High-speed notch filters

Introduction Active notch filters have been used in the past for applications like elimination of 50and 60-Hz hum components. They have proven to be somewhat problematic from the standpoints of center frequency (f0) tuning, stability, and repeatability. The advent of high-speed amplifiers opens the possibility of higher-speed notch filters—but are they actually producible? This article will show what is presently possible and what design trade-offs a designer will face with real-world components. As a review, the reader should remember some characteristics of the notch filter: • The depth of the notch obtainable in simulations like that shown in Figure 1 is not the depth that can be achieved with real-world components. The best that the designer can hope for is 40 to 50 dB. • Instead of focusing on notch depth, the designer should focus on center frequency and Q. The Q for a given notch filter is the –3-dB point, not the notch depth or a point 3 dB above the notch depth, as shown in Figure 2. Remember that the designer’s objective is not a notch filter but the rejection of a specific interfering frequency. Any filter that does not reject that interfering frequency because it misses the frequency or has too little rejection at that frequency is not much use. The best way to avoid missing the interfering frequency is to select the best values of R and C from the start. The RC Calculator under “Filter Design Utilities” in Reference 1 should be used to find the correct values of R0 and C0 for the circuits in the following discussion.