Subsidiary communication authorization (SCA) radio turner

Abstract

A complete process is described for demodulating subsidiary communications authorization (SCA) radio signals. This process takes as an input an FM radio signal from a commercial FM telescoping type antenna. The signal is optimally processed through filtering, amplification, and dual detection circuitry to provide a high quality sub-carrier audio signal. Special matching is provided to set the signal's gain, noise figure, and injected distortion levels as a function of the antenna being extended or collapsed. Also, filtering of RF image and IF baseband signals is uniquely and efficiently performed in a radio receiver having an telescoping antenna and audio out line, an SCA radio turner has a matching filter between the antenna and a first FM discriminator to match the level of extension of the antenna, a phase-lock-loop circuit within a second FM discriminator following the first FM discriminator, and sub-carrier audio processing between the FM discriminator and the audio out line. A filter for image rejection is included between the matching filter and the first FM discriminator. The first FM discriminator includes a local oscillator and mixer to convert the incoming RF signal to an IF signal at approximately 10.7 MHz and a square wave detection circuit to convert a main broadcast signal into an audio signal and a sub-carrier signal from a doubly modulated signal to a singly modulated signal.

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