Krohn-Hite 3945 3Hz to 25.6MHz High-Pass/Low-Pass Butterworth/Bessel Programmable Filter

The Krohn-Hite 3945 is a three-channel programmable analog filter delivering precise signal conditioning, anti-aliasing, and noise reduction across 3 Hz to 25.6 MHz. Channels 1.1 and 1.2 provide independent high-pass, low-pass, band-pass, and band-reject filtering from 3 Hz to 2 MHz with Butterworth or Bessel transfer functions. Channel 2.1 extends low-pass filtering to 25.6 MHz with Butterworth response and gain capability up to 40 dB. All channels employ 4-pole filters with 24 dB/octave attenuation slope and support both AC and DC coupling. Input-referred noise remains below 250 µV.

Technical Specifications Frequency Performance:

Channels 1.1 & 1.2: 3 Hz to 2 MHz tunable range

Channel 2.1: 170 Hz to 25.6 MHz tunable range

Butterworth -3 dB gain at cutoff; Bessel -7.58 dB

Frequency resolution: 1 Hz (3 Hz–1 kHz), 10 Hz (2 kHz), 100 Hz (100 kHz), 1 kHz (1 MHz), 10 kHz (2 MHz) for channels 1.1/1.2

Cutoff accuracy: ±2% to 2 MHz, ±5% above 2 MHz Filtering & Gain:

Passband response: ±0.2 dB to 256 kHz; ±0.5 dB to 2 MHz (channels 1.1/1.2)

Stopband attenuation: 100 dB to 1 MHz; 80 dB at 10 MHz; 70 dB at 30 MHz; 50 dB to 100 MHz

Amplifier gain: Up to 40 dB per channel

Rise/fall time: <7 ns (0 dB input gain, 6 dB output gain); <10 ns (+20 dB gain); <5% overshoot

Input/output noise: Typically <250 µV referred to input Coupling & Termination:

AC or DC input/output coupling available

AC coupling: ~16 Hz input cutoff, ~10 Hz output cutoff at 50 Ω termination

Key Features

Three independent channels with flexible filter topology

Butterworth (maximally flat) or Bessel (linear phase) options

Band-pass and band-reject configurations via channel cascading

Channel 2.1 gain mode for signal amplification without filtering

Programmable cutoff frequencies with 1 Hz resolution at low frequencies

Typical Applications Telecommunications signal conditioning, audio/video processing, EMI testing, medical device development, and laboratory measurement systems requiring calibrated analog filtering and gain control.

Compatibility & Integration Three independent signal paths allow flexible cascading and multi-stage filtering architectures. AC/DC coupling selection supports both AC-coupled analog systems and DC-biased signal chains.