Design and Construction of A Low-Power Multi-channel FM Transmitter Abstract The main aim of the project is to develop a miniaturised low power FM transmitter to be used in specialized application such as hearing aid for a tour guiding system, room monitoring (such as baby device), small area security system and entertainments. No license is required for this low power- range limited transmitter designed according to FCC regulation. Frequency modulation has several advantages over the system of amplitude modulation (AM).

The most important is that an FM has greater freedom from interference and static. Various electrical disturbances such as those caused by thunderstorms and car ignition systems. A well designed FM receiver is not sensitive to such disturbances when it is tuned to FM signal of sufficient strength. Also, the signal-to-noise ratio is much higher than AM. World’s broadcast band falls within the VHF part of the radio spectrum. Commercial FM radio stations are assigned frequencies between 87. 5 to 108 MHz.

The whole report will reflect on 3 issues, background of frequency modulation, basic transmitter building blocks and its circuit component characteristics and finally an analysis of the finished design as regards construction and performance. Typical models have been considered, which would be suitably applied. The final design and its parameters may differ with some little modification according to the market availability of components and instruments to be used. Transistor Circuit Design:

This two transistor simple FM transmitter is built around two amplifiers, Q1 is a common emitter with a dc gain of 1 and Ac gain that can be set by potentiometer R4 and this will amplify the signal from the electret and pass it to the next stage by coupling capacitor C3. Circuit diagram of a two transistor FM Transmitter: Q2 is the heart of the RF section, because of C4 (which ac grounds the base) and feedback cap C7 (that splits the capacitance C5 AND C6) the RF section is a colpitt oscillator in the common base mode . The inductance L. var and the effective capacitance C5 and C6 work out the center frequency.

The base collector junction capacitance (which acts like a varactor diode) is varied as the amplified base band signal changes it’s reverse bias voltage, this capacitance will inevitably be a part of the overall tuning capacitance of the resonant tank circuit. The antenna (very short end fed wire) can be resistively matched by an ordinary low- value resistor. This is quite effective little transmitter that can be easily made and has a range of about 60 feet indoors. [N. B. This circuitry here given may face some improved modification, changes in parameter and its components value based on future prospect and time. ]