Variable Low Power FM Variable Low Power FM Transmitter for use with Transmitter for use with Portable Audio PlayerPortable Audio Player

Communication Electronics – Dr. Pao-Lo LiuCommunication Electronics – Dr. Pao-Lo Liu

TA – Mr. Saurav BandyopadhyayTA – Mr. Saurav BandyopadhyayJason BurgessJason Burgess

Kee Soon LimKee Soon Lim

Terry HudsonTerry Hudson

Objectives & GoalsObjectives & Goals

The device will function as a portable The device will function as a portable wireless FM transmitter with an input from wireless FM transmitter with an input from the mini jack of a portable audio device.the mini jack of a portable audio device.

The device will adhere to Title 47 of the The device will adhere to Title 47 of the Code of Federal Regulations (Part 15, Code of Federal Regulations (Part 15, subpart C), which states, “the power limit subpart C), which states, “the power limit for unlicensed FM transmissions is a signal for unlicensed FM transmissions is a signal strength of 250 microvolts per meter, strength of 250 microvolts per meter, measured 3 meters from the transmitting measured 3 meters from the transmitting antenna” antenna”

Design SpecificationsDesign Specifications

Transmitter’s frequency range: 88-107MhzTransmitter’s frequency range: 88-107Mhz Effective Transmitter distance: 2-5 metersEffective Transmitter distance: 2-5 meters Frequency Response:Frequency Response: 10khz 10khz Easily PortableEasily Portable Variable Transmitted FrequencyVariable Transmitted Frequency

Final DesignFinal Design

Fig 1. The schematic of the final design.

Bread boarded CircuitBread boarded Circuit

The Pre-emphasis circuit

The oscillator circuit

The amplifier circuit

Fig 2. Picture of the bread boarded circuit.

Operation and AnalysisOperation and Analysis

FM frequency response 30-15kHz +0.0, -2.1 dBFM frequency response 30-15kHz +0.0, -2.1 dB SNR 74.4 dB mono, 71.2dB stereoSNR 74.4 dB mono, 71.2dB stereo All measurements taken at 89.1 MHz carrier frequency.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 AnalysisOperation and Analysis

Fig 3. Output at 100 Hz

Fig 4. Output at 30 Hz

SNRSNRsignal (V) noise (mV) snr (dB)

1.3 3.8 50.6831951.5 3.8 51.926153

1.75 3.8 53.2650891.82 3.8 53.6057561.9 3.9 53.753782.1 4 54.4031862.2 4.2 54.383468

2.35 4.3 54.751988

Average SNR (dB) = 53.346577

-To measure the SNR an To measure the SNR an audio analysis file was audio analysis file was acquired of a 1kHz sine acquired of a 1kHz sine wave.wave.

- The peak to peak The peak to peak voltage of a 1kHz sine voltage of a 1kHz sine wave was measured wave was measured against the no signal against the no signal noise level at different noise level at different volumes of the input.volumes of the input.

SNR=20 log (Vs/Vn)

Fig 5. SNR data

Effective Transmitter DistanceEffective Transmitter Distance

The same 1kHz sine wave was used as the input.The same 1kHz sine wave was used as the input. The output was measure by the receiver with three foot The output was measure by the receiver with three foot

whip antenna attached.whip antenna attached. A simple 18 in. long 20 gauge wire was used as the A simple 18 in. long 20 gauge wire was used as the

transmitter antenna. transmitter antenna. The effective transmission distance was determined to be The effective transmission distance was determined to be

the straight line, antenna to antenna distance at which an the straight line, antenna to antenna distance at which an attenuation of 3dB occurred.attenuation of 3dB occurred.

The lab measurement was 6.4m (21 feet)The lab measurement was 6.4m (21 feet)

Frequency ResponseFrequency 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.

Frequency Response (w/ Pre- emphasis)

0

0.5

1

1.5

2

10 100 1000 10000 100000

Frequency (Hz)

Vo

ltag

e (

V)

Frequency Response (w/o Pre-emphasis)

0

0.5

1

1.5

2

10 100 1000 10000 100000

Frequency (Hz)

P-P

Vo

ltag

e (V

)

Possible ImprovementsPossible 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.

ScheduleActual Choose Topic/Research Design/Order Parts Construction Debugging Analysis/Comparison

Week of 2/7

Week of 2/14

Week of 2/21

Week of 2/28

Week of 3/7

Week of 3/14 Break

Week of 3/21

Week of 3/28

Week of 4/4

Week of 4/11

Week of 4/18

Week of 4/25

Time Line

Thank YouThank You

Any Questions?Any Questions?

REFERENCES

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

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