Variable low power fm transmitter for use with portable audio player
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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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Variable low power fm transmitter for use with portable audio player l.jpg
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


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Objectives & Goals Audio Player

  • 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”


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Design Specifications Audio Player

  • Transmitter’s frequency range: 88-107Mhz

  • Effective Transmitter distance: 2-5 meters

  • Frequency Response: 10khz

  • Easily Portable

  • Variable Transmitted Frequency


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Final Design Audio Player

Fig 1. The schematic of the final design.


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Bread boarded Circuit Audio Player

The Pre-emphasis circuit

The amplifier circuit

The oscillator circuit

Fig 2. Picture of the bread boarded circuit.


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Operation and Analysis Audio Player

*The analysis was conducted with use of a Clarion DXZ735MP receiver, the specs are listed below.

  • 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.


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Operation and Analysis Audio Player

Fig 3. Output at 100 Hz

Fig 4. Output at 30 Hz


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SNR Audio Player

  • 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.

Fig 5. SNR data

SNR=20 log (Vs/Vn)


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Effective Transmitter Distance Audio Player

  • 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)


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Frequency Response Audio Player

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.

Fig 6.

Figure 7- The output from the transmitter with out the pre-emphasis. The –3 dB points are 80 Hz, 9.3 kHz. The bandwidth is then 9.2 kHz.

Fig 7.


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Possible Improvements Audio Player

  • 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.


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Time Line Audio Player


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Thank You Audio PlayerAny Questions?

REFERENCES

[1] http://sound.westhost.com/project54.htm

[2] Electronic Communications Systems 5th ed., Tomasi, Wayne, Prentice Hall, 2003