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Understanding Digital Signal Processing, Third Edition by Richard G. Lyons

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13.34 Filtering Narrowband Noise Using Filter Nulls

Here we present two filter design tricks that take advantage of the frequency-domain magnitude nulls of simple FIR filters. These schemes are particularly useful when used in AM and FM demodulation systems.

The first example uses a simple 3-tap nonrecursive FIR filter. Such a filter can be both computationally efficient, and useful, for narrowband-noise reduction. Here’s how. Think about the x(n) time-domain signal, contaminated with high-frequency noise, shown in Figure 13-82(a) with its spectrum provided in Figure 13-82(b). The sample rate of the signal is 10 kHz. Let’s assume we want to recover the low-frequency signal of interest (centered at zero Hz) without inducing phase distortion, ...

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