FM Remote Speaker System

A high quality, noise free, wireless FM transmitter/receiver may be made using the LM566 VCO and LM565 PLL Detector. The LM566 VCO is used to convert the program material into FM format, which is then transformer coupled to standard power lines. At the receiver end the material is detected from the power lines and demodulated by the LM565.

National Semiconductor Application Note 146 June 1975

The important difference between this carrier system and others is its excellent quality and freedom from noise. Whereas the ordinary wireless intercom uses an AM carrier and exhibits a poor signal-to-noise ratio (S/N), the system described here uses an FM carrier for inherent freedom from noise and a PLL detection system for additional noise rejection.

Carrier System Transmitter

Two input terminals are provided so that both LEFT and RIGHT signals of a stereo set may be combined for mono transmission to a single remote speaker if desired. The input signal level is adjustable by R1 to prevent overmodulation of the carrier. Adding C2 across each input resistor R7 and R8 improves the frequency response to 20 kHz as shown inFigure 5. Although casual listening does not demand such performance, it could be desired in some circumstances.

Carrier System Receiver

The receiver amplifies, limits, and demodulates the received FM signal in the presence of line transient interference sometimes as high as several hundred volts peak. In addition, it provides audio mute in the absence of carrier and2.5W output to a speaker