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FM Radio Transmitters

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Jaclyn Wilson

on 5 June 2014

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Transcript of FM Radio Transmitters

Built a "Super Simple FM Transmitter" using instructions from makezine.com
Researched different components of the transmitter

FM Dipole Antenna
orientation of waves
natural light is unpolarized
three main types
Field Regions
FM Radio Transmission
Capacitor Function and Capacitance
Valuable because of their ability to store charge
Become charged when other circuit elements create a potential difference across it
Capacitance is property of every capacitor
Geometric property of the capacitor.
Inductor Function in Circuit
Stores energy in a magnetic field which is generated by the presence of the current flow through the inductor (Faraday's Law of Induction)
Stores energy for the uses of different parts of the circuit
An important part of the resonator (RLC circuit) that varies frequencies of output wave signal
(discussed shortly)
Resonant Circuit (RLC circuit)
Essentially the main part of a resonator which includes resistor, inductor and capacitor in parallel
Makes output wave signal adjustable
Resistor should be kept in a small range in order not to cause difficulty for current to oscillate
Huy Tran, Gabrielle Glynn, Steven Smith, Jaclyn Wilson
Team Soundwave
Charge on a Capacitor
Charge stored depends on the capacitor and the circuit
Parallel-plate capacitors are the simplest and most widely known type of capacitor
A capacitor can be made from any two arbitrary electrodes.

Energy in Capacitors
Capacitors are also valuable in their ability to store energy in a circuit
Potential energy increases as more charge is added
Or Uc=.5C(deltaV)^2.

Capacitor Potential Energy vs Spring Potential Energy
k=spring constant
deltax=distance from equilibrium
Energy in Capacitors
The stored energy in a capacitor is held in the electric field between the electrodes
Capacitors are a good source of charge due to their ability to discharge at nearly instantaneous rates.
Why Does this matter?-Capacitors in our Circuit
• Our radio transmitter contains five capacitors:
one 1-33 microfarad electrolytic capacitor (C1)
two 0.01 microfarad ceramic disk capacitors (C2 and C3)
two 10 picofarad ceramic disk capacitors (C4 and C5).

Our actual circuit as well as the circuit diagram is pictured below with the five capacitors indicated.
Capacitors are represented in circuit diagrams as two parallel lines.
• the capacitors used in our circuit are AC (alternating current) ceramic capacitors and an electrolytic cap
Capacitors in Our Circuit
Capacitors in the Resonance Circuit
A vital part of the fm transmitter is its resonance circuit
Resonance circuit fixes the sensitivity and selectivity of the radio
Purpose: use a little bit of energy over and over with intentional timing
Capacitor functions as the electromagnet
Capacitors and Alternating Current
Current is created one direction through the coil
Process is reversed, opposite current created
Changing the capacitance changes the frequency of the transmitter
Capacitance can be changed by using different capacitors, or playing in series or parallel
The Electrolytic Capacitor
Has largest capacitance of all our capacitors
C1 in circuit diagram
Capacitance is increased through the use of dielectrics.

Electrolytic Capacitor
• Purpose: set the frequency of the oscillator, so a certain frequency can be expected and picked up on a receiver
• Placed at the audio input to transfer the oscillations from the audio source into the transistor

Ceramic Disk Capacitors
This includes all the other capacitors: C2, C3, C4, and C5
Ceramic disk capacitors also use dielectrics
Voltages are low, does not significantly change the voltage of the circuit

Purpose: to balance the circuit to ground
When the voltage of the circuit is kept balanced, the balanced portions of the circuit are all at the same voltage

C2, C3, and C4
C1 and C5
The only capacitors in the circuit that do not balance to ground
Filter the DC current
Also set the oscillation for the system
Capacitors set the oscillation for the entire transmitter.

Capacitors in Radio Transmitters
General function of the transmitters: short circuit to high frequencies
Transmitter must oscillate at a high frequency
Frequency is proportional to current, need higher current to produce higher frequency
Overall, capacitors are vital in:
Establishing AC current and frequency
Balancing the circuit to ground
Filtering out DC current and low frequency

Capacitors in Circuit
Linear Polarization
Circular Polarization
Elliptical Polarization
Near Field
Near Field Communication (NFC)
reactive region
Fresnel region (radiative)
Most commonly utilized field
EM Modeling (radiation patten)
Far Field
3D Model: Dipole Antenna
Radiation (Far field) Pattern
FM frequency band: 88 to 108MHz
Centered in FM frequency band
1/4 Wavelength: c/(4*f)
300/(4*(88+(108-88)/2))) = 7.65 meters
ratio of power produced in far field to power produced by hypothetical lossless isotropic antenna in the direction of maximum radiation
dBi ("decibels-isotropic")
How directional an antenna is relative to a hypothetical isotropic antenna.
How RLC circuits work
How RLC circuits work
As soon as one cycle is completed, another cycle occurs.
The process repeats until the system runs out of energy.
It seems like there is a sine wave which propagates from the circuit as long as oscillating current exists
Our FM model has more complicated design; based on this idea, its RLC circuit generates wave signal
Frequency and Inductance
Factors Affect Inductance
Radius of coil
Permeability of the core that the coil wound around
Length of coil
L (μH) = (4π∙#Turn∙#Turn∙A∙μ)/(10 length)
Decrease inductance by making coil's radius smaller, then the new magnetic field is stronger.
=>Inductor has better ability to store energy in this magnetic field
=>Wave signal travels with higher frequencies.
Change physical appearance results in inductance change
Frequency and Inductance
Increase inductance by shortening the length of coil significantly
=> Larger amount of energy is transferred back and forth in the same period of time
=>Wave signal is able to travel further, wavelength increases
=> Frequencies decreases
"Decreases inductance, increases frequency"
"Increase inductance, decrease frequency"
What is an Inductor?
A coil of wire wound around air or cylindrical core
Inductance represents how good an inductor is; measured in Henry
Inductor works effectively when there is a non-constant current flowing through
L = 0.1316 μH
V = L (di/dt)
What Are Radio Waves?
Part of the electromagnetic spectrum
Longest wavelengths
Occur naturally and artificially
Naturally Occurring
Astronomical bodies
Radio telescopes used to study composition, structure, and motion, must be large to get useful resolution
Cell Phones (824-849 megahertz)
Baby monitors (49 megahertz)
Garage Door Openers (about 40 megahertz)
GPS (1127-1575 megahertz)
Deep space radio communication (2290-2300 megahertz)
FM/AM Radio
Man-Made Radio Devices
Regulation in the United States
Federal Communication Commission (FCC)
National Telecommunications and Information Administration (NTIA)
FCC Office of Engineering and Technology (OET) maintains a table of frequency allocations
Only frequencies between 9 kilohertz to 275 gigahertz currently allocated
Bibliography (continued)
How Do Radio Waves Transmit Information?
Transmitter will convert info to sine waves and broadcast using radio waves
Receiver picks up wave and decodes it
Creating Sine Waves: Radio Transmission
Rapidly vary electric current in a wire using an oscillator circuit
Oscillator circuit comprised of and inductor and capacitor
Energy will continuously move back and forth between the two
Pulse Modulation
Amplitude Modulation
Frequency Modulation
Created by turning wave on and off
Signal itself does not contain and info, but can be use for Morse code
Imposes a sine wave produced by information being sent on transmitter's sine wave
Used for AM radio and picture portion of TV signals
Varies frequency
FM radio, sound portion of TV signals, cell phones, many other wireless devices
Decoding Sine Waves: Radio Receivers
Most radio receivers are made to tune into a specific range of frequencies
Receiver uses an antenna (increases metal signal can interact with) and a tuner (create resonance) to pick up a frequency
Signal is then decoded with a demodulator, and amplified
AM and FM Radio
FM radio bandwidth (88 to 108 megahertz)
AM radio bandwidth (540 kilohertz to 1.7 megahertz)
FM has shorter wavelengths, travel shorter distances, less vulnerable to interference, has higher frequency range (30 hertz to 15 kilohertz compared to AM's 200 hertz to 5 kilohertz)
Our FM Transmitter Circuit
Based on design popularized by Tetsuo Kogawa
Instructions from Sean Ragan from makezine.com
Should transmit a monaural signal up to 30 feet away
Unable to successfully pick up a signal in the FM range, too complicated to calculate exact frequency
Use receiver with higher range
Modify circuit (vary length of coil), exact resistance, variable capacitor
Project still helped us to gain insight into how radio waves are used to transmit information, learned practical circuit construction
Battery clip
Phone plug, mono, 1/8 inch
Stranded copper wire, 22 gauge
2 capacitors (0.01 uF)
2 capacitors (10 pF)
Capacitor (1 uF - 33 uF)
9V battery
Solid copper wire, 18 gauge
Resistors (470, 10K, 27K ohms(22K + 4.7K))
Copper clad board (ground, nodes)
Soldering iron and solder
FM receiver
Wire strippers
Audio Source
Utility knife
1/4-20 bolt

radiate (or absorb) EM waves between the transmitter (or receiver) and free space
directs and polarizes
Radiated Effectivity: Ratio of radiated power to input power

Total effectivity: Product of radiated effectivity and antenna's loss due to impedance mismatch

RLC Circuit/Alternating Current
Full transcript