The annual “Top Twelve Articles from 2012” was just published by Interference Technology Magazine. My article, “The HF Current Probe – Theory and Application” was the number one most read article last year! Thanks everyone!
Figure 1 – A pair of DIY current probes constructed using split-core ferrite chokes.
I recently received the following question on how to calibrate current probes and thought you’d be interested.
Question: Good morning. I read your article, “HF Current Probe: Theory and Application”, but now I have a question I’m hoping you can help me answer. I am attempting to measure the transfer impedance of a current monitor probe using a probe calibration fixture or jig. To keep the setup simple, I am using a signal generator and a power meter. As in your example, I am setting the generator to source 0dBm and I will verify it with the power meter and sensor through an adapter; I will then connect the generator’s output and current probe to the calibration fixture and measure the probe’s output using the same meter and sensor. This all works fine until the input SWR of the calibration jig reaches about 1.3 at 100MHz. From that point up to 400MHz, the SWR of the jig reaches 3.4. It appears that one would be measuring both the current probe’s insertion loss and the calibration jig’s mismatch loss. Would it then be best to establish the reference by measuring the output of the signal generator while it is connected to the calibration fixture (without the probe inserted), so as to include the jig’s mismatch loss in both the reference and measurement sweeps?
Answer: You’re on the right track. You need to normalize out the effect of any mismatch from the jig setup. There are several methods for calibrating current probes. If you have the jig, that’s great. Basically, what you’re trying to do is measure the current accurately versus frequency – a not so trivial task to keep the current fixed as frequency changes. The problem is that any parasitics (R, C, L) in the wire to be measured can greatly influence the current value. That was the problem I was running into when measuring the wire in the referenced article. I tried to keep the value of current fixed by inserting a small resistor in series and measuring the voltage drop, keeping the this voltage drop steady by adjusting the RF generator output. It’s much better to use the 50-Ohm jig, but there will still be mismatch errors, which may be somewhat alleviated through the use of 6 to 10 dB attenuators. The goal is to measure the current through the probe versus the voltage at the probe terminals. Dividing the terminal voltage by the current gives you the transfer impedance. I’ve attached a few references.
Here’s a recent article from Interference Technology.
Teseq also has a calibration procedure within the instructions for their test jig, and look under the “downloads” tab.
Dr. David Pommerenke, of Missouri University of Science and Technology (MST), authored a paper with Ram Chundru and Sunitha Chandra on “A New Test Setup and Method for the Calibration of Current Clamps“, which runs through the historical calibration methods and then suggests an improved method.
I just received my matched set of Fischer Custom Communications (FCC) model F-33-1 current probes! Yipee!
This is actually a pretty big deal, as matched current probes will allow me to make some very unique measurements during troubleshooting. Many of these techniques were pioneered by fellow colleagues Michael Mardiguian (EMI Troubleshooting Techniques) and Doug Smith (High Frequency Measurements and Noise in Electronic Circuits).
I’ll be showing you some of these techniques in future installments.
Fischer F-33-1 current probes (matched set).
What do you do after returning to your workbench with a product that has just failed radiated emissions? In this multi-part series, I’ll describe simple and low-cost ways I use to help my own clients solve these issues. Most of the time, it’s possible to set up a simple 1 to 3 meter “measurement range” and determine whether a potential fix is required, or not.
PHILOSOPHY
First, a little troubleshooting philosophy. In many cases, you’ll run into more than one emission source causing the same harmonic frequency. The result is that you might apply a fix and the harmonic will do three things – either get reduced, have no change or better yet…get larger! It won’t be until you apply fixes to ALL the sources that you’ll yield positive results. This is what makes chasing down emission problems such a joy(!), I mean “challenge”!
There is also the issue of “balloon effect”; that is, you’ll beat down one frequency, only to have one, or more, pop up higher! It’s like squeezing a balloon in the middle – both ends get bigger! Often times, this is the result of resonances within your cabling or on the PC board.
In this installment, I’ll describe some simple reference antennas I use (you’ll be surprised) as well as setting up an area on your workbench where you can troubleshoot and apply potential fixes and really see whether you’re making progress, or not.
ANTENNAS
The antenna you select should ideally be somewhere near resonance for the frequencies of concern, however, it’s not really that critical for troubleshooting purposes. So long as the antenna is fixed in length and fixed in place on the bench, you’ll receive consistent results. During troubleshooting, it’s more important to know whether the fix is “better” or “worse” or “no change” and as long as the test setup doesn’t change, the results should be believable.
Low-cost EMC antennas I use for troubleshooting, based on television "rabbit ears" and a UHF folded dipole.
Now, EMC antennas are not inexpensive, as you might imagine, so for general troubleshooting, I tend to use a couple television antennas – a pair of “rabbit ears” and a UHF “bowtie” (with TV balun to match 50-ohm coax). If the workbench is wooden, I’ll extend the antenna to approximate resonance (if possible) and tape it down to the bench with duct tape. If the bench is metallic, I’ll find a non-conductive support and position it some distance away from (or above) the bench. I usually use a test distance of about a meter, but as long as you can see the product’s harmonics on a spectrum analyzer, you’ll be able to determine your progress. Sometimes I need to insert a low-noise wide-band preamp between antenna and analyzer.
Now, obviously, ambient signals from broadcast radio, television mobile phones and two-way radio services will tend to interfere with observing the product harmonics. You may need to bring the antenna closer or set up the troubleshooting measurement in a basement or building interior away from outside windows. I usually record the known harmonics of concern and try to characterize them in relation to other nearby ambients.
CURRENT PROBES
If I know that one, or more, cables are the dominant radiation source, I might use a current probe to monitor the common-mode currents flowing on the cable, rather than an antenna. This also helps reduce the ambient signals, because current probes are generally shielded against e-fields and tend to be poor antennas. I’ll attach the probe to the cable with dominant emissions, moving it back and forth along the cable to achieve a maximum, and then fix it in place.
Commercial current probes can measure rf currents flowing on I/O cables - a very typical issue for radiated emissions issues.
BENCH
Next, I’ll clear off the workbench and (assuming the product is small) find a convenient place for it where I can work on it without moving it around much. I also place reference marks on the bench with tape, so I can reposition it for repeatable measurements. At that point, I’m ready to begin the troubleshooting and fixes while watching the emission levels.
Now don’t make the mistake of assuming that a 10 dB reduction on the bench with a one-meter test distance translates to the same reduction when measured at the test facility at a ten-meter test distance! During troubleshooting, we’re likely working in the “near field” where test distance is determined by terms of 1/r squared and 1/r cubed. At ten meters, we’re likely in the “far field” (plane waves) and the distance factor is closer to 1/r. Where “r” is the test distance. You can be fairly confident, though, that a reduction on the bench will equate to some reduction at the site.
More in the next posting!
Electromagnetic Compatibility (EMC) design, troubleshooting and training.
design-4-emc.com 