Doug's picture
High Frequency Measurements Web Page
Douglas C. Smith

 Address:  P. O. Box 1457, Los Gatos, CA 95031
 TEL:      800-323-3956/408-356-4186
 FAX:      408-358-3799
 Mobile:   408-858-4528

Technical Tidbit - August 2006
A Measurement Technique for Locating EMI "Hot" Areas on Boards or Systems

checking a connector ground pin for noise

Figure 1.
Checking Circuit Ground at a Connector for Noise

Abstract: An area of a circuit board or system that has significant noise present on ground can often drive common mode currents on cables or other structures causing excessive emissions. A simple technique is described to identify areas of a circuit board or system that are at risk for emissions problems.

Discussion: One way of  finding  "hot"  areas of a circuit ground is to attach a wire to act like an antenna and see if it is driven with substantial common mode current. Common mode current on a conductor can be directly related to radiated emissions. Click here to read the Technical Tidbit for March 2006, "Predicting Cable Emissions from Common Mode Current" on this site for a discussion on common mode currents and emissions. If a wire attached to a board is driven with enough current to cause excess emissions (15 uAmps or more is generally enough), then there is a risk that a nearby cable or other structure attached near the same point on the board could radiate excessive emissions as well.

Figure 1 shows a square magnetic loop attached to a wire checking a gound pin of a connector for the ability to drive common mode current on a cable. Figure 2 shows the whole loop and wire assembly. The loop is attached to the wire by paper masking tape and the wire length is about one foot or about 30 cm, a length useful in the 150 to 200 MHz range. A spectrum analyzer  with a 50 Ohm input impedance is then usually used to measure the loop output voltage into 50 Ohms.

magnetic loop attached to wire
Figure 2. One Inch Magnetic Loop Attached to "Antenna" Wire

Figure 3 shows a close-up of the loop. I can remember Henry Ott using a current probe at the base of a one meter wire many years ago to check for "hot grounds." This is essentially the same technique, except adapted for today's higher frequencies and smaller products. The wire used should be about one quarter wavelength in the center of a frequency band of interest. A typical frequency range might be 150 to 200 MHz, a ratio of about 4:3. For this frequency range, a 30 cm (one foot) wire works well.

If a frequency is scanned that is too far from the quarter wave frequency of the wire, one might not see much current even though noise is present, it is just that the wire is not providing a low enough impedance to accept current. For a wire that is one half wavelength, it will present a very high impedance and little current can flow at that frequency.

Close-up of loop attachment to wire

Figure 3. Close-up of Magnetic Loop Attached to Wire

A shielded magnetic loop works best, but a simple wire loop like that in Figure 3 is often adequate. The loop is really being used as a current probe in this technique. The transfer impedance of the loop as a current probe (ratio of voltage out to current in the attached wire) in the "flat response" frequency range of the loop (see below) is given by:

ZT = (M/L)RL

  1. ZT is the loop probe transfer impedance.
  2. L is the self inductance of the loop.
  3. M is the mutual inductance between the loop and attached wire.
  4. RL is termination resistance of the loop, usually th 50 Ohm input of a spectrum analyzer in this case.
This equation is derived in Chapter 8 of my book, High Frequency Measurements and Noise in Electronic Circuits. It is the result of placing an L-R voltage divider (loop L and 50 Ohm load) on the open circuit output of the loop, Mdi/dt, and grinding equations for a page or two making substitutions along the way.

For a one inch (~2.5 cm) per side square loop, the flat region of frequency response ranges from about 100 MHz to just above 1 GHz. In this frequency range, the transfer impedance is often near 5 Ohms for thin insulation and typical wire sizes (L~80 nH and M~8 nH). This means the output of the loop is comparable to an F-33-1 current probe and the calculations in the March, 2006 Technical Tidbit, "Predicting Cable Emissions from Common Mode Current" can be applied to the loop. Thus a loop output above 100 MHz of ~38 dBuV indicates enough common mode current on the wire that ground in that area is "hot" and may cause nearby attached cables to radiate excessive emissions. Above a few hundred MHz, emissions limits are several dB higher and levels  above ~38 dBuV from the loop may be acceptable.

In a typical current probe, a magnetic core is used. The self inductance of the probe's winding, L, and mutual inductance to the wire, M, can be calculated and are relatively constant. Thus the current probe can be calibrated. When using a loop as a current probe, M is a function of insulation thickness and is variable. The equations that describe both probes are identical,  M and L are just different for the two cases, generally lower for the loop because there is no magnetic core. See my Technical Tidbit for September 1999 for more information or chapter eight of my book for even more details on current probes (probably too many details!).

The advantage of using a loop as a current probe is its small size and ease of attachment to a wire. The loop could even be considered a "handle" to hold the wire. This allows access to crowded circuit boards at the expense of calibration accuracy. Sometimes, only a relative measurement is needed, so calibration is not an issue. A wire on the order of one quarter wavelength is also small in the hundreds of MHz frequency range so the whole assembly is small enough to get in tight spaces on a circuit board in an enclosure.  Once the frequencies get above one GHz, a smaller loop is needed. The circumference of the loop should be smaller than one half wavelength to avoid loop resonance.

Summary: The combination of a small square loop and a piece of wire can be a powerful tool to find "hot" areas of ground on a circuit board or in a system.

Other articles on this website related to this topic are:
  1. March 2006, Predicting Cable Emissions from Common Mode Current
  2. September 1999, Using a Current Probe to Directly Measure Voltage Drop in a Circuit
  3. Signal and Noise Measurement Techniques Using Magnetic Field Probes (~600K)
    • (1999 IEEE EMC Symposium paper)
  4. Current Probes, More Useful Than You Think (~600K)
    • (1998 IEEE EMC Symposium paper)
If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems.

Click here for a description of my latest seminar to be available in the Fall titled:

EMC Lab Techniques for Designers
(How to find EMC problems and have some confidence your system will pass EMC testing while it is still in your lab).

D-104 mike
Check out my podcast containing mp3 format short tutorials, tech news and more! Just click on the microphone to see the listing of free audio programs. Content is added every week  on technical topics so check back frequently.
Top of page

Questions or suggestions? Contact me at
Copyright © 2006 Douglas C. Smith