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.

Holdoff

A trigger event is a state that would normally result in the oscilloscope triggering an acquisition.  Holdoff should be interpreted as a condition that suppresses the trigger for a specified period time or trigger event count following the initial arming of the trigger.  Holdoff is restricted to use with Edge and Pattern triggers, but it can be useful when there are multiple potential trigger events per acquisition that otherwise vary in ways that could make synchronization difficult, such as with complex signals. It allows the oscilloscope to ignore the “extra” trigger events and stabilize the display as if there was only a single trigger event.    

Figure 2: Using holdoff by time to trigger
on a complex signal. Click image to expand.

Figure 2 shows an example of holdoff by time.  The waveform being acquired is a packet of 21 RF pulse bursts from a keyless entry fob with a duration of 41 ms, so that there are 21 possible Edge trigger events occurring at different intervals.  

Holdoff was set to ignore all edges for the 41 ms duration of the entire packet.  Any trigger will be followed by a 41 ms interval.  Since this is the duration of the packet, the next trigger cannot occur until the beginning of the next packet.  

SmartTriggers

Modern oscilloscopes usually include a powerful set of "smart triggers" that are based on timing and amplitude characteristics of the trigger signal.  Often, Boolean conditions can be added to include or exclude different events from the trigger. Teledyne LeCroy SmartTrigger® types include Glitch, Width, Window, Interval (Period), (signal) Drop Out, Runt and Slew Rate.  

As an example of how a SmartTrigger can be used to help stabilize the display, we’ll focus on the Width trigger.  The Width trigger is sensitive to the pulse width of a signal defined as the time difference between two edges with opposite slopes and is generally applied to a rectangular pulse, as is often used for serial data signals.  

The Width trigger can be refined to the occurrence of measured pulse widths that meet the conditions of equal to, less than, greater than, within a range, and outside of a range of values, making this a powerful tool for triggering on complex signals.  

Figure 3: Setup for a Width trigger based on pulse
width being within the range of 2.3 to 2.7 µs.
Click image to expand.

Figure 3 shows the waveform to which we will apply our Width trigger. This is a Pulse Width Modulated (PWM) waveform with eight distinct widths from 500 ns to 4 µs that normally occur, as measured by the Width parameter P1. An Edge trigger would fail on this waveform because there are eight possible trigger events that would all occur at different time intervals, resulting in an unstable display.

Figure 3 shows the setup for a Width trigger based on the width of the pulse being anywhere in the range between 2.3 and 2.7 µs. Triggering on any of the normative pulse widths in this waveform would help to stabilize the display, but we picked a pulse in the middle of the burst to center it at the trigger point. Centering the pulse burst on the time axis makes it simple to change the timebase or zoom without having to shift the waveform. 

Beneath the parameter P1 readout is a histicon showing the distribution of the width values measured over many acquisitions. This can be expanded into a histogram display to see the range of each pulse width (or other parameters), which is one of the best ways to determine what range of values to use when setting up the trigger conditions.  The widths of the pulses in the burst increase from 0.5 µs to 4 µs in increments of 0.5 µs.  The variation of the central 2.5 µs pulse width is confirmed by a gated Width measurement in parameter readout P2, showing only the width of the selected pulse over many acquisitions. We set the trigger range from 2.3 to 2.7 µs, as much buffer as we can to ensure we trigger on all the 2.5 µs pulses, but not so much we begin to also trigger on the previous or next pulse in the burst.

The Width trigger dialog box in the figure shows the conditional setup for triggering on a width ‘within a range’.  As a result, any pulse with a width of 2.5 µs ±2 µs is the trigger event that stabilizes the acquisition.  

The other SmartTriggers behave in a similar fashion, offering a wide range of conditions to refine triggers based on signal timing or amplitude characteristics.

See also:

Oscilloscope Basics: Stabilizing Waveform Display, Pt. 1