Constructing FM transmitters (89MHz - 109MHz)

This project provides the schematic and the parts list needed to construct FM transmitters with an operating frequency of 89MHz - 109MHz. You need a receiver to receive the signals from this transmitter. Typical radio tuned to FM range will be able to receive this signal.

A Frequency Modulated wave is a sine wave with a periodically varying instantaneous frequency and a constant amplitude. The average frequency is called the carrier frequency and the instantaneous frequency changes at the modulation frequency . The maximum excursion of the instantaneous frequency from the average is related to the modulation depth .

For a FM radio transmission, the carrier frequency would be the station you tune to, and you would hear a pure audio tone at the modulation frequency, with a loudness derived from the modulation depth.

Frequency modulation (FM) is the encoding of information in either analog or digital form into a carrier wave by variation of its instantaneous frequency in accordance with an input signal. This is typically accomplished using radio waves. The most typical use is radio broadcasting.

Frequency modulation requires a wider bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio". The FM modulation illustration is as shown in the diagram below.

When assembling the components into the PCB, be careful to cut the leads of components as short as possible because at high frequencies, leads will alter the capacitance and inductance of the circuits.

The output of the FM transmitters is approximately 9mW at 9V with the antenna tapped at position B. Tapping the antenna at position A will triple the range to 27mW.

You can find the CK217 9V FM Transmitters kit here under the SURVEILLANCE AND SECURITY (SPY) category.

ref: electronics-project-design.com/FMtransmitters.html