This transmitter circuit has been built and tested and it has a range of a few feet if no antenna is connected.
- The input transducer is an electret microphone. R1 provides DC power for the FET pre-amplifier built into the electret microphone.
- Audio signals cause changes in the FET current and this causes the voltage across R1 to vary.
- C1 couples the audio AC output from the microphone to the next amplifier stage. This coupling capacitor blocks DC potentials.
- R2 and R3 form a voltage divider providing a 6 Volt line (50% of the power supply).
- C2 is a decoupling capacitor. It guarantees that there are no AC voltages at point A in the circuit.
- R4 ensures that the DC voltage is 6 Volts at the non inverting input (pin 3) of the op amp.
- R5 and R6 control the gain of the op amp.
- Gain = 1 + Rf / R1 = 1 + 220 / 3.3 = 67.7
- In a normal op amp circuit, R5 would be connected to ground and a -12V supply line would be needed.
- In this circuit, R5 is connected to point A and the -12 Volt line is not needed.
- The op amp is a 741 but almost any pin compatible device should work.
- R7 couples the op amp audio output to D1 and provides DC reverse bias across D1.
- R7 is large to prevent RF signals escaping from the oscillator circuit.
- D1 is a 5 amp rectifier diode. It is being used as a variable reactance diode. When the diode is reverse biased, virtually no current flows but there is some capacitance. If the reverse bias voltage is increased, the capacitance decreases. This makes a voltage controlled capacitor. The DC output of the op amp reverse biases the diode. The audio output from the op-amp varies the bias voltage and causes the capacitance of the diode to change. This changes the transmission frequency. In this way, Frequency Modulation is produced. Various diodes were tried and this one worked best. It is possible to buy varactor diodes which would work better.
- D1, C3 and C4 with the coil form a tuned circuit which resonates at about 100 MHz in the middle of the FM radio band.
- The MPF102 (available from Farnell Electronics) is connected as a source follower with a useful current gain.
- The source current flows through one turn of the four turn coil. This current induces a voltage across the whole coil and this is fed to the gate. This is positive feedback and the circuit oscillates at the resonant frequency of the tuned circuit.
- This is a Hartley oscillator with modified tuning capacitors to allow frequency modulation with the varactor diode.
- C4 is a fixed 22 pF capacitor. The circuit would be easier to get working with a trimmer capacitor covering 5 to 30 pF.
- C5 is a decoupling capacitor. It prevents RF voltages on the FET drain.
- R8 limits the DC current through the FET and ensures the device operates inside safe limits.
- D2 is a polarity protection diode. It protects the circuit if the power supply is connected the wrong way round.
- C6 and C7 are decoupling capacitors. C7 works well at low (audio) frequencies and C6 works well at VHF radio frequencies.