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Zero Phase Shift Variable Frequency Selective Filter

IP.com Disclosure Number: IPCOM000073784D
Original Publication Date: 1971-Feb-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Belman, HJ: AUTHOR [+2]

Abstract

The circuit shown utilizes a phase-locked oscillator to extract a low-level sine wave Fs from an input signal containing other higher-level components without introducing a phase shift. The input signal is applied at input 10. A control signal is generated at phase detector I2 which after being filtered in low-pass filter 14 controls the frequency of voltage-controlled oscillator 16. Oscillator 16 runs at four times the frequency Fs of the sine wave being detected. Flip-flop 18 divides the frequency of oscillator 16 by two and generates two outputs of opposite phase. The first output is applied to the AC input of the flip-flop 20 and the second output is applied to the AC input of flip-flop 22.

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Zero Phase Shift Variable Frequency Selective Filter

The circuit shown utilizes a phase-locked oscillator to extract a low-level sine wave Fs from an input signal containing other higher-level components without introducing a phase shift. The input signal is applied at input 10. A control signal is generated at phase detector I2 which after being filtered in low-pass filter 14 controls the frequency of voltage-controlled oscillator 16.

Oscillator 16 runs at four times the frequency Fs of the sine wave being detected. Flip-flop 18 divides the frequency of oscillator 16 by two and generates two outputs of opposite phase. The first output is applied to the AC input of the flip-flop 20 and the second output is applied to the AC input of flip-flop 22. Flip-flops 20 and 22 again divide the frequency of oscillator 16 by two but, flip-flop 20 is 90 degrees out-of-phase with flip-flop 22. A phase-locked oscillator normally runs at the same frequency as the reference signal Fs at the input but lags by 90 degrees. Since, flip-flop 22 is 90 degrees out-of-phase with flip-flop 20, the outputs of flip-flops 22 are in-phase with the low-level sine wave at the input.

To convert the output of flip-flop 22 to a sine wave, a sine-wave filter 30 is provided. Sine-wave filter 30 inherently introduces a phase delay Theta F which is not constant over the wide range of frequencies that the phase-locked oscillator is capable of following.

This phase delay Theta F also causes the signal a...