FM Transmitter Matt Baker Kevin Van Dyke

Design Three stage, 9V FM transmitter Three stage, 9V FM transmitter Audio amplificationAudio amplification OscillationOscillation RF amplificationRF amplification Electret microphone for input Electret microphone for input Dipole antenna – 160 cm Dipole antenna – 160 cm

Circuit Diagram

Circuit Description Radio frequency oscillator (100MHz) Radio frequency oscillator (100MHz) Microphone picks up and amplifies audio and then feeds it to first transistor for amplification Microphone picks up and amplifies audio and then feeds it to first transistor for amplification Output from collector is fed into base of the second transistor where it modulates the resonant frequency of the tank circuit Output from collector is fed into base of the second transistor where it modulates the resonant frequency of the tank circuit This is done by varying the junction capacitance of the transistor.This is done by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the second transistor.Junction capacitance is a function of the potential difference applied to the base of the second transistor.

Oscillation occurs Oscillation occurs Final stage: Third transistor amplifies the output RF signal. Final stage: Third transistor amplifies the output RF signal.

Electret Microphone What is an electret? What is an electret? It is a permanently charged dialectric.It is a permanently charged dialectric. The diaphragm of the microphone acts as one of the plates in the capacitor.The diaphragm of the microphone acts as one of the plates in the capacitor. The movement of the plate changes the capacitance.The movement of the plate changes the capacitance. Amplified by a FET amplifier.Amplified by a FET amplifier.

First amplification stage Standard self-biasing common emitter amplifier Standard self-biasing common emitter amplifier The 22n capacitor isolates the microphone from the base voltage of the transmitter by blocking DC. The 22n capacitor isolates the microphone from the base voltage of the transmitter by blocking DC.

First Transistor

Amplification

Oscillator Stage Oscillator necessary to generate RF carrier waves Oscillator necessary to generate RF carrier waves Feedback signal makes the base-emitter current vary at the resonant frequency. Feedback signal makes the base-emitter current vary at the resonant frequency. This causes the emitter- collector current to vary at the same frequency. This causes the emitter- collector current to vary at the same frequency. This signal then radiated as radio waves. This signal then radiated as radio waves.

Final Amplification Stage This stage amplifies the RF signal. This stage amplifies the RF signal. A Zetex ZTX320 RF transistor was used to do this efficiently. A Zetex ZTX320 RF transistor was used to do this efficiently. L2 and the 10p capacitor in parallel with it are designed to reduce harmonics from the circuit. L2 and the 10p capacitor in parallel with it are designed to reduce harmonics from the circuit. Output power from this stage will be maximum when it is tuned to oscillate at the same frequency as the previous stage. Output power from this stage will be maximum when it is tuned to oscillate at the same frequency as the previous stage. How?How? Peaking Circuit Peaking Circuit

Peaking Circuit Uses diodes to charge a capacitor. Uses diodes to charge a capacitor. Measure voltage across cap w/ voltmeter. Measure voltage across cap w/ voltmeter. To tune circuit, move turns of coil L3 further apart or closer together until the reading on the voltmeter is maximum. To tune circuit, move turns of coil L3 further apart or closer together until the reading on the voltmeter is maximum.

Third Transistor

Final Amplification

Signal Power