Ateneo de Naga UniversityCollege of Engineering

ECE and CPE DepartmentS/Y 2014-2015

Project in ECEM416LABFM TRANSMITTER

September 29, 2014

Prepared by:John Kristoffer E. Villareal

BS ECE - GE41

Submitted to:Engr. Gilbert Detera

I. Introduction

A radio transmitter is an electronic circuit which transforms electric power from a battery or electrical mains into a radio frequency alternating current, which reverses direction millions to billions of times per second. The energy in such a rapidly-reversing current can radiate off a conductor (the antenna) as electromagnetic waves (radio waves). The transmitter also impresses information, such as an audio or video signal, onto the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver, the waves excite similar (but less powerful) radio frequency currents in it. The radio receiver extracts the information from the received waves. A practical radio transmitter usually consists of these parts:• A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output.• An electronic oscillator circuit to generate the radio frequency signal. This usually generates a sine wave of constant amplitude often called the carrier wave, because it serves to "carry" the information through space. In most modern transmitters this is a crystal oscillator in which the frequency is precisely controlled by the vibrations of a quartz crystal.• A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave. The information is provided to the transmitter either in the form of an audio signal, which represents sound, a video signal, or for data in the form of a binary digital signal.• In an AM (amplitude modulation) transmitter the amplitude (strength) of the carrier wave is varied in proportion to the modulation signal.• In an FM (frequency modulation) transmitter the frequency of the carrier is varied by the modulation signal.• In an FSK (frequency-shift keying) transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two binary digits, 0 and 1.Many other types of modulation are also used. In large transmitters the oscillator and modulator together are often referred to as the exciter.• An RF power amplifier to increase the power of the signal, to increase the range of the radio waves.• An impedance matching (antenna tuner) circuit to match the impedance of the transmitter to the impedance of the antenna (or the transmission line to the antenna), to transfer power efficiently to the antenna. If these impedances are not equal, it causes a condition called standing waves, in which the power is reflected back from the antenna toward the transmitter, wasting power and sometimes overheating the transmitter.

In higher frequency transmitters, in the UHF and microwave range, oscillators that operate stably at the output frequency cannot be built. In these transmitters the oscillator usually operates at a lower frequency, and is multiplied by frequency multipliers to get a signal at the desired frequency.

II. ObjectivesThis project, FM Transmitter, aims to:

To be able to generate and transmit voice signals as well as audio signals of the correct frequency at the desired point without interfering other radio stations.

To be able to modulate any inputted baseband signals through frequency modulation of the distinguished carrier signal.

To be able to develop an FM transmitter that is capable of transmitting audio and voice signals for over a distance of 20 meters. This will be actually take place at the Fourth Floor of the Dolan Building down to the Philosophy Department at the Ground Floor.

III. MethodologyThis section seeks to show the establishment of the design parameters and

to observe the followed design procedures. It also seeks to explain the entire circuit as a whole, including its parts and purposes. The design procedure is the one who determines the values of the components like the capacitor or the resistor to be used.

a. Description

The range of this FM transmitter is around 100 meters at 9V DC supply. The circuit comprises three stages. The first stage is a microphone preamplifier built around BC548 transistor. The next stage is a VHF oscillator wired around another BC548. (BC series transistors are generally used in low-frequency stages. But these also work fine in RF stages as oscillator.) The third stage is a class-A tuned amplifier that boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter.

b. Frequency Selection

The process of selecting the desired frequency was one of the things that needed careful planning and analytic considerations. The

frequency to be used was based in the radio stations located in our province and also their respective time of operation in Camarines Sur,

namely:

The following reasons were considered in the decision of the desired frequency to be used:

Should avoid FM stations that interfere with the transmission at the area around Dolan Building.

Should be according to the frequencies that is not occupied in the digital receiver to be used.

Should be near the frequencies used by the others.

In the process of careful tuning and determining the right frequency, the frequency to be used was at the range of 88-91MHz. More specifically, the one used in these project was 90 MHz.

c. Circuit Diagram

In this circuit from www.extremecircuit.net, all other parameters including the resistors, capacitors and transistors has been specified except for the coil.

Coil L1 comprises four turns of 20SWG enameled copper wire wound to 1.5cm length of a 4mm dia. air core. Coil L2 comprises six turns of 20SWG enameled copper wire wound on a 4mm dia. air core. Use a 75cm long wire as the antenna. For the maximum range, use a sensitive receiver. VC1 is a frequency-adjusting variable capacitor. VC2 should be adjusted for the maximum range. The transmitter unit is powered by a 9V PP3 battery.

Also, the audio input jack was put in parallel with the position of the microphone and is operated with a switch.

IV. Sample Computations

a. INDUCTANCEParameters:N(number of turns) = 4,6

l=0.015mA=π(2mm)2

μ=4 π ×10−7H /m

L= N2μAl

L=(42)(4π ×10−7 H

m )(π (2mm )2)

0.015m

L1 = 16.8441 nHL2 = 37.8992 nH

b. CAPACITANCEParameters:f = 90 MHzL1 = 16.8441 nH

f= 12π √LC

C= 1

L (2πf )2

C= 1

16.8441nH (2 π (90M ))2

C=18.28007 pF

This was the capacitance value at the oscillator circuit. We assumed that this was achieved by adjusting the variable capacitor.

V. PCB Layout

VI. Related Literarturea. Data Sheets

BC548

b. 2SC2570

VII. Photos

The actual transmitter (microphone, antenna and input jack are visible).

Inside of the casing (actual circuit and the battery are visible).

VIII. Observation and AnalysisIn class discussions, characteristics of an FM transmitter was already

tackled. Hence, through this project, it is now time to know and test if the basis of

this facts are really much effective with most circuits design.

FM transmitters typically offers some advantages or benefits over AM

transmitters. Although, at the beginning I can say that the circuit is somewhat

complex, and it even required three stages built around a transistor. But, the first

thing I saw was its “noise immunity” at the receiving end. Perhaps, in the whole

process of testing, when the receiver is at the proper frequency of the transmitter

and the transmitter is already properly calibrated, the audio/voice signals

transmitted is just like the original one thrown by the one who talks and will now

depend on the quality of the receiver used.

Second, is its somewhat rejection of the interfering signals because of the

phenomenon of capture effect. The transmitter used in this project is only a low

power one, and the other broadcast stations doesn’t really follow the standards set

by NTC, so this is a somewhat a fifty-fifty condition. During the testing process,

we had tried several time frames to observe if which conditions satisfy most the

need for proper transmission. During night and early mornings, the above table

for the supposed to be transmission of the broadcast stations are not really strictly

followed. As a result, the entire range of FM is covered by a certain station. Now,

the transmitted signals from the project is sometimes interfered by these

frequencies and is sometimes overpowered at certain scenarios. So, I prefer that

testing is to be done from early afternoons until six in the evening. At this time

frames, minimal interferences has been obtained. Thus, transmitting audio and

voice signals is a much easier task.

Now, considering that the transmitting distance is somewhat far, and the

angle of transmission not being in the best case scenario for a 1 watt FM

transmitter. Although, a class A amplifier was really used instead of a Class C,

perhaps, when we talk about efficiency, the project is considerably okay with it.

IX. RecommendationsThis project, overall, has good rating. But then, when we look at it in a more strict sense, it is somewhat not yet perfect and should need to be improved. For instance, since the FM signal transmitted is constant in amplitude, and therefore, it is not really necessary to use linear amplifiers to increase its power level. It is suggested that a class C amplifier should be used to have a better transmitting efficiency and for the battery to even last for a longer period of time. Also, proper installation of the input jack should be followed because it is very observable during the testing proper that improper plugging leads to very unlikely audio result.X Sources:www.extremecircuits.nethttp://www.eeweb.com/blog/extreme_circuits/medium-power-fm-transmitterhttp://www.asiawaves.net/philippines/camarines-sur-radio.htm#camarines-sur-fm-radio