B+K precision 2510B Series User Manual page 58

DC Coupling
Allows both DC and AC components into the trigger path.
AC Coupling
Blocks all the DC components and attenuate signals lower than 8 Hz.
Use AC coupling to get a stable edge trigger when your waveform has a large DC offset.
LF Reject
Blocks the DC components and reject the low frequency components lower than 2 MHz. Low frequency
Reject removes unwanted low frequency components from a trigger waveform, such as power line
frequencies, etc. that can interfere with proper tirggering.
Use LF Reject coupling to get a stable edge trigger when your waveform has low frequency noise.
HF Reject
Reject the high frequency components higher than 1.2 MHz.
Trigger 58