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Oscillating Notch Filter

IP.com Disclosure Number: IPCOM000087774D
Original Publication Date: 1977-Mar-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 2 page(s) / 43K

Publishing Venue

IBM

Related People

Palmer, RS: AUTHOR

Abstract

A notch filter (Fig. 1) comprises a bandpass filter 1 with a positive feedback loop through amplifier 2 and nonlinear limiting element 3 so that the loop oscillates at the pass band (notch) frequency. The limited feedback signal is summed with the input signal V(IN) in summing junction 4, and the resultant waveform is applied to junction 5. The output of bandpass filter 1 is subtracted from the output of junction 4 to produce a notched output signal V(OUT). A counter or frequency discriminator 6 can be added for notch indication or control purposes.

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Oscillating Notch Filter

A notch filter (Fig. 1) comprises a bandpass filter 1 with a positive feedback loop through amplifier 2 and nonlinear limiting element 3 so that the loop oscillates at the pass band (notch) frequency. The limited feedback signal is summed with the input signal V(IN) in summing junction 4, and the resultant waveform is applied to junction 5. The output of bandpass filter 1 is subtracted from the output of junction 4 to produce a notched output signal V(OUT). A counter or frequency discriminator 6 can be added for notch indication or control purposes.

In Fig. 2, the concept of Fig. 1 is implemented as a state variable notch filter comprising high pass stage 10, bandpass stage 11, and low pass stage 12. The bandpass and low pass stages 11 and 12 are integrators, and the high pass stage 10 consists of a difference amplifier to which the low pass output is fed back. A tuning stage 13 determines the notch frequency Omega(o) by means of an FET (field-effect transistor) 14 to which a notch control signal is applied. Finally, the notched output signal

A positive feedback loop is formed by nonlinear diode stage 16, having the characteristic shown, and a notch width determining stage 17. The output and input of the notch filter are related by the expression:

(Image Omitted)

where Omega(o) is the notch frequency, 2 Delta Omega(o) is the notch width, and * = 0 or 1 for small and large amplitudes, respectively. When * = 0, the circuit oscillates at not...