Transcript FM Transmitter

FM Transmitter
Michael Boersma
Engr 311
12/6/02
Schematic
Taken from http://zeus.eed.usv.ro/misc/mirrors/cc/index2.htm
Basic Operation
• Electric microphone translates audio signal
to small voltage signal
How Microphone Works
• Sound waves causes
charged diaphragm to
vibrate over a
conductive back-plate.
• This causes a change
in capacitance, and in
turn, an electric signal
Basic Operation
• Electric Microphone translates audio signal
to small voltage signal
• First transistor amplifies the signal
First Transistor
Simulated Circuit
8.0V
6.0V
4.0V
2.0V
0V
0s
V(Q3:b)
1ms
2ms
V(C2:1)
3ms
4ms
5ms
Time
6ms
7ms
8ms
9ms
10ms
Basic Operation
• Electric Microphone translates audio signal
to small voltage signal
• First transistor amplifies signal
• Frequency Modulation
Frequency Modulation
Resonant Circuit is the Key
Circuit will discharge energy at its resonant
frequency
Z
jL
jL
jL
j C
1 
jC   jL 

jC


1    L C
1
j C
2
 J L
1  LC

1
LC
2f
f
2
1
2  LC
Circuit Response
100mV
50mV
0V
1.0MHz
V(L1:1)
3.0MHz
10MHz
30MHz
Frequency
100MHz
300MHz
1.0GHz
Basic Operation
• Electric Microphone translates audio signal
to small voltage signal
• First transistor amplifies signal
• Frequency Modulation
• Signal Power
Signal Power
VE  7.907 V
VC  9
P  IC  VCE
3
IC  7.867  10
A
VCE  VC  VE
3
P  8.599  10
W
Antenna
Optimum antenna length is a function frequency
Speed_of_light  300000 
Wavelength 
km
s
Speed_of_light
Frequency
Antenna_lenght 
Frequency  100000000  Hz
Wavelength  3 m
2
2
Wavelength
4
sin( x)
0
2
2
Antenna_length  .75m
0
10
20
0
x
20
Implementation
Difficulties in Implementation
• Needed to make the inductor
• Adjusting the capacitor to tune the
frequency
Improvements
• Add capacitors to supply low impedance
path to ground for AC signals
• Better tool for adjusting capacitor