What is a RF Current Probe?

An RF current probe is a very versatile tools. Essentially, a current probe is just a current transformer, where the current in the wire you are measuring forms a single turn winding and your measuring tool sits on the secondary side. Especially in the field of EMC/EMI, they can be very helpful to measure the conducted emissions of your product, and to trace down your problems with radiated emissions. This article shows you how to use them, what types of probes exist, what properties you would want, and how to build one using only limited resources.

How to use a current probe to measure conducted emissions.

Conducted emissions are mostly about so called common-mode-currents. In the figure below, I have shown you a cable with 2 conductors, a current is flowing. This current has a portion which is called differential-mode current (DM) and some common-mode current (CM).

The differential-mode current is what you would expect if you power any load, represented by the resistor. Since the current loop is closed "inside" the probe, you can expect the magnetic fields produced by the curents in both wires to cancel each other - at least as a first-order approximation. The closer the two wires to each other and the bigger the aperture of the probe, the better this approximation will hold.

Now reality get us. You will have some sort of coupling (often, capacitance is the problem) at your load. This capacitance is formed by e.g. the conductors, or the filters inside your load. Usually, you cannot do something about that.

In the ideal case, that wouldn't be too much of a problem, but in reality, you will have a small common-mode current going to the load and returning via your ground wire. Whenever you have something switching on your PCB, you'll get some of these current due to imperfections and stray capacitance to the environment.

So how do you measure with a current, probe? Well, that's simple. Just put it over any "current path" to/from your product! Remember, you're after currents which will get back to your product via the ground "wire". If you're luck, and your product is connected to ground (e.g. via the PE wire on your mains plug), you can just put the current probe over it to measure the sum of all emissions.

When doing such measurements, you may want to follow some standards to some extent. We will have a look at some details down below.

Some Requirements Engineering

As for every project, you should always think about requirements right at the beginning. In our case, it is easy:

• somewhat standard-compliant
• we want to go cheap
• and it should be easy to build

Now let's start with the standards...

Standards

You may know, I'm a professional standards user and writer. This means, I have access to the paid stuff. However, there are plenty of resources, which you can (and should) us.

It happens regularly, that I get asked where to get all the specs for EMC testing for free. One of my go-to-resources for freely available information are MIL-STDs. For basic measurement instrumentation, MIL-STD-462 would be the right one for you. It has an annex about current probes, which tells us, that we (typically) want a transfer impedance of 5Ω, or less and an add a series impedance of max. 1Ω.

A newer, but vastly bigger one is MIL-STD-461. In its edition G (that's why the ATM newest document one is called MIL-STD-461G), you get the very same information hidden in Annex A.

Now, there's another go-to resource I checked, which you maybe cannot: it's CISPR 16-1-2. This adds to our requirement list, that we want some electrostatic shielding, and we want to test it in some coaxial fixture. MIL-STD-461 isn't too precise about calibration current probes, but you get some information on the current injection fixture. That one is not too coaxial for my feeling and the construction is not easy to replicate at home.

Additionally, CISPR 16-1-2 tells us, that will need around 7 turns and presents us with some typical performance curves. These suggest, we should shoot around 100MHz for our upper usable frequency. For conducted emissions, 30MHz would be fine already, but for conducted immunity, 100MHz would help us to calibrate our up-to 80MHz test signals as well.

Types of current probes

I really like clamp-on current probes. However, those require strict tolerances when building one, and require more care when using them. If something get crushed between the 2 halfs it could destroy (or degrade) the performance by scratching or breaking the interfaces.

If you're doing EMC-Testing a lot, I highly recommend those because you are that much faster when modifying the setup compared to the next type.

Fixed aperture RF current monitoring probes are just "donuts" where you get your cable through the center. Those can have way better RF-performance just because the core is not split. While this isn't really from interest for EMC (we're using current probes for up to maybe 100MHz or so), it can (and is) crucial to know when you're dealing with higher frequency currents. However, this is a niche inside a niche. The most interesting thing about them is that their construction is simple compared to the split core type.

Summary of Constraints

• easy and cheap to build (this is probably the driving requirement)
• usable up to 80MHz (30MHz if we get to our goal quicker).
• a fixed aperture probe is what we build, about 25mm would be nice to get an Ethernet connector through it.
• want a transfer impedance of 5Ω or less, and a series-impedance of max. 1Ω.
• electrostatic shielding would be great
• for calibration, we should come up with something easy, cheap and repeatable as well.

How to diy a RF current probe

Choosing components

Let's start with the ferrite core. We know, we need something which is fine above 100MHz, and we want maybe 30mm of inner diameter. From my research, I got Kemet's ESD-R-57D-1 and 74270097 by Würth Elektronik as my best options.

Both are NiZn and have an ID of 32.4mm (the Kemet one) and 33.4 (the core by Würth). The cross-section of the core sold by Kemet is way larger. This should allow for higher currents before we get saturation. Würth has far better pricing (at least at my sources). To leave the choice to you, I'll use both...

Next is the electrostatic shielding. An easy way to implement this on any transformer is a coaxial cable. This may sound wired, but just connect the shield on only 1 side, and you get a very nice shielding. There are plenty of options for coax cables - I choose RG174 because it's just 2.74mm in diameter.

Probe construction

I opted to use my 3d printers to print me a shell.

This shell embeds some grooves so that the coax is tightly constraint, and we get consisting readings. When closed up, everything looks pretty tidy and presents some sturdiness.

It is not well visible in the rendering, but the 4 cover pieces (2 on top and bottom) are separated from the middle piece. That way, this is printable without supports and easy to assemble.

Construction of a coaxial calibration fixture

Building the probe is one part of our problem. The second one is the calibration fixture. It took me quite a while to figure out a simple but clever solution. We're using PCBs and simple headers!

Firing up KiCad (and FreeCad for the 3d assembly), I came with the following construction:

The idea is to have a 2x2 connection with headers in the center for the inner conductor of our coaxial connection. On the outside, a bunch of 1x6 headers make for a nice outer conductor. A cheap 2-layer 10x10cm PCB gives stability and makes for a solid ground connection.

Since the length is short, there's no real need to match the dimensions to some impedance. Getting the probe in there would change everything anyways.

When you look for these very long pin-headers. Those are called board stackers and available in various length.

Improvements

A limiting factor of the presented design is the self-resonance due to the capacitance by the coax. Lowering the capacitance per unit length possible by increasing the characteristic impedance. A simple change is to use 75Ω RG179 instead of the 50Ω RG174 coax. In my experiments, this increases the resonance frequency to 245MHz.

Update

Finally, I made some products from this design. One of these is free. You can download the 3d model and the assembly instructions to evaluate the design. A full BOM is included in the assembly instructions.

Further, I sell an enhanced design and a version with a split core

modified: 2024-11-18