FM TransmitterFM Transmitter

Matt BakerMatt Baker

Kevin Van DykeKevin Van Dyke

DesignDesign

Three stage, 9V FM transmitterThree stage, 9V FM transmitter• Audio amplificationAudio amplification• OscillationOscillation• RF amplificationRF amplification

Electret microphone for inputElectret microphone for input Dipole antenna – 160 cmDipole antenna – 160 cm

Circuit DiagramCircuit Diagram

Circuit DescriptionCircuit Description

Radio frequency oscillator (100MHz)Radio frequency oscillator (100MHz) Microphone picks up and amplifies audio Microphone picks up and amplifies audio

and then feeds it to first transistor for and then feeds it to first transistor for amplificationamplification

Output from collector is fed into base of Output from collector is fed into base of the second transistor where it modulates the second transistor where it modulates the resonant frequency of the tank circuitthe resonant frequency of the tank circuit• This is done by varying the junction This is done by varying the junction

capacitance of the transistor.capacitance of the transistor.• Junction capacitance is a function of the Junction capacitance is a function of the

potential difference applied to the base of the potential difference applied to the base of the second transistor.second transistor.

Oscillation occursOscillation occurs Final stage: Third transistor amplifies Final stage: Third transistor amplifies

the output RF signal.the output RF signal.

Electret MicrophoneElectret 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 The diaphragm of the microphone acts

as one of the plates in the capacitor.as one of the plates in the capacitor.• The movement of the plate changes the The movement of the plate changes the

capacitance.capacitance.• Amplified by a FET amplifier.Amplified by a FET amplifier.

First amplification stageFirst amplification stage

Standard self-biasing Standard self-biasing common emitter common emitter amplifieramplifier

The 22n capacitor The 22n capacitor isolates the isolates the microphone from the microphone from the base voltage of the base voltage of the transmitter by blocking transmitter by blocking DC.DC.

First TransistorFirst Transistor

AmplificationAmplification

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(Q5:c) V(V5:+)

-400mV

0V

400mV

800mV

Oscillator StageOscillator Stage

Oscillator necessary to generate RF carrier Oscillator necessary to generate RF carrier waveswaves

Feedback signal makes Feedback signal makes the base-emitter current the base-emitter current vary at the resonant vary at the resonant frequency.frequency.

This causes the emitter-This causes the emitter-collector current to vary at collector current to vary at the same frequency.the same frequency.

This signal then radiated This signal then radiated as radio waves.as radio waves.

Final Amplification StageFinal Amplification Stage This stage amplifies the RF This stage amplifies the RF

signal.signal. A Zetex ZTX320 RF transistor A Zetex ZTX320 RF transistor

was used to do this efficiently.was used to do this efficiently. L2 and the 10p capacitor in L2 and the 10p capacitor in

parallel with it are designed to parallel with it are designed to reduce harmonics from the reduce harmonics from the circuit.circuit.

Output power from this stage will be maximum Output power from this stage will be maximum when it is tuned to oscillate at the same frequency when it is tuned to oscillate at the same frequency as the previous stage.as the previous stage.• How?How?

Peaking CircuitPeaking Circuit

Peaking CircuitPeaking 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 To tune circuit, move turns of coil L3

further apart or closer together until the further apart or closer together until the reading on the voltmeter is maximum.reading on the voltmeter is maximum.

Third TransistorThird Transistor

Final AmplificationFinal Amplification

Time

0s 1ms 2ms 3ms 4ms 5ms 6ms 7ms 8ms 9ms 10msV(C7:2) V(L6:2)

2V

4V

6V

8V

10V

Signal PowerSignal Power

VE 2.537 IC .0252

VC 9 VCE VC VE

P IC VCEP 0.163 Watts