Interference factors that should be noted when using PCB design oscilloscopes

Oscilloscope is a delicate tool that does not display what you are really looking for. Oscilloscopes always display something, but sometimes you may not realize what you are missing. Let's go into detail one by one.

Improper installation

If the sampling rate is less than three times the bit or clock frequency, the input signal you see will be a distorted signal. If the input signal is a sine wave, you will still see a good sine wave, but in reality, this sine wave is only a part of the input signal frequency. From one point, it can be seen that there is an error, which is that the sine wave appears to have not been triggered. The solution is to enable peak detection or envelope mode.

Insufficient capture rate

You know that there is a problem with a specific input signal because the output signal is corrupted, but you cannot see where the problem lies. This situation occurs during the process of low occurrence of anomalies and occasional errors in the waveform that cannot capture data. One solution is to increase the waveform capture rate of the oscilloscope. Once the characteristics of intermittent faults are observed, the selected trigger can be used to capture anomalies. In advanced trigger mode, the oscilloscope is always in monitoring mode and will not trigger until abnormalities are detected, thus capturing all intermittent faults that occur.

Change the operating conditions of the device under test (DUT)

The installation of probes always changes the operating environment of the tested device. Sometimes device failure occurs due to degradation of rise time and establishment/maintenance time margin. Sometimes, due to the reduction of crosstalk and electromagnetic interference, the device is able to "pass" detection. In the latter case, if the device passes the detection, it should be ensured that the impact of the probe is taken into account in the design to meet the final specification requirements, including the influence of the probe material medium and the design medium, and suitable probes should be equipped for each product.

What can be seen on an oscilloscope largely depends on the type of probe and its installation position on the circuit. In the next post, I will discuss various detection defects and techniques used to avoid the permanent use of probes in the final design.