Model 601 is a tunable high pass/low pass filter. The instrument contains a tunable 8-pole Butterworth high pass filter followed by a tunable 8-pole Butterworth low pass filter to easily implement high pass, low pass and bandpass filters. Both filter responses are flat in the pass band (unity gain) and roll off at 96 dB per octave beyond the corner frequency. Corner frequencies can be set from 100 Hz to 40 kHz at the front panel or over a serial link from a PC computer. The unit is controlled by a Microchip PIC microprocessor and includes an input analog anti-aliasing low passing filter, a variable gain output amplifier, and an analog low pass output filter. The microprocessor is in sleep mode when not programming the filter, so its clock does not add any output noise.