Capture the Waveform You Want & Skip the Rest

April 17, 2015 //By Arthur Pini
Capture the Waveform You Want & Skip the Rest
With these handy hints, you can use an oscilloscope to measure what you want and ignore the rest.

Modern digital oscilloscopes include a variety of automatic measurement parameters such as amplitude, frequency, and delay that help you interpret the waveforms. In most cases, these measurements let you quickly complete your measurement task. Sometimes, however, the oscilloscope doesn't measure exactly what you want. When that happens, you need to use the oscilloscope's features to get the desired information.

Suppose you're looking at a bus and you need to measure the frequency of a signal when a specific device is talking. If your oscilloscope measurement supports waveform gating, you can use the chip-select signal to gate the measurement, limiting it to only times when the specific device is on the bus.

The measurement setup in Figure 1 gates the frequency measurement of parameter P2 by the waveform in channel 2 (C2). The measurement is only made when the C2 waveform is above 50% of its full amplitude yielding a reading of 5 MHz. Parameter P1 reads the frequency across the entire waveform and shows a range of values from 300 kHz to 5 MHz.

Figure 1. The gated frequency measurement in parameter P2 occurs only when the chip enable trace, C2, is high. The parameter P1 shows the frequency reading over the entire waveform.

Waveform gating is one of three tools for limiting the range of a measurement. The others are fixed gates which make the measurement between fixed horizontal limits on the display and Accept, which only accepts measurement value within a user specified range. This selection are shown on the Gate and Accept tabs of the measurement setup in Figure 1.

Accept measurements by value

Acceptance criteria can be used to separate measurements that fall into fixed ranges. Take, for instance, the DDR timing measurement shown in Figure 2. The oscilloscope is measuring the hold time between the DDR strobe and data.

Figure 2. Acceptance criteria lets you separate the hold time measurements of read-and-write operations in a

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