Full-color, full-resolution graphics from the pages of QRP Homebrewer

QHB #10 -- April 2003

 

Tunable Audio Filter -- Another "20 Minute Project!", by Phil DeCaire, WB7AEI

Figure 1 Basic Active Bandpass Filter Circuit

Figure 2 Complete Active Bandpass Filter Schematic

This plot is interesting, because it shows how Q and frequency track each other, resulting in a constant bandwidth regardless of frequency setting. It also shows that gain remains constant as the frequency is tweaked. It is also obvious that the peak frequency varies inversely with the square root of R1b. This means R1b should be an audio taper pot rather than linear for smoothest adjustment.

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