FM Transmitter
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FM Transmitter Dec06-01 Client Iowa State University – Senior Design Team Grant Blythe Luke Erichsen Tony Hunziker Jacob Sloat Advisors Dr. John W. Lamont Prof. Ralph E. Patterson III. Definitions. FCC: Federal Communication Commission LCD: liquid crystal display

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Fm transmitter dec06 01 client iowa state university senior design team

FM TransmitterDec06-01


Iowa State University – Senior Design


Grant Blythe Luke Erichsen

Tony Hunziker Jacob Sloat


Dr. John W. Lamont

Prof. Ralph E. Patterson III


  • FCC: Federal Communication Commission

  • LCD: liquid crystal display

  • MP3 player: digital music player, (i.e. ipod)

  • Satellite radio: subscription radio signal sent via satellite, (i.e. XM radio, Sirius radio)

  • Transmission frequency: The frequency at which the device is transmitting the FM modulated signal to the FM radio.

  • Wall wart: AC power transformer designed to plug into a standard wall outlet

General problem statement
General Problem Statement

The objective of this project is to design a FM transmitter that will:

  • Connect to a standard headphone output jack of an mp3 player

  • Transmit a minimum of 12ft

  • Transmit between frequencies of 88MHz to 108MHz

  • Have 4 programmable preset buttons

  • Receive power from a cigarette lighter/power socket

  • Automatic on/off function

General solution approach
General Solution Approach

  • The FM transmitter will modulate the signal, send it through an amplifier, and finally through an internal antenna.

General solution approach1
General Solution Approach

  • Internal memory will store four programmable station presets from the frequency band.

  • A LCD screen will display the current transmission frequency. This LCD will be backlit for easy night use.

  • The transmission frequency and presets will be accessible through a button/knob interface that may also include a lighting system.

  • There will be a microcontroller to access memory, automatic turn off/on function, possible lighting, display frequency, and any other controlling concerns.

Operating environment
Operating Environment

The finished device will operate within a personal vehicle or a household room that could be exposed to:

  • Moisture

  • Dust

  • Dirt

  • Impacts

  • Reckless negligence

Intended user s use s
Intended User(s) / Use(s)

Intended User(s)

  • The intended user for this product is anyone owning MP3 players or satellite radio devices. It does assume the amount of operating knowledge associated with MP3 player/satellite radio users.

    Intended Use(s)

  • The FM transmitter is intended to make personal music players more accessible to listen to through home and car stereos.


  • The transmitter will be used for all varieties of personal music players.

  • The transmitter will be used in a variety of environments including varying temperatures, humidity, seismic conditions, and electromagnetic noise.

  • The transmitter will be used at all hours of the day.

  • Similar products will come directly from the personal music players producers as competition.

Assumptions continued
Assumptions Continued…

  • The device will be used with standard North American FM radio equipment.

  • The input audio signal will consist of standard music with a frequency range of 20 Hz to 20 kHz.

  • The device design will implement solid state electronics.

  • The user will have access to a steady power source able to supply the rated voltage and frequency (if AC) within a 10 percent tolerance.


  • The cost to purchase this product shall not become uncompetitive.

  • The transmitter must conform to FCC regulations.

    • The FCC has regulation broadcast strength of .1kW. The transmitter must not exceed this strength.

    • The device shall conform with FCC rules Part 15 concerning unlicensed FM broadcasting.

    • The RF field strength 3 meters from the device should not exceed 250uV/m.

  • The transmission frequency band must stay within 88-108 MHz.

  • The device shall be capable of obtaining power from both a 120 V AC and a 12 V DC source.

Limitations continued
Limitations Continued...

  • The form of the device must be manageable for ease of transportation and storage.

  • There must be at least 4 programmable preset transmission frequencies.

  • The device must transmit at least 12 feet.

  • Transmission frequency must be adjustable.

  • Transmission frequency must be displayed.

  • The size shall not exceed 6 in. by 6 in. by 3 in.

  • The weight shall not exceed 1 lb.

  • The device shall be compatible with both digital and analog tuned radios.

Expected end product
Expected End-Product

  • The device case will be made of plastic

  • The case will allow for easy hand manipulation and transportation

  • The device will implement an LCD screen displaying the transmission frequency.

  • The user input interface will consist of six buttons.

    • “up” and a “down” button to adjust transmission frequency

    • 4 buttons will each access a programmable preset frequency

Detailed functionality
Detailed Functionality

  • Pushing and releasing a preset button, in less than 3 seconds, will adjust the transmission frequency to the stored frequency assigned to that button.

  • Pushing and holding a preset button for greater than 3 seconds will assign and store the current transmission frequency to that preset button.

  • Both the LCD display screen and the buttons may be backlit for use in low ambient light environments.

  • This device shall come with an adapter for use with an American standard wall outlet. This will be a simple wall wart device. The output from this adapter will be a male power jack.

Detailed functionality continued
Detailed Functionality Continued…

  • The transmitter will also come with an adapter for the standard cigarette outlet for automobile use. The output of this adapter will be a male power jack.

  • Both adapters will connect to the device through a common female jack. The adapters will be designed so that the device will receive the same power input regardless of which adapter is being used.

  • An instruction manual will also be included.

  • The device and accessories should be deliverable by December 2006.

Detailed design
Detailed Design

This includes the different systems within the design, the parts currently considered to implement these systems, and preliminary prices of these components.


Manual Inputs

These inputs allow the user to adjust and fine tune the transmission frequency manually.

  • There will be two buttons for the manual input function.

    • one to adjust the transmission frequency up and one button to adjust the transmission frequency down.

  • Adjustment of the transmission frequency will happen in increments of 200 kHz.

    • For example 102.7 MHz will increase to 102.9 MHz or decrease to 102.5 MHz in response to the received input.

Inputs continued
Inputs Continued…

Preset Inputs

  • The four programmable preset frequencies shall be stored in non-volatile memory in the device.

  • The transmission frequency shall be adjusted to any of these frequencies by pushing one of the four corresponding buttons.

  • A preset shall be programmed to the current frequency by pushing and holding the corresponding button for greater than 3 seconds.

  • These four buttons will be backlit for use in low light environments.

Inputs continued1
Inputs Continued…

Power Supply

  • The device shall receive its power input from a 12 V DC cigarette lighter/power socket in an automobile or a standard 120 V AC wall outlet. An adapter will be provided for each of these two sources.

    • Both adapters will provide a common 5 V DC output to the device.

    • The device will have one power input jack that will be capable of connecting to either adapter.

Inputs continued2
Inputs Continued…

Power Supply Continued…

  • The adapter for the cigarette lighter will be a single stage adapter that plugs directly into the standard cigarette lighter/ power outlet of an automobile.

    • The adapter will convert the 12V output of the cigarette lighter to the 5V DC input needed for the device

  • The source from the wall outlet will be a wall wart

    • Converts the 120V AC to a 5V DC output

    • Provided by the EE/CprE Senior Design

Inputs continued3
Inputs Continued…

Input Signal

  • The input signal may come from an mp3 player, or a satellite radio system.

  • The audio signal input is expected to consist of audio frequencies within the range of 20Hz to 20 kHz.

  • The device shall accept a stereo input audio signal through a standard 3.5 mm stereo jack.


Microcontroller Design Requirements

  • It will take in the manual and preset inputs and tune the device to a transmission frequency depending on which buttons are pushed.

  • It will allow the device to store transmission frequencies into the preset buttons.

  • It will also implement the auto turn on/turn off function with respect to the input signal.

  • The LCD display will also be controlled by the microcontroller.

Processing continued
Processing Continued…

Microcontroller Solution

  • The PIC processor PIC16F873A was chosen along with the LM555 clock.

    • It contains a 28 pin IO interface and the memory and processing power needed for our application.

    • PIC processors will give us the necessary computing power to control all components of the device including the LCD.

    • At the same time PIC processors are inexpensive and will fall within the project budget.

    • PIC processors also possess the non-volatile memory necessary to store the preset stations.

Processing continued1
Processing Continued…

Microcontroller Solution Continued…

PIC16F873A Microcontroller

Processing continued2
Processing Continued…

Auto On/Off Implementation

  • The microcontroller will be in a continuous loop awaiting the input in order to control the peripherals.

    • Upon detection of an input signal, the device shall power on and begin transmitting within 1 second.

    • After detecting no input signal for 1 minute, the device shall power down within 1 second.

    • This count down is a period to wait for signal before actually powering down. This prevents premature power down.

Modulation transmission

Modulation/Transmission Block

After examining the available technologies, an integrated solution was found to provide the modulation, amplification, and transmission.

  • The device will implement a Rohm Electronics BH1415F Wireless Audio Link IC to provide stereo modulation and FM transmission.

Modulation transmission2

Rohm BH1415F

  • The BH1415F consists of a stereo modulator for generating the stereo composite signal and a FM transmitter for broadcasting an FM signal on the air.

  • The IC transmits on a frequency range of 88 MHz to 108 MHz. The transmission frequency is set by the microcontroller.

Modulation transmission3

Composite Stereo Signal

Modulation transmission4

FM Modulation

  • The composite stereo signal is then modulated with a carrier signal.

    • This carrier signal is the frequency you tune to with an FM radio.

  • The carrier signal is set on the BH1415F by the microcontroller using a data packet.

Modulation transmission5

Data Packet to BH1415F

Modulation transmission6

Carrier Frequency Data

For Example: in the case of 99.7 MHz carrier frequency.

99.7 MHz / 100 kHz (fref) = 997  3E5 (HEX)


There are two outputs from the device.

  • An LCD screen that will display the frequency that the device is currently transmitting at.

  • The other output is the FM audio output transmitted by the antenna.

Outputs continued
Outputs Continued…

LCD Display Requirements

  • When a frequency change occurs, the microcontroller will send the appropriate control signals to the LCD display.

  • Each of the 4 digits plus the decimal point displayed will be controlled individually, and will only need to be refreshed when the frequency is change.

  • The format of this display will be “XXX.X” (i.e. “102.7”).

  • The display is also backlit.

Outputs continued1
Outputs Continued…

LCD Display Solution

The LCD chosen was the VIM-404-DP-FC-S-HV LCD manufactured by Varitronix.

  • It is a transflective backlit display

  • The way it accepts inputs is dependant upon its programming making it a versatile choice.

  • Its 20 I/O pin interface is typical for LCD components on the market.

Outputs continued2
Outputs Continued…

LCD Display Solution

Outputs continued3
Outputs Continued…

LCD Display Solution

Outputs continued4
Outputs Continued…


  • The device will use a small wire antenna that will remain inside the device case. Because the case is plastic and the transmission range is small, there should not be a need for an external antenna.

Unforeseen issues
Unforeseen Issues


  • Interfacing with microcontroller

    Rohm BH1415F Wireless Audio Link IC

  • Phase Lock Loop