Agilent/Keysight 3581C

The Agilent/Keysight 3581C is a selective voltmeter engineered for precise signal analysis in audio and baseband applications. Operating from 15 Hz to 50 kHz, this portable instrument delivers accurate amplitude measurements across linear and logarithmic scales with a 5-digit LED display. Three selectable input configurations—unbalanced, balanced-bridged, and balanced-terminated—accommodate diverse measurement scenarios. The 3581C measures signals from 30 V to 100 nV (linear) or -150 dBm/dBV to 30 dBm/dBV (logarithmic). Calibration options include dBm/900 ohms, dBm/600 ohms, and rms Volts. Frequency accuracy reaches ± 3 Hz with 1 Hz resolution; typical stability is ± 10 Hz/hour, improving to ± 5 Hz/°C after one-hour warm-up. Automatic Frequency Control (AFC) holds signal lock within ± 800 Hz. Built-in scanning capability spans 50 Hz to 50 kHz with adjustable sweep times from 0.1 S to 2000 S in single or repetitive modes. The instrument features a balanced 600-ohm output, speaker monitor, and phone-jack input connectors (WECO Type 310). Input impedance varies by configuration: 1 megohm (40 pF) unbalanced; approximately 15 kilohms balanced-bridged; 900 or 600 ohms balanced-terminated. Integration with X-Y recorders enables spectral display generation. This is a legacy instrument no longer sold or supported by Keysight; operating and service manuals remain available.