SPLIT 3-POLE ACTIVE FILTER WITH LOW RF SUSCEPTIBILITY
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2001-Oct-10
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.