Special band-stop filter: Double T filters are used in many 11 circuits. This type of filter can be made into a perfect band-stop filter, at least in theory. In the present circuit (Fig. 1), a double T filter is used in a different manner. It can have the characteristic of the combination of either a band-stop filter and a low-pass filter (switch S1 in position 1) or a high-pass filter and a band-stop filter (S1 in position 2). The characteristic curves are given in Fig. 2 and 3 respectively.
A band Stop Filter known also as a Notch Filter, blocks and rejects frequencies that lie between its two cut-off frequency points and passes all those frequencies on either side of this range.
The Band Stop Filter, (BSF) is another type of frequency selective circuit that functions in exactly the opposite way to the Band Pass Filter we looked at before. The band stop filter, also known as a band reject filter, passes all frequencies with the exception of those within a specified stop band which are greatly attenuated.
If this stop band is very narrow and highly attenuated over a few hertz, then the band stop filter is more commonly referred to as a notch filter, as its frequency response shows that of a deep notch with high selectivity (a steep-side curve) rather than a flattened wider band.
The variable Kin Fig. 2 and 3 is determined by the setting of P1. Variable M depends on the setting of P2 and determines the Q factor. A drawback of the present design is that the maximum suppression is diminished slightly. At M= 0.75, maximum attenuation is (calculated) 50 dB. With component values as in Fig. 1, the frequency of that attenuation is I kHz. Other frequencies can be computed easily. The band-stop frequency, ƒbs, with S1 in position 1 is
ƒbs = 1/2Π RCK
With S1 in position 2,
ƒbs = K/2Π RC.
In all cases, 0< k <1; C is in farad and R is in ohms. Various types of IC may be used for the operational amplifier.
The current drawn by the circuit is around 2 mA.
