Oscilloscope Basics: Stabilizing Waveform Display, Pt. 2

Figure 1: A 50 kHz low-pass filter eliminates a
93 kHz interfering signal from a 10 kHz signal (top two grids)
and a 50 kHz high-pass filter cleans up a 93 kHz signal
with an additive 10 kHz interfering signal (bottom two grids).
Click image to expand.

In Pt. 1, we discussed the fundamental cause of unstable waveform displays. In this post, we’ll discuss how to use signal conditioners and conditional triggering to help the oscilloscope ignore extraneous samples when determining where the acquisition trigger event actually occurs.

Coupling 

In the Setup section of the Trigger dialog, Trigger input sources can be conditioned using AC or DC coupling, high-pass filters (LFREJ for low-frequency reject) and low-pass filters (HFREJ for high-frequency reject). The frequency selective coupling paths are used to attenuate extraneous signals. The low-frequency reject inserts a 50 kHz high-pass filter in the trigger signal path, which is useful for eliminating low-frequency interference such as 60 Hz power mains signals. This low-frequency noise can cause erroneous triggers, resulting in an unstable display. The high-frequency reject inserts a 50 kHz low-pass filter. This coupling mode finds use in applications such as troubleshooting switch-mode power supplies, where it suppresses signals at the power supply switching frequency. Like any extraneous signal, high frequency pickup can leak into the input signal and cause trigger instability. Figure 1 provides examples of how the HFREJ and LFREJ coupling filters eliminate interfering signals from the trigger source.