Introduction |
A Band-Reject Filter, or Notch Filter, has a single stop-band (ω1 < ω < ω2) and two pass-bands (0 < ω < ω1 and ω > ω2). The frequencies ω1 and ω2 are the cutoff frequencies (the point at which the amplitude equals 1/√ 2 = 0.707 times its maximum value).
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2nd Order Active Band-Pass Filter (MFB/VCVS) |
The figures on the left and right show the equations used for the MFB and VCVS calculations.
When you are using standard 1%, 2%, 5% or 10% resistors, the filter parameters will be slightly different than specified. The center frequency (F0) may be slightly higher or lower than intended. And/or the Gain might be slightly higher or lower than intended. For the MFB filter, these parameters (F0 and Gain) can be fine tuned using an adjustable resistor for R1 and R2, respectively. To do this, reduce the resistor's value by about 10% and then add a variable resistor in series with that resistor. The value of the variable resistor should be about 20% of the initial resistor value. This should provide you with about a +/- 10% adjustment to the resistor value.
As an example, suppose that a calculated value for R2 is 1,378Ω, and a 1.3KΩ resistor (5%) is selected. If you then replace that resistor with a 1.2K resistor (next lower standard 5% resistance) and a 200Ω variable resistor in series. This should provide you with ample range to adjust the filter's center frequency (F0). A similar approach could be used to adjust the Gain (using R1) to a more precise value, should the need arise.
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