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Null Experiments While Measuring Signals

When measuring signals in electronic circuits sometimes what you see on the screen is not what you have in the circuit. Parasitic couplings, mismatching, noise pickup, and many other anomalous effects can give you the wrong picture of your problem.

As usual, I am analyzing the problem of radiated emissions from a product. This time, emissions are at approximately 400MHz, and I was able to identify that the origin of the problem was a small DC/DC converter with 10-12Vdc input voltage, 3.3Vdc output voltage, and approximately 200mA output current.

The converter was a buck topology switching in hundreds of kHz so I was interested in analyzing the switching waveforms looking for some ringing or parasitic oscillation.

I prepared one of my scopes with voltage and current probes to analyze switching node voltage Vo and current iL through the inductor L1 (see Figure 1).

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Figure 1: Schematic of the DC/DC converter


I used a Picoscope 5444 with 200MHz bandwidth and four channels, one of the x10 passive probes included in the scope, and one CWT Ultramini Rogowsky current probe. This probe, typical from power electronic designers, is ideal to fit between legs of small devices. It is thin, flexible, and clip-around easy to insert in confined spaces. My probe is a CWT06B model with 50mV/A and 20MHz bandwidth.

Important: Rogowsky probes cannot measure DC so my signal will not show the DC level (my probe has an LF -3dB bandwidth of 32Hz). This is not important for me because I am interested in the AC waveform and a bigger probe with DC capabilities is not easy to introduce without including parasitics.

I prepared my setup and I obtained the waveforms as in Figure 2. The voltage was measured without pigtail using CH1, and the current was measured in CH2 with a small wire prepared to open a loop to insert the current probe.

Figure 2: Measuring voltage and current in the DC/DC converter


Then, I noted an interesting effect as you can see in Figure 2 in the current waveform (green). The general waveform is correct but two small glitches appear in two specific points of the waveform (marked with a red “?” in the figure).

What was the origin for those glitches? Saturation? Parasitics in the layout of the PCB?

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Then I remembered one suggestion I received many years ago from my good friend Doug C. Smith: try to make a “null experiment.” A null experiment must give you a known response in known conditions.

For example, if you are measuring current in a wire, put the probe close to the wire in a very similar position to the position where you will be measuring the current BUT without enclosing the wire. If the output of the probe is not zero, then you have some parasitic coupling that creates a non-desired effect in your measurement.

In Figure 3 you can see the probe is close to the original position but OUT OF THE WIRE. The result is the red trace iLB different from the original trace iLA. You can appreciate how the glitches (and some small ringing) appears when the theoretical response must be zero. The origin for those effects is the electric field coupling because the strong dV/dt in the switching node (see the ramp in voltage waveform in those moments).

Figure 3: Null experiment for our measure of voltage and current in the DC/DC converter


To solve the problem we could try a probe less sensitive to electric fields, some shielding in the probe, or, as you can see in Figure 4, inserting the probe close to the node of the coil that is not connected to an important dV/dt node.

Figure 4: Measuring with the current probe far from the noisy area


My final advice: when measuring (any variable), do not forget to prepare some “null experiment” to be sure the setup is correct. Thanks, Doug, for the many things I have learned from you!


author mediano-arturoArturo Mediano
received his M.Sc. (1990) and his Ph. D. (1997) in Electrical Engineering from University of Zaragoza (Spain), where he has held a teaching professorship in EMI/EMC/RF/SI from 1992. From 1990, he has been involved in R&D projects in EMI/EMC/SI/RF fields for communications, industry and scientific/medical applications with a solid experience in training, consultancy and troubleshooting for companies in Spain, USA, Switzerland, France, UK, Italy, Belgium, Germany, Canada, The Netherlands, Portugal, and Singapore. He is the founder of The HF-Magic Lab®, a specialized laboratory for design, diagnostic, troubleshooting, and training in the EMI/EMC/SI and RF fields at I3A (University of Zaragoza), and from 2011, he is instructor for Besser Associates (CA, USA) offering public and on-site courses in EMI/EMC/SI/RF subjects through the USA, especially in Silicon Valley/San Francisco Bay Area. He is Senior Member of the IEEE, an active member from 1999 (Chair 2013-2016) of the MTT-17 (HF/VHF/UHF) Technical Committee of the Microwave Theory and Techniques Society, and a member of the Electromagnetic Compatibility Society. Arturo can be reached at a.mediano@ieee.org. Web:  www.cartoontronics.com.

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