Brick-wall lowpass audio filter needs no tuning

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.

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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