Ithaco, Inc., has developed the Model 4251 Variable Electronic Filter, designed for state-of-the-art accuracy of all parameters. The filter can operate either as a high-pass or low-pass filter, and, according to the manufacturer, the corner frequency may be set with 3-digit resolution and 1% accuracy from .01 Hz to 111 kHz. Phase and amplitude accuracy are reportedly 20 and 1%, respectively. Att enuation slope is 24 dB/octave. Total hum and noise is 50 J.Lvolts or less. If the Model 4251 is used as a low-pass filter, response can be set to be maximally flat (Butterworth) or linear phase (Bessel).