21-07-2012, 12:51 PM
FM Transmitter
FM Transmitter.ppt (Size: 865.5 KB / Downloads: 412)
Carrier generation
Colpitts Oscillator is used
to produce the fm signals
to be transmitted at
RF.Crystal oscillator can’t be
used as it is invariable.
Circuit Description
Radio frequency oscillator (100MHz)
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
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.
Electret Microphone
What is an electret?
It is a permanently charged dialectric.
The diaphragm of the microphone acts as one of the plates in the capacitor.
The movement of the plate changes the capacitance.
Amplified by a FET amplifier.
Oscillator Stage
Oscillator necessary to generate RF carrier waves
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 signal then radiated as radio waves.