High-frequency reject mode
The 390B series is equipped with an AC LPF (low-pass filter) to help reduce unwanted electronic noise when measuring AC voltage or AC frequency. The LPF can improve measurement performance on composite sine waves that are typically generated by inverters and variable frequency drives.
Set up the multimeter to measure AC voltage. Press the HFR button to enable the LPF. The multimeter will continue measuring in the chosen AC mode, but now the signal diverts through a filter that blocks voltages above 800 Hz.