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Improved Filter

IP.com Disclosure Number: IPCOM000040039D
Original Publication Date: 1987-Sep-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Ett, AH: AUTHOR

Abstract

The classical filter used for suppression of unwanted line-conducted emissions is an L-section filter consisting of an inductor and capacitance. When used with low impedance digital circuits such as those used for peripheral interface, these circuits "ring" on line level transitions. This "ring" produces a damped oscillation with a peak- to-peak amplitude of twice the normal logic level, centered about the new logic level. This "ringing" provides signals below ground The first method described is the use of a silicon diode across the inductor in the circuit. The circuit of the classical filter is illustrated in Fig. 1, and the circuit for the modified circuit is illustrated in Fig. 2, along with the input and output waveforms.

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Improved Filter

The classical filter used for suppression of unwanted line-conducted emissions is an L-section filter consisting of an inductor and capacitance. When used with low impedance digital circuits such as those used for peripheral interface, these circuits "ring" on line level transitions. This "ring" produces a damped oscillation with a peak- to-peak amplitude of twice the normal logic level, centered about the new logic level. This "ringing" provides signals below ground The first method described is the use of a silicon diode across the inductor in the circuit. The circuit of the classical filter is illustrated in Fig. 1, and the circuit for the modified circuit is illustrated in Fig. 2, along with the input and output waveforms. It is observed that the capacitor in the circuit becomes fully charged, from the positive logic level. When the input level shifts to low, the input end of the inductor is in effect transferred to ground, forming a parallel LC tank circuit. Since the capacitor is charged, this tank will produce a damped oscillation with the initial excursion being negative, with an amplitude nearly equal to the initial voltage across the capacitor. A similar condition exists on positive transitions except that the energy causing the oscillation is stored in the inductance as a magnetic field. By placing the diode across the inductor as illustrated in Fig. 2, it conducts as soon as its threshold is reached, dissipating the stored energy in th...