National Semiconductor Application Note 147 Jim Sherwin June 1975

A preassembled front-end was selected as the cost-effective approach to minimum parts count and minimum factory adjustments. This model features an FET input stage providing excellent distortion performance. High selectivity is obtained through the use of two cascaded ceramic filters yielding an approximate 6-pole response with less than 12 dB insertion loss.

The LM3089 FM IF System does all the major functions necessary for FM processing, including a three stage amplifier/ limiter and balanced product detector, as well as an audio preamplifier. A single quadrature coil was used for ease of ailgnment; yielding recovered audio with THD less than 0.5%, however a double coil may be used to diminish THD to 0.1% if required. Carrier level detectors provide delayed AGC, SIGNAL strength meter drive, and adjustable interstation mute control R11. The internal AFC amplifier was used to drive the TUNING meter, giving a visual indication of center tuning.