The Models 751 A, 752A and 753A are programmable Brickwall™ Filters offering near-ideal passband and stopband characteristics. The instruments employ 7th-order (7 poles and 6 zeroes) Elliptic or Cauer filters with design values of0.3dBpeak- to-peak passband ripple and 85 dB stopband attenuation. The transition from passband to stopband has a 115 dB/octave rolloff rate. For low pass response, the stopband is at 1.7 x cutoff frequency; high pass response, the stopband is 0.6 x cutoff for frequency. The cutoff frequency may be programmed from 10 Hz to 99 kHz with 2-digit resolution and 1 Hz to 9 Hz with 1-digit resolution. Pre-filter, and (where available) postfilter gain may be programmed in 10 dB steps, up to the maximum available. These instruments are extremely valuable in signal- processing applications for band-limiting prior to sampling to prevent aliasing, or for separating a narrow spectrum of interest from noise or other interfering signals. In addition to the extremely sharp filtering, they provide programmable low noise amplification for applications involving low-level signals and overload indicators to ensure operation within their linear dynamic range. The basic differences of the three models are summarized here. Model 752A is a dual-channel, low-pass filter. The two channels are amplitude-and phase-matched. They may be used independently, or cascaded for a rolloff rate of 230 dB/octave. Each channel has prefilter gain of up to 40 dB, input overload indicators and selectable ac/dc input coupling.