Transcript Variable Low Power FM Transmitter for use with Portable Audio

Variable Low Power FM
Transmitter for use with
Portable Audio Player
Communication Electronics – Dr. Pao-Lo Liu
TA – Mr. Saurav Bandyopadhyay
Jason Burgess
Kee Soon Lim
Terry Hudson
Objectives & Goals
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
The device will function as a portable
wireless FM transmitter with an input from
the mini jack of a portable audio device.
The device will adhere to Title 47 of the
Code of Federal Regulations (Part 15,
subpart C), which states, “the power limit
for unlicensed FM transmissions is a signal
strength of 250 microvolts per meter,
measured 3 meters from the transmitting
antenna”
Design Specifications


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Transmitter’s frequency range:
Effective Transmitter distance:
Frequency Response:
Easily Portable
Variable Transmitted Frequency
88-107Mhz
2-5 meters
10khz
Final Design
Fig 1. The schematic of the final design.
Bread boarded Circuit
The Pre-emphasis
circuit
The amplifier
circuit
The oscillator circuit
Fig 2. Picture of the bread boarded
circuit.
Operation and Analysis
*The analysis was conducted with use of a
Clarion DXZ735MP receiver, the specs are listed
below.

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FM frequency response 30-15kHz +0.0, -2.1 dB
SNR 74.4 dB mono, 71.2dB stereo
All measurements taken at 89.1 MHz carrier frequency.
Operation and Analysis
Fig 3. Output at 100 Hz
Fig 4. Output at 30 Hz
SNR
-To measure the SNR an
audio analysis file was
acquired of a 1kHz sine
wave.
- The peak to peak
voltage of a 1kHz sine
wave was measured
against the no signal
noise level at different
volumes of the input.
SNR=20 log (Vs/Vn)
signal (V) noise (mV)
1.3
3.8
1.5
3.8
1.75
3.8
1.82
3.8
1.9
3.9
2.1
4
2.2
4.2
2.35
4.3
Average SNR (dB) =
snr (dB)
50.683195
51.926153
53.265089
53.605756
53.75378
54.403186
54.383468
54.751988
53.346577
Fig 5. SNR data
Effective Transmitter Distance
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The same 1kHz sine wave was used as the input.
The output was measure by the receiver with three foot
whip antenna attached.
A simple 18 in. long 20 gauge wire was used as the
transmitter antenna.
The effective transmission distance was determined to be
the straight line, antenna to antenna distance at which an
attenuation of 3dB occurred.
The lab measurement was 6.4m (21 feet)
Frequency Response
Frequency Response (w/ Pre- emphasis)
2
Voltage (V)
Figure 6- The output from
the transmitter with the
pre-emphasis. The –3 dB
points are 85Hz, 14.5
kHz. The bandwidth is
then 14kHz.
1.5
1
0.5
0
10
100
1000
10000
100000
Frequency (Hz)
Fig 6.
Frequency Response (w/o Pre-emphasis)
2
P-P Voltage (V)
Figure 7- The output
from the transmitter
with out the preemphasis. The –3 dB
points are 80 Hz, 9.3
kHz. The bandwidth is
then 9.2 kHz.
1.5
1
0.5
0
10
100
1000
Frequency (Hz)
Fig 7.
10000
100000
Possible Improvements
-Soldering to a printed circuit board
-Making use of a better antenna
-Indicating transmission frequency on dial
-Indicating transmission frequency using 7 segment
LED displays.
Time Line
Schedule
Actual
Choose Topic/Research Design/Order Parts Construction Debugging
Week of 2/7
Week of 2/14
Week of 2/21
Week of 2/28
Week of 3/7
Week of 3/14
Week of 3/21
Week of 3/28
Week of 4/4
Week of 4/11
Week of 4/18
Week of 4/25
Break
Analysis/Comparison
Thank You
Any Questions?
REFERENCES
[1] http://sound.westhost.com/project54.htm
[2] Electronic Communications Systems 5th ed., Tomasi, Wayne,
Prentice Hall, 2003