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SPLIT 3-POLE ACTIVE FILTER WITH LOW RF SUSCEPTIBILITY

IP.com Disclosure Number: IPCOM000005443D
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 2 page(s) / 70K

Publishing Venue

Motorola

Related People

Richard J. Vilmur: AUTHOR

Abstract

A standard 3-pole high pass filter configuration in series with a high gain amplifier had a serious problem of oscillation when exposed to a strong RF field as would be experienced in a mobile telephone application. The circuit is given in Figure 1. The frequency of the oscillation would occur at the peaking frequency of the 2.pole part of the 3.pole filter. The 2-pole part consists of C2, C3, R2, R3, R4, and 01. The oscillation could not be induced without RF energy and would not continue when the RF energy was removed. Normal RF bypassing and cure methods had no effect on the problem. Reducing the gain Of the amplifier by 10dB. which was more than could be tolerated, had no effect on the problem. Space and per- formance requirements prevented RF shielding and/or filter order reduction from 3 pOlSS to 2 poles to remove the peak.

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Volume 2 January 1982

M070ROlA Technical Developments

SPLIT 3.POLE ACTIVE FILTER WITH LOW RF SUSCEPTIBILITY

By Richard J. Vilmur

STATEMENT OF PROBLEM

   A standard 3-pole high pass filter configuration in series with a high gain amplifier had a serious problem of oscillation when exposed to a strong RF field as would be experienced in a mobile telephone application. The circuit is given in Figure 1. The frequency of the oscillation would occur at the peaking frequency of the 2.pole part of the 3.pole filter. The 2-pole part consists of C2, C3, R2, R3, R4, and 01. The oscillation could not be induced without RF energy and would not continue when the RF energy was removed. Normal RF bypassing and cure methods had no effect on the problem. Reducing the gain Of the amplifier by 10dB. which was more than could be tolerated, had no effect on the problem. Space and per- formance requirements prevented RF shielding and/or filter order reduction from 3 pOlSS to 2 poles to remove the peak.

SOLUTION

   The solution to the RF induced oscillation problem consists of decomposing the original 3pole filter design into separate sections, a 2.pole peaking section and a RC roll-off section. The circuit is shown in Figure 2. The RC roll-off section Rl and Cl has been made part of the gain amplifier Ul for a net saving of two parts while still accomplishing both the filtering and gain functions. The reason that the composite 5 pole filter in Figure 1 could sustain an RF induced oscillation...