The Model 31 Series are high-pass, 7-pole, 6-zero elliptical Plug-In Filter providing 115dB/octave rolloff, a minimum stopband attenuation of >80dB, and a typical passband ripple of 0.22dB peak-to-peak. The Model 31-1 covers the range from 1Hz to 99kHz. The Model 31-2 operates from 0.1Hz to 9.9kHz and from 0.01Hz to 990Hz in the Model 31-3 With its 115dB/octave rolloff rate, The Model 31 provides a much narrower transition region than an 8-pole Butterworth, and much less passband attenuation near cutoff. The transition region from -3dB to -85dB is 38% of fc (cutoff), compared to 71% for the 8-pole Butterworth. From -0.3dB to -85.6dB, it becomes 41% of fc compared to 89%. This means the fast transition from passband to stopband of the Model 31 makes it the ideal Filter for the rejection of interfering signals near the passband. The Model 31 Series is part of an ever expanding line of Plug-In Filter cards for use in the Models 3905 and 3916 Mainframes, which accommodates 5 to 16 Filter cards respectively.