The kit is constructed on a single-sided printed circuit board (PCB).ﾠ It has a silk screen overlay on top to aid construction.ﾠ On the bottom there is a solder mask to help in soldering.
The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz.ﾠ Audio picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor.ﾠ Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (L1 coil and the red trimcap) by varying the junction capacitance of the transistor.ﾠ Junction capacitance is a function of the potential difference applied to the base of the transistor T2.ﾠ The tank circuit is connected in a Hartley oscillator circuit.ﾠ The final stage built around T3 amplifies the output RF signal.
Let us look at the individual blocks of the circuit more closely:
The electret microphone: an electret is a permanently charged dielectric.ﾠ It is made by heating a ceramic material, placing it in a magnetic field and then allowing it to cool while still in the magnetic field.ﾠ It is the electrostatic equivalent of a permanent magnet.ﾠ In the elctret microphone a slice of this material is used as part of the dielectric of a capacitor in which the diaphragm of the microphone forms one plate. ﾠ Sound pressure moves one of its plates.ﾠ The movement of the plate changes the capacitance.ﾠ The electret capacitor is connected to an FET amplifier.ﾠ These microphones are small, have excellent sensitivity, a wide frequency response and a very low cost.
First amplification stage: this is a standard self-biasing common emitter amplifier.ﾠ The 22n capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current signals to pass.
Oscillator stage: every transmitter needs an oscillator to generate the RF carrier waves.ﾠ The tank circuit, the transistor and the feedback capacitor are the oscillator circuit here.ﾠ An input signal is not needed to sustain the oscillation.ﾠ The feedback signal makes the base-emitter current of the transistor vary at the resonant frequency.ﾠ This causes the emitter-collector current to vary at the same frequency.ﾠ This signal fed to the aerial and radiated as radio waves.
The name "tank" circuit comes from the ability of the LC circuit to store energy for oscillations.ﾠ In a pure LC circuit (one with no resistance) energy cannot be lostﾠ(in an AC network only the resistive elements will dissipate electrical energy).
The purely reactive elements, the C and the L, just store energy to be returned to the system later.ﾠ Note that the tank circuit does not oscillate just by having a DC potential put across it.ﾠ Positive feedback: must be provided.
The slots inside the trim cap are shaped like the head of an arrow.ﾠ The maximum capacitance value is when the arrow is in pointed to the 12 o'clock position.ﾠ A 180 turn brings the trimcap value to its minimum rated value. ﾠ With experimentation you will be able to build up a table of total capacitance value (remember to add in the 10pF) to FM frequency.ﾠ You can also change the frequency by altering the space between the coils of L1.
The 10pF ceramic capacitor is parallel to the red trim cap will enable you to tune the Tx in the 98 MHz to 105 MHz range of the commercial FM band.ﾠ If you use a higher value (for example, 27pF) you will move the frequency down towards the other end of the FM band.ﾠ
This end gereraly has more commercial stations in it.
Final Amplification Stage: this RF stage adds amplification to the RF signal.ﾠ It needs an RF transistor to do this efficiently.ﾠ We use a ZTX320 or 3563.ﾠ L2 (an RFC - radio frequency choke) and the 10p capacitor in parallel with it are disigned to reduce harmonics from the circuit.
A small (10pF) coupling capacitor on the aerial is optional to minimise the effect of the aerial capacitance on the final stage LC circuitﾠ(we have not used one in this circuit).
Greater range from the transmitter can be obtained by replacing the half-wave antenna (the length of wire about 160cm long) with a dipole antenna.ﾠ This is basically two wires attached to two points in the circuit which are oscillating 180 out of phase with each other.ﾠ Two such points are the antenna point and the positive rail (the + 9V track.)ﾠ You can experiment by cutting the antenna wire in half, leaving half soldered into the antenna point and soldering the otherhalf to the +9V pad.ﾠ Point the two wires in opposite directions.
Output power is also increased by using a higher operating voltage.ﾠ
9V is better than 6V.ﾠ The maximum operating voltage for this kit is determined by transistor T3.ﾠ This is 15V but if you try this then the values of some resistances will have to change.ﾠ You can experiment with this.