The document describes a project to build a portable FM transmitter and receiver. Key requirements were that the devices be handheld, use a portable power source, transmit and receive wireless signals, and be able to change frequencies. Prototypes were created and tested. The transmitter was able to broadcast between 89.5-93.5 MHz and the receiver could receive signals from 105.5-107.5 MHz. Future improvements discussed include adding casing and external antennae to both devices.

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EE 490 – Special Problems
FM Transmitter & Receiver Project
Todd Cook
010425296

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Table of Contents:
1. Objective 3
2. Level 1 Requirements 3
3. Level 2 Requirements 4
4. Design Solutions 4
a. Antenna Problems 4
b. Grounding 5
5. Prototyping 5
6. Materials Trade-off Study 7
7. Subsystem Design 8
8. PCB Board 8
9. Block Diagram 9
10. Verification and Validation 11
11. Project Schedule 12
12. Cost Budget 12
13. Results 13
14. Future Project Improvements 14

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Objective:
The project objective is to build a portable transmitter and receiver to simulate larger
scale radio transmitters and consumer radios. The project FM transmitter will send out audio
signals through the FM waveform and picked up by a radio. The project FM receiver will be able
to receive FM radio frequencies. The project budget was created to be cost-effective for the
general consumer.
Transmitter & Receiver Level 1 Requirements:
Requirements and Verification:
1. The project will be completed by the end of the semester which is December 8, 2016
a. Verification: CSULB calendar & Professor deadline
2. Documentation will be completed by the end of the semester by December 8, 2016.
a. Verification: CSULB calendar & Professor deadline
3. FM transmitter will be able to change frequencies.
a. Verification: Variable parts to change frequency.
4. FM receiver will be able to change frequencies.
a. Verification: Variable parts to change frequency.
5. FM transmitter & receiver will be handheld.
a. Verification: FM devices will can be held in one hand.
6. FM transmitter & receiver will have a portable power source.
a. Verification: FM devices will not require wall socket.
7. FM transmitter & receiver will use wireless signals.
a. Verification: FM devices will not be connected to produce or receive signals.

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Transmitter & Receiver Level 2 Requirements:
Level 1: FM transmitter will be able to change frequencies.
Level 2: The FM transmitter will make use of the LC frequency property.
Level 2: The inductor will be hand-made, so a trimmer variable capacitor is needed.
Level 1: FM Receiver will be able to change frequencies
Level 2: The FM receiver will make use of the LC frequency property.
Level 2: The inductor will be hand-made, so a trimmer variable capacitor is needed.
Level 1: transmitter & receiver will be handheld.
Level 2: The devices need to use the smallest parts possible for devices to fit in the hand.
Level 1: FM transmitter & receiver will have a portable power source.
Level 2: The design will require a small battery power source.
Level 1: FM transmitter & receiver will use wireless signals.
Level 2: The devices will use an antenna to send a broadcast out or receive broadcasts.
Design solutions:
Antenna problems:
I received much feedback on fixing the problem of requiring a 10-foot antenna as the parts sizes
and power source have a large area. However, the cost of any large antenna was unreasonable for
intended consumers, and most cheap antennas did not stand on their own. Which means a cheap
wire or a long piece of copper wire may be necessary. My solution was to use a long 3-foot
copper enameled wire to act as an antenna. However, the main problem was the range of the
transmitter and the ability for the radio to catch a clear signal. That said, the copper wire was
acceptable for consumer use.

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Ground problems:
There is a grounding problem with the transmitter and receiver because the devices use a
common ground with no ground source body touching the metal of the devices battery and audio
devices causing the circuit to short out. The solution was to use electric tape and isolate the
circuit from all metal contacts.
Prototyping & trade-off studies:
Prototyping:
The inductor coil had to be handmade, thus for this project an enameled copper wire was
used to craft it. The coil will be open air with 5 coils. With a diameter of 5mm and a length of
1cm.
𝐿 =
𝑁∗𝑁0 ∗𝑁𝑟 ∗𝐴
𝑙
𝐴 =
𝑝𝑖
4
𝑑2
𝑁 = 5𝑡𝑢𝑟𝑛𝑠 𝑙 = 1𝑐𝑚 𝑑 = 5𝑚𝑚 𝑓𝑐 =
1
2𝑝𝑖√ 𝐿∗𝐶
From the formula, the inductance of the coil is .05uH by using the current measurements, but
when using a multimeter, it was .15uH. The target frequency carrier is to be between 90-120Mhz
which on the radio is 90.0 -120.0 FM. So, my target capacitance based on the frequency equation
will be around 20-50 pF. Circuit shown below is the LC circuit used to obtain the carrier
frequency resonance.

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My audio signal needs an initial amplifier stage fulfilled by the IPod and the connection
to the base of the transistor. Which then passes to the frequency oscillator using a simple LC
loop and connected to my antenna to send out a signal. An example is posted below of a class A
single transistor amplifier which I used to boost my input signal for the FM transmitter. (1)
For the receiver, I used a 2 transistor Darlington pair connected to my demodulator LC
loop to increase signal strength to my op amp. My op-amp used a simple operational voltage
gain as seen below. Components are soldiered as close as possible to decrease interference.
The first attempt at prototyping failed as the printed boards were incorrect and a mistake
were made in the pin hole diameter resulting in the boards being discarded.
The second attempt at prototyping resulted in a burned transmitter due to bad soldering.
The final prototype alleviated prior issues and mistakes and was put out as the final product.

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Materials trade-off study:
The trade-off study of materials focused overall on affordability because they all follows
the same dimensions and tolerances of circuit components only varied by a small percentage of
tolerance. The bulk items costed less and were more reasonable to use in the project than to buy
the more expensive name branded products like Radio Shack or Fry’s. Headphone jacks were
cheaper if I used my own wires to it, and cheap small double sided protoboards.
My transistors the 2N3904 was a replacement when researching radio systems. A
transistor BF494 was recommended for the FM radio frequency from 90-150 MHZ, but the
2N3904 can go up to 270MHZ and many of hobbyists debated over efficiency problems. On the
other hand, the 2N3904 is more common than the BF494 audio transistor although it has been
discontinued.

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Subsystem Design:
PCB Design:
Transmitter:

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Receiver:
Block Diagram:

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Verification and Validation Test Plans:

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Project Status:
Project Schedule:
Cost Budget:

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Results:
The transmitter was tested with the variable capacitor to see if the transmitter can achieve
different frequencies. By adjusting the capacitance in the LC circuit resulting in the transmitter
achieving broadcast from 89.5 to 93.5 MHz or FM. The FM receiver achieved tuning to
frequencies between 105.5 to 107.5 picking up a blues station and a classic rock station. The
initial test had a potentiometer on the radio speaker which caused the speaker to output low audio
and increasing the resistance also reduced the volume. Results without the potentiometer causes
the speaker volume to be higher but due to the incoming signal not amplified enough caused the
speaker to have enough volume so the 8-ohm speaker can act as a headset speaker. Another
solution to have increased the sound was to use a larger speaker which in turn would be louder
but the use of the 8-ohm speaker may have a use which will be covered in future project ideas.

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Future Project Improvements:
There are many improvements that can be done which will mostly improve aesthetic
qualities and improve the quality of the signal being received and sent out by both devices. The
FM receiver can be improved with a body case like AM/FM sport headsets so a body for my
radio can be designed to hold the device in a casing attached to a headband while the speaker is
facing inwards towards a consumer’s ear.
To allow tuning with the current trimmer capacitors a mold of a shaft that will fit and turn
the small variable screw will need to be long enough from the capacitor to the outside of the case
for user access. A potentiometer paired with another op-amp to boost the signal for a much
higher voltage to the speaker will allow adjusting volume for the consumer. And to simplify the
battery add a simple switch so the battery doesn’t have to be removed from the 9v clip when the
device is no longer in use. Instead of the current wire antenna since time will not be a factor a
cheap but effective metal extending antenna can be bought for the device.
The FM transmitter can be improved as well by using a metal extending antenna. Like the
changes that would improve the FM receiver the transmitter would use a case and the battery
would need a switch for convenience. A shaft would need to be used for the capacitor to make
easy access and tuning.