One of the main blocking aspects of EMC is, in general, the material cost. Anechoic chambers are highly expensive so only a few companies can afford them for internal development. Furthermore, all the extra components such as spectrum analyzers, antennas, current probes, among others, are also not easily affordable for most. For this reason, within a development cycle, many teams only test their products when it is in its final stage, and unfortunately, when there is not much room left for fixes.

Having a set of tools for verifying a design, even if they are not the best in the market or not even calibrated, can be very handy and can save a lot of hours in certified laboratories, which in the end means, to save a lot, a lot, of money.

Conducted emissions is one of the main tests performed during an EMC test campaign. The goal is to measure the emissions introduced to the power or communications network of electronic equipment. One of the methods to do it uses a current probe that surrounds the wires to analyze, converting the current into voltage thanks to Ampere’s law.

This article describes how to build a current probe and how to use it for EMI troubleshooting. This probe will not provide certified measurements, so the results should not be used to determine whether or not a system will pass the tests in a certified laboratory, but it will be perfect for troubleshooting and design validation.

Commercial probes

First of all, it is good to keep an eye on what is on the market. Commercial probes are sold according to their frequency range, impedance, and size. For example, the A.H. Systems model BCP-615 works between 10 kHz and 500 MHz and has an aperture (maximum wire size). The impedance is normally indicated with a graph like the following:

As can be seen, the impedance is quite linear until reaching 400 MHz, where the capacitive components become big so the probe is less effective.

We will see that the difference between a commercial probe and a home-made one is the linearity and the curve smoothness.

Gathering all the material

First of all, we need a ferrite core. You can buy many of them from different suppliers or even ask for a free sample from some of them. I used a ferrite core of Würth Elektronik, model 74271251. It is a NiZn ferrite core frequently used for mitigating radiated noise. Its impedance is as shown below:

The black line shows the impedance when winding once around the core, while the red one shows the impedance when doing it twice. This is useful also to remind that the number of turns will increase the sensitivity of the core, but it will reduce the resonant frequency, i.e. will the probe bandwidth.

The extra components we need are a coaxial connector, a wire, and some glue. I used a BNC connector, so it is easy to connect to an oscilloscope or any other measurement instrument.

Measuring the frequency response

Using a VNA

Even if we are not looking for calibrated measurements, it is convenient to have an idea of the probe frequency response. Also, it will be helpful for our measurements to have the probe characterized.

There are different ways to do it. The fastest and, the most expensive, is to use a Vector Network Analyzer (VNA). VNAs stimulate the input of a system and measure its response, obtaining the transfer function. We had the chance to use a VNA to characterize my probe which. We are perfectly aware that this is not coherent with the principle of keeping a low budget, but we could not permit this opportunity to pass.

Using a signal generator

There is a second way of characterizing the probe, more tedious and cheaper. The steps to follow are:

• Connect a 50 Ω resistor to the signal generator output. Any other value can be used, but it is very convenient since most of the instrumentation is adapted for 50 Ω.
• Configure the signal generator to provide a known signal.
• Introduce the resistor in the probe, so it can measure the current circulating through it.
• While varying the frequency, measure the induced voltage at the current probe. Since the resistance is 50 ohm, it is easy to convert from voltage to current.

Conclusions

Work in EMC should not be extremely expensive. If we sacrifice some accuracy we can build some great tools to start searching the root of possible EMC problems. A current probe is fundamental to find conducted emissions and to see what is happening inside cables.

This article has been inspired by the great work of Kenneth Wyatt: https://interferencetechnology.com/the-hf-current-probe-theory-and-application/