cannot deal with any more than 5mA collector-emitter current. Any more than 5mA is wasted. That's why you need an RF transistor in the output.

The buffer is taking excess current

7. EMITTER TAP The following circuit taps the emitter of the oscillator stage. We have already explained the collector or the emitter can be tapped and produce about the same results.

Tapping the emitter of the oscillator transistor

8. CLASS "C" OUTPUT The following circuit uses no biasing on the output transistor. It gets all the energy to activate the base from the oscillator stage. While this is possible, the amount of energy needed is very large and the oscillator cannot provide enough energy to fully drive the output stage.

Class "C" output

9. ADJUSTABLE OSCILLATOR COIL To make a circuit more compact or cheaper (of if you don't have a trimmer), the oscillator coil can be adjusted by stretching or compressing. When the coil is stretched, the frequency increases. The buffer circuit has to be adjusted too, to get the greatest output.

Adjustable oscillator coil (no trimmer capacitor)

10. POOR DESIGN Here is a circuit with poor design. It goes against 6 of the things we have mentioned above. The first poor design is the low value resistance for the microphone. For 9v, the microphone load resistor should be 47k. If a low value is used, the microphone will will over-amplify and create a background noise similar to bacon and eggs frying. The 100k separating the microphone from the base of the audio transistor is not needed. If the microphone is operated at its correct level of current, this resistor will not be needed. The BC 547 is self biased via the 1k and 100k, so the 22k resistor on the base is not needed. The 100R on the emitter of the BC338 is a very low value for 9v supply. The coil is on the PC board has a very low "Q" and the current taken by the circuit is excessive when the emitter resistor is 100R. The 22p in series with 2-30p gives a value about 2p to 12p and this is very small for this tank circuit. Series capacitors make it very difficult to adjust the frequency as the trimmer is having a lot of effect on changing the frequency when it is in series with another capacitor. The final unusual feature is the 10u and 100u electrolytics. We have already mentioned that electrolytics do not have any effect with frequencies around 100MHz. It seems the designer had difficulties with audio instability, due to the low value of resistance on the microphone and tried to fix the problem with electrolytics. The circuit has 4 unnecessary components and if you are going into manufacturing, with will be a costly mistake. This project is to be avoided if you want a good range with low current consumption. Some of our other circuits are a better choice. The photo below shows the assembled circuit.

This covers all the possible combinations for the greatest output with three transistors using a 3v to 9v supply. If you want to improve any of the circuits we have covered, here are some helpful tips: It's handy to know the effective reactance (resistance) of a capacitor at the operating frequency of the circuit. If we assume 100MHz, the resistance is as follows:

Capacitance reactance

@100MHz reactance @2kHz

5.6p 300R

10p 166R

22p 75R

47p 35R

1n 1R6

22n much less

than 1R 3k7

1u ----- 82R

Now we come to understanding what the values mean. It depends where the capacitor is placed. A 22n across the power rails will be like a small battery equal to the voltage of the supply, but with an internal resistance of less than one ohm. When a battery has a low internal impedance, a high current can be taken without the voltage dropping. You may not think the oscillator circuit takes a high current but if the average is 10mA, there will be times when the circuit requires 20mA, and times when it needs 1mA. If the voltage dips when the circuit is trying to charge a capacitor, for example, the capacitor will not get charges to its maximum. This is what happens with the circuits above. AS soon as you put a 22n across the battery, the output increase a small amount. Not only does the output increase, but the increase stays throughout the life of the battery, especially when it is getting flat. So the 22n across the battery is very important. A ceramic capacitor is able to supply this tiny amount of charge very quickly and this is needed as the circuit is working at 100,000,000 times per second. An electrolytic is not able to supply a tiny amount of charge at this fast rate and so an electrolytic is not suitable for the supply "decoupler." A decoupler is the name given to any capacitor that is placed across the supply rails to suppress spikes or prevent the effects of one stage from interfering with another stage. It "decouples" or "separates." When a capacitor is used to "couple" one stage to the next, such as the 22n between the microphone and base of the audio amplifier, the capacitor has a certain resistance at the frequency of the signal and since this is audio, it has an effective resistance of about 4k. If you put a 4k resistor in place of the 22n, you can see any signal produced by the microphone is only a few kilo ohms away from the base of the audio transistor. The audio transistor has an input impedance of about 4k and thus the two resistances can be seen as joined together in series with the input of the transistor at their middle. They form a voltage divider in which 50% of the signal produced by the microphone is delivered to the transistor. This is a very simple way to see the situation, so that if the 22n is replaced by a 1n, very little of the signal produced by the microphone will be delivered to the transistor. But if the 22n is replaced by a 1u, abut 95% of the signal will be delivered. That's a choice you have to make. Experiment with the two values and see if the improvement is noticeable. When a capacitor is used to stabilize a voltage in a building block, such as the 1n on the base of the oscillator, it is acting just like the 22n across the supply and it appears as tiny battery with a voltage of about 2v and a resistance of about 2 ohms. This type of battery will deliver 1 amp, so you can see the 1n will keep the base very stable. The 10p to 47p coupling the oscillator to the output stage, is equivalent to a very low resistor so nearly all the energy of the oscillator is being passed to the output stage. This is only a very simple way to look at the operation of each capacitor but it gives an idea of why each value has been chosen. It's a pity the designer of circuit #10 did not read these notes before trying to design a kit for the electronics market.

GOING FURTHER The next stage to improve the output, matches the impedance of the output stage to the impedance of the antenna. The impedance of the output stage is about 1k to 5k, and the impedance of the antenna is

about 50 ohms. This creates an enormous matching problem but one effective way is with an RF transformer. An RF transformer is simply a transformer that operates at high frequency. It can be air cored or ferrite cored. The type of ferrite needed for 100MHz is F28. The following circuit uses a small ferrite slug 2.6mm dia x 6mm long, F28 material. A slug is the screwed rod that screws into a coil and is adjusted to change the matching of the windings or the frequency of the coil or transformer. To create an output transformer for circuit 6 above, wind 11 turns onto the slug and 4 turns over the 11 turns. The ferrite core will do two things. Firstly it provides a high amount of energy to pass from the primary winding to the antenna. and secondly it will prevent harmonics passing to the antenna. The only way to prove the effectiveness of the transformer is with a field test and the range increased nearly 100%, over the tuned output design in circuit 9.

Matching the output to the antenna via a transformer