For a lowpass prototype of order N, the bandpass filter has order 2N and requires N biquads. The function biquad_bp (listed in the Appendix) computes biquad coefficients and gains given the order of the lowpass prototype, center frequency, bandwidth, and sampling frequency. There can be one or more cascaded biquad sections in a bandpass filter. It is defined by two feedback coefficients a, three feed-forward coefficients b, and gain K. This becomes important when designing narrowband filters.Ī biquad section block diagram using the Direct Form II structure is shown in Figure 1. Compared to conventional implementations, bandpass filters based on biquads are less sensitive to coefficient quantization. This post provides a Matlab function to do the same for Butterworth bandpass IIR filters. In an earlier post, we implemented lowpass IIR filters using a cascade of second-order IIR filters, or biquads.
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