NEW TUNING SYSTEM FOR USE IN AM STEREO RECEIVERS.

The system, termed a tuning stabilizer, is a form of frequency-locked loop (FLL) with two modes of operation. In the tuning mode, the loop utilizes the existing tuner oscillator coil as a frequency discriminator to generate a dc voltage that is used to steer a voltage-controlled oscillator (VCO). The output of the oscillator is applied to the discriminator closing the loop, and forcing the VCO to track the resonant frequency of the tuner oscillator coil. It is the output of this oscillator that is used as the receiver heterodyne oscillator. On acquiring a usable carrier, the system switches from the previously described tuning mode to a tuned mode. In this mode, the FLL steering voltage is derived from the error voltage line of the AM stereo decoder PLL, forming an automatic frequency control. Thus, with the addition of one IC, a small number of external components, and no additional adjustments, a conventional manually-tuned AM receiver can achieve the noise and tuning accuracy of the best electronically-tuned receivers.