Scope Measurement Hints

An Easy Way to Isolate the Cause of Noise Problems

In todayís digital systems, noise coupled from other parts of the system is a common problem. Whether the noise is caused by a switching power supply, CRT interference, or the system clock, this measurement technique will help isolate the cause of the noise. Noise signals are tricky to analyze on a scope. Typically the scope is triggered on a signal other than the noise itself, since many times noise signals are so small that it is difficult to trigger on them directly. Figure 1 shows a noisy ground signal with the scope auto triggered.

Figure 1

A solution to this problem is to trigger on the suspected noise source. See Figure 2. In this case, the 516 kHz clock signal was suspected. Triggering on the clock signal (channel 2) and viewing the noisy ground signal on channel 1 results in a trigger synchronous to the noise. Now you can use averaging to average out the asynchronous noise. Using this technique, itís easy to see that this noise is indeed due to the 516 kHz clock.

Figure 2

If there are multiple noise sources, the same technique can be used to isolate the noise components that are due to each noise source, with the contributions from all other asynchronous noise sources eliminated through averaging.