1. Automatically Tuned FM Transmitter
Dan Fishman
Saim Jafri
Michael Thylur
Electrical Engineers 2007

2. Project Idea
FM transmitters to transmit from digital music devices to car antenna
Between cities the unused radio frequency in one city might be used by a broadcasting station in the next
The synchronization of the transmitter and receiving radio frequencies can be quite time consuming
An automatic FM radio frequency scanner that finds unused stations will be developed

3. Project Goals
Develop a full understanding of frequency synthesizers that use a Divide-by-N and PLL
Successfully detect an unused station in the FM band
To better understand communication systems
Investigate the marketability of our system

4. Existing Technologies Griffin iTrip
Power source: receives power from ipod
Modulation: FM Stereo
Frequencies: MHz
High stability crystal oscillator, phase-locked loop control
Transmits Audio frequencies: 50Hz to 15KHz
Operating range: ft

5. Monster® iCarPlay™ Wireless 200 FM Transmitter with AutoScan
This device automatically finds the clearest available FM station
3 programmable station presets
Simple, intuitive 3 button control

6. A Stereo FM Transmitter
The two channels are encoded and multiplexed into one channel
This is fed into the frequency generator
The frequency generator works as an FM frequency modulator, it receives a voltage and produces an oscillating wave based on the input
The output of the oscillator will be very low, so it is sent to an amplifier to increase its broadcasting range

7. Introduction to Design
Radios are tuned by using local oscillators
Want to make an automatic scanning system using a frequency synthesizer
Impractical to use different oscillators
Makes sense to use a phase-locked loop as a frequency synthesizer

8. Phase-Locked Loop

9. To control the digital divider a microcontroller will be used
The frequency of the voltage at the output will be equal to the product of the frequency divide and the reference frequency.
To control the digital divider a microcontroller will be used
We will use two dividers: A N-divide and a Pre-Scaler
Need to generate frequencies in the FM range 200 kHz apart ,

10. The System Frequency synthesizer Detector Super heterodyne PLL
Receiver
Microcontroller
Output Voice
or Display

11. Superheterodyne Receiver
The mixer multiplies the local oscillator with a signal.
The intermediate frequency is 10.7 MHz.

12. Frequencies FCC allows transmission from 88 MHz-108MHz
Allowable band of 200KHz
Mixer moves a wave to 10.7MHz
Local oscillator will generate 98.8MHz and 118.6MHz
Ex. 98.9MHz -88.1MHz= 10.7MHz

13. PLL Operation
In lock mode, the input signal and voltage controlled oscillator signals are identical
When input equals free-running frequency of VCO no voltage is supplied to the VCO

14. VCO ff is the free running frequency Vd is the input voltagefo ff Vd
ff is the free running frequency
Vd is the input voltage
fo is the output frequency of VCO
Ko is the transfer function of the VCO

15. Phase Detector
The phase detector’s voltage output is a function of the phase difference between Fr and Fvco
Gain of phase detector:
θe is the shifted phase difference between the signals at the phase detector
Ve is the voltage signal proportional to the phase difference (It is the output of the phase detector)
A sinusoid can be approximated to be linear for small phase differences

16. Digital phase lock loop
Constructed from both analog and digital devices
The most simple phase detector is EXOR gate
Phase error is positive when output lags the reference signal.
RC filter can be used to remove AC components from the phase error signal
If phase error is zero, the phase detector’s output will also be zero and VCO operate at its center frequency

17. Digital PLL For FM Band Synthesizer

18. LM565CN Phase-Locked Loop
We will insert an N divide and pre-scaler between VCO and phase comparator
Free running frequency is:
The VCO is set using an external capacitor and resistor

19. High Frequency Phase-Locked Loop
Has a lock frequency between 43MHz and 100MHz when supplied 5 volts.
A 2.4 kΩ resistor was placed between the power supply and the VCO to adjust the oscillation frequency range.
Several by-pass capacitors were placed between power supply and ground to minimize the noise ratio.
The system was successfully able to track a sinusoidal input between 64MHz and 102MHz.

20. Divide-By-N Has 24 pins, 16 jam inputs
Outputs a pulse with the desired frequency
Testing found that it works for both square waves and sinusoids
Programmable

21. N-divide CD54HC4059 Three mode-select inputs
The mode times the jam inputs select the choice of the divisor N.
To divide by 493 in the divide by ten mode, the chip needs to be setup in the divide by ten mode by applying appropriate high voltages to the mode-select inputs. 493/10=49.3

22. Divide-by-n Presets

23. Divide by 493

24. 8051 Microcontroller Programmed using C language
Both controls divide-by-n and determines if a station is unused
Used to detect voltages
Add picture of microcontroller? Or add more text

25. Control of divide-by-n via μC
Each ribbon can control 8 jam inputs
12 jam inputs need to be controlled
Therefore 2 ribbon cables
Measured 3.27V from each pin

26. Divide-by-n oscillation
Voltage oscillates when using μC
No effect on frequency

27. Frequency Synthesizer
Test system built which outputs a desired frequency
Divide-by-n set to divide-by-2 mode
Microcontroller programmed to set divide-by-n to various divisors to control the output of the PLL
Test system successfully worked
Tested at 10 KHz

28. Peak Detection AM stations used
The input to the peak detector was from the base of the transistor at the audio amplifier
Op amp used with gain of ~62.5 so diode would be in active mode
Experimented and found RC value of one second showed good results

29. Station
Noise
peak to peak (mv)
max (mv)
1131.7
1481.7
288
1052
max/peak ratio
1.3
3.7
Station (detect)
Noise (detect)
383.3
901.7 50
504
2.4
10.1

30. Ratio of Noise to Station
Check to see if there was an advantage of using detector circuit.
Output of amplifier: 2.79
Output of peak detector: 4.29

31. Prescaler
Need to scale the FM frequency down because, through experiment, we found the n-divide will not operate above 7 MHz
Divide by 64
Operates from 50 MHz to 1.1 GHz

32. Project Timeline Start Date Completed Remaining
Start Date
Completed
Remaining
Make Divide-by-N operational
10/24/2006 16
Test PLL
11/8/2006 10
Integrate PLL with Divide
11/16/2006 4
1/3/2007 7
Program Microcontroller
1/10/2007
Design Detection System
1/20/2007 5
Test Detection System
1/25/2007
2/1/2007
Test system as a whole
2/11/2007 8
Design output display
2/19/2007
2/26/2007 3
Mount on PCB board
3/1/2007

33. Acknowledgements Professor Hassib Professor Hedrick Emad Andarawis
Gene Davison

34. References
Barret, Curtis. "Fractional/Integer-N PLL Basics." Texas Instruments. 25 May 2006 <
Best, Ronald E. Phase-Locked Loops. 3rd ed. New York: McGraw-Hill,
"ITrip Auto." Griffin Technology. Griffin Technology. 19 May 2006 <
Ward, Darrin. “FM Transmitter.” How A Stereo FM Transmitter Works

35. AM Regulations The AM broadcast band range is 530 kHz to 1700 kHz
AM is limited to a carrier signal power of 375 watts

36. Phase-Locked Loop

37. PLL Top View External low-pass filter Lock Frequency
43 MHz to 100 MHz (VDD= 5V +5%, TA= -20˚C to 75˚C, x1 Output)
37 MHz to 55 MHz (VDD= 3V +5%, TA= -20˚C to 75˚C)

38. Prescaler The prescaler operates at a frequency of 50 MHz
It is able to divide by 64 and 128
By testing the prescaler it was concluded that device works in the specified manner

39. PLL and Prescalar Integration
When the system was run the PLL went out of its lock range to 128MHz, the prescaler divided this by 64, hence returning an input of 2MHz to the phase detector.
Subsequent to this response the PLL would not return to its lock range, unless the system was reset and the prescaler was set in the disable mode.
A possibility of the system not working is because the bandwidth range of the phase detector was not large enough to accommodate a large bandwidth range.

40. 810 WGY

41. Local Oscillator MHz

42. Noise

43. Energy Detector
The AM/FM radio kit allowed access to the super heterodyne receiver stages.
Voltages were taken after peak detector circuit in the AM reciever.
The signal is smoothened by a resistor and capacitor, that act as lowpass filter and a peak detector.