Download FM Transmitter

FM Transmitter
Matt Baker
Kevin Van Dyke
Design

Three stage, 9V FM transmitter
• Audio amplification
• Oscillation
• RF amplification


Electret microphone for input
Dipole antenna – 160 cm
Circuit Diagram
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.


Oscillation occurs
Final stage: Third transistor amplifies
the output RF signal.
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.
First amplification stage


Standard self-biasing
common emitter
amplifier
The 22n capacitor
isolates the microphone
from the base voltage
of the transmitter by
blocking DC.
First Transistor
Amplification
800mV
400mV
0V
-400mV
0s
V(Q5:c)
1ms
2ms
V(V5:+)
3ms
4ms
5ms
Time
6ms
7ms
8ms
9ms
10ms
Oscillator Stage

Oscillator necessary to generate RF carrier
waves
 Feedback signal makes
the base-emitter current
vary at the resonant
frequency.
 This causes the emittercollector current to vary at
the same frequency.
 This signal then radiated
as radio waves.
Final Amplification Stage




This stage amplifies the RF
signal.
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.
Output power from this stage will be maximum
when it is tuned to oscillate at the same frequency
as the previous stage.
• How?

Peaking Circuit
Peaking Circuit



Uses diodes to charge a capacitor.
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.
Third Transistor
Final Amplification
10V
8V
6V
4V
2V
0s
V(C7:2)
1ms
V(L6:2)
2ms
3ms
4ms
5ms
Time
6ms
7ms
8ms
9ms
10ms
Signal Power
VE  2.537
VC  9
P  IC VCE
IC  .0252
VCE  VC  VE
P  0.163
Watts