Advanced FM-IF

Abstract

The trend to electronically-tuned radios (ETRs) has placed new requirements on FM-IF and detector circuits. A new high performance FM-IF integrated circuit* has been developed for use in ETRs as well as traditional radio designs. This circuit has been designed to reduce external components while improving the overall radio performance. For example, this I.C. implements a new quadrature detector which not only reduces distortion but broadens the off frequency distortion characteristic using only a single- tuned quadrature coil. This type of performance was previously only attainable, with a double-tuned quadrature coil which has proved costly and difficult to adjust in production. Further improvements include a dual threshold AGC which eliminates the need for a local/distance switch and offers improved immunity from third order intermodulation products due to tuner overload. The signal level stop function has been simplified and improved by providing a user adjustable signal level stop threshold, controlled either by ultrasonic noise in the recovered audio or by the meter output. In order to eliminate still more external components this IC utilizes an on-chip buffer, which properly terminates two ceramic filters and provides gain to make up for losses in these filters.