Click here to purchase the entire book in PDF format. Notch Filter There is a special breed of band-reject filter that is designed to have almost infinite attenuation at a single frequency, leaving all others intact. This, of course is impossible, but we can come close. If we have a band-reject filter with a very high Q, the result is a frequency response like the one shown in Figure 6.5. The shape is basically a flat frequency response with a narrow, deep notch at one frequency - hence the name notch filter Figure 6.5: The frequency response of a notch filter with a centre frequency of 1 kHz.