Agilent/Keysight 400FL

The Agilent/Keysight 400FL is an AC voltmeter and dB meter engineered for precision RMS voltage and decibel measurements across a wide frequency spectrum. Built around dual-scale metering - linear dB scale and logarithmic AC voltage scale - this instrument serves audio, RF, and general AC measurement applications where accurate voltage characterization is critical. The 400FL delivers voltage measurements from 100 µV to 300 V full scale, with frequency response spanning 20 Hz to 4 MHz.

Accuracy varies by frequency: ±1% across the 100 Hz to 1 MHz band, with ±2% at both 40–100 Hz and 1–2 MHz ranges, and ±4% at the frequency extremes. The meter responds to absolute average signal values, calibrated in RMS volts. Input impedance is specified at 10 MΩ with input capacitance less than 30 pF (reducing to <15 pF on selected ranges).

An AC amplifier output provides 1 V RMS open circuit at full-scale meter deflection, enabling integration with external measurement systems and recording equipment.

Technical Specifications

• Measurement Type: AC voltage and decibels


• Voltage Range: 100 µV to 300 V full scale


• Frequency Range: 20 Hz to 4 MHz


• Accuracy (% of reading): - 20–40 Hz: ±4% - 40–100 Hz: ±2% - 100 Hz–1 MHz: ±1% - 1–2 MHz: ±2% - 2–4 MHz: ±4%


• Input Impedance: 10 MΩ


• Input Capacitance: <30 pF (down to <15 pF on specific ranges)


• AC Amplifier Output: 1 V RMS open circuit at full-scale indication


• Calibration Basis: Absolute average value, calibrated in RMS volts

Key Features

• Dual-scale display combines linear dB and logarithmic AC voltage scales


• High input impedance minimizes loading effects on circuits under test


• RMS-calibrated measurement ensures accurate AC characterization across wide voltage range


• Dedicated AC amplifier output for external instrumentation interfacing

Typical Applications

• Audio frequency signal analysis and level measurement


• RF and IF signal monitoring


• AC voltage verification across component testing and troubleshooting workflows

Compatibility & Integration The 400FL integrates within Agilent and Keysight instrument ecosystems, pairing effectively with signal generators, oscilloscopes, and spectrum analyzers for comprehensive AC measurement workflows.