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  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  Transmitter’s frequency range: Mhz  Effective Transmitter distance: 2-5 meters  Frequency Response: 10khz  Easily Portable  Variable Transmitted Frequency

Final Design Fig 1. The schematic of the final design.

Bread boarded Circuit The Pre-emphasis circuit The oscillator circuit The amplifier circuit Fig 2. Picture of the bread boarded circuit.

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

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) Fig 5. SNR data

Effective Transmitter Distance  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 Fig 6. Fig 7. 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. 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.

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

Thank You Any Questions? REFERENCES [1] [2] Electronic Communications Systems 5th ed., Tomasi, Wayne, Prentice Hall, 2003