Brick-wall lowpass audio filter needs no tuning

Posted on Dec 3, 2012

The filter should provide adjustable gain to maximize SNR at the audio processor's first stage. The filter's frequency response should also include a notch at 19 kHz to achieve maximum attenuation at the FM-subcarrier pilot-tone frequency and thus minimize phasing problems. To reduce manufacturing costs, the filter should require no in-process adjustments. Conventional analog active-filter designs cannot meet these goals at reasonable cost and complexity without time-consuming adjustments. This Design Idea outlines an active-filter-synthesis approach that reduces a filter's sensitivity to passive-component tolerances and enables construction of inexpensive, high-order and highly selective filters.

Brick-wall lowpass audio filter needs no tuning
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When a system's specifications call for a lowpass filter with a steep frequency-cutoff characteristic, an engineer can opt for a "brick-wall"- filter design that features a sharp transition band. For example, in an FM stereophonic-broadcast system, the lowpass filter in the baseband audio's left and right channels should have a –3-dB cutoff frequency of at least 15 kHz, a passband ripple of less than 0.3 dB, a stopband start frequency of at least 19 kHz, a stopband attenuation greater than 50 dB, and identical phase response for both channels.

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