E-Field Probing: Filtering 50 Hz Signal From Radiated Noise

Introduction

This article shows a use of simple passive oscilloscope probe (1X/10X) which can be used to pick radiations from Device Under Test (DUT).

During near field probing, one is interested to pinpoint the location of radiation on a PCB. If the radiation is caused by high dV/dt, E-field probe is used whereas, for radiations due to high dI/dt, H-field probe is utilized. If nature of radiation is unknown, both probes are used to identify cause of radiation and locate components or traces responsible for it.

In the following example, I have not used a standard E or H-field probe, instead, passive oscilloscope probe is made to pick E field.

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Problem

The device had two PCBs stacked on each other. One PCB radiated so well that it manipulated the PCB sitting over it which caused problems.

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Setup

Exposed tip of probe can pick E-field easily. The noise can be seen on oscilloscope as amplitude vs time. By looking at shape of noise, it becomes easier for an engineer to know the noise source. For example, noise radiated from switching regulators has a certain signature.

Diagnosis

1.????The noise picked up from right above the power supply on the PCB, had 50 Hz sine wave and a high frequency signal riding over it. The 50 Hz sine wave is of no interest as it the ambient noise due to presence of AC mains (220Vac 50Hz). However, high frequency signal needs to be examined only.

?2.????I made a simple RC high pass filter. The high frequency noise had frequency of 350 KHz. So, I made filter with cut-off frequency much away from 50 Hz, like for example 100KHz.

?3.????Electric shield is implemented on the filter components so that E-field coupling can take place on input of filter only (i.e. one leg of capacitor). It is done by wrapping the filter components with copper tape and copper tape is connected to probe return (ground).

4.????After these changes to the probe, the results were very clean with high frequency signals only.

Conclusion

Near field probing can help identify noise type (such as electric or magnetic) and radiation location. Mostly, these are used with spectrum analyzer to give view of whole spectrum in which radiation is happening, but when used with oscilloscope, it can allow us to see shape of signal. This is sometimes more helpful to identify circuit part on a PCB that could generate that specific signal.

The bottom line is to know what is real and what is artifact when measurement is performed. For that reason, understanding the capabilities/limitations of your equipment, mechanism for noise coupling and having good intuition (knowing what to expect) are very important.