Switch between AC coupling and DC cou­pling by toggling the softkey AC/DC.

Fig. 3-3 AC coupling a symmetrical signal.

Use the AC coupling feature to eliminate un­wanted DC signal components. Always use AC coupling when the AC signal is superim­posed on a DC voltage that is higher than the trigger level setting range. However, we rec­ommend AC coupling in many other measure­ment situations as well.

When you measure symmetrical signals, such as sine and square/triangle waves, AC cou­pling filters out all DC components. This means that a 0 V trigger level is always cen­tered around the middle of the signal where triggering is most stable.

Fig. 3-4 Missing trigger events due to AC coupling of signal with varying duty cycle.

Signals with changing duty cycle or with a very low or high duty cycle do require DC coupling. Fig. 3-4 shows how pulses can be missed, while Fig. 3-5shows that triggering does not occur at all because the signal ampli­tude and the hysteresis band are not centered.

NOTE: For explanation of the hysteresis band, see page 4-3.

Fig. 3-5 No triggering due to AC coupling of signal with low duty cycle.