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 751A is a single-channel band pass filter which consists of a high-pass section followed by a low- pass section. Low-pass and high-pass cutoff frequencies may be independently programmed. Prefilter gain, up to 40 dB, may be programmed, as well as postfilter gain, up to 20 dB. In addition, there are individual input and output overload indicators.