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Proposed technologies for increasing FM selectivity do not address all relevant performance parameters or manufacturing issues. Switching to a narrow ceramic filter has drawbacks. The extra narrow-band filter must be carefully selected to match the rest of the channel, taking into account tolerances of other filters and the PLL crystal reference. In the narrow state, THD is high and data on ultrasonic sub-carriers is lost. Furthermore, switching back and forth between the two states causes audible disturbances. The digital filter solution has good static selectivity, but discontinuity in bandwidth control limits its perceived dynamic selectivity. This integrated time-continuous adaptive filter has instantaneous bandwidth determined by all relevant system parameters. The combination of filter structure and bandwidth-control algorithm delivers higher dynamic selectivity, increased sensitivity and low THD at high frequency deviation without audible artifacts. The automatic alignment of center frequency eliminates IF channel tolerances and makes it suitable for global receiver applications.
Date of Conference: 9-9 Feb. 2000