2007/04/15 ELEN 4314 Communications Circuits - Design Project A Three Transistor Discrete FM Transmitter
Russell Mohn, Member, IEEE
Abstract This paper explains the design and construction of a simple FM transmitter. The transmitter is made from identical discrete bipolar junction transistors. Continuous tuning is achieved with a variable capacitor. The designed tuning range was 88MHz to 108MHz. The transmitter can transmit music on a 91.7MHz carrier over a distance of 50feet.
I. Introduction Frequency modulation (FM) is a technique for wireless transmission of information where the frequency of a high frequency carrier is changed in proportion to message signal which contains the information. FM was invented and developed by Edwin Armstrong in the 1920’s and 30’s. Frequency modulation was demonstrated to the Federal Communications Commission (FCC) for the first time in 1940, and the first commercial FM radio station began broadcasting in 1945. FM is not a new concept. However, the concept of FM is essential to a wide gamut of radio frequency wireless devices and is therefore worth studying in 2007.  The objective of this paper is to explain the design and construction of a simple FM transmitter. The salient specifications for the transmitter are as follows: one 9V supply, DC power consumption less than 80mW, transmit frequency at a vacant spot in FM band (88MHz ..108MHz), transmitted power not to exceed 10uW, and the only active device type allowed is the 2N2222A NPN BJT. In addition, a pre-emphasis filter is required with a 3dB corner frequency equal to 1 / 75us. A variable capacitor with value 4pF .. 30pF is used for tuning. The design has been simulated and constructed. The built transmitter can transmit music at a distance of 50 feet. This paper will explain the design decisions that were made in the process of design and construction.
II. System Design A