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Douglas C. Smith

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Technical Tidbit - August 2008
The Square Shielded Loop - Part 4, Coupling to a PCB
(From Shielded and Unshielded Magnetic Loops)

Unshielded wire loop held to circuit board

Figure 1.
Coupling a Signal into a Circuit Path Using a Square Unshielded Wire Loop
(covered with heat shrink tubing)

Abstract: Shielded loops are often used to minimize electric field (capacitive) coupling. A case is shown where using a shielded loop to inject signals into a path on a circuit board results in a significant resonance whereas using an unshielded wire loop results in a relatively flat frequency response of the injected signal. Unshielded wire loops are thus shown to be more useful than shielded loops in some cases.
Discussion: Figure 1 shows a square unshielded wire loop held up to a path crossing a break in the ground plane of a test board that is used for many experiment on this website. The injected signal was measured at the BNC connector on the board (left side) for the cases where the loop is positioned as shown and for a 180 degree rotation of the loop and similarly for a square shielded loop (embedded in plastic for strength) as shown in Figure 2. The construction of the shielded loop is shown in the May 2008 Technical Tidbit, The Square Shielded Loop - Part 1.

Shielded loop held up to circuit board
Figure 2. Coupling a Signal into a Circuit Path Using a Square Shielded Loop
(embedded in a plastic housing for strength)

Figures 3 and 4 show the measured signal at the BNC connector on the board for the unshielded wire loop in the normal position (as in Figure 1) and for the 180 degree rotated position of the loop respectively. The data was taken using an Agilent N1996A spectrum analyzer as a two port insertion loss measurement. The square loop was connected to the tracking generator output and the BNC connector on the board was connected to the receiver input of the analyzer.

Capacitive coupling between the loop and the board will cause either a resonance effect (dip or peak in the response) or a directional effect when the loop is rotated 180 degrees because the phase of the inductive coupling changes by 180 degrees whereas the capacitive coupling remains the same. As can be seen in Figures 3 and 4, there is no resonant effect, the frequency response is nearly flat. The capacitive coupling itself is very low compared to the inductive coupling because the difference between Figures 3 and 4 is only a few dB and then only at the higher frequencies above 600 MHz. Contrast the responses in Figures 3 and 4 for the unshielded loop to the responses in Figures 5 and 6 for the shielded loop.

Scope plot for signal on board from unshielded loop

Figure 3.
Injected Signal for Unshielded Loop

Scope plot for signal on board from reversed unshielded loop

Figure 4.
Injected Signal for Reversed Unshielded Loop

In both Figures 5 and 6, a resonant dip in the response is seen similar to that shown for coupling between shielded loops in the June 2008 Technical Tidbit, The Square Shielded Loop - Part 2, Parasitic Coupling. The reason for this resonance is described in that Technical Tidbit. In this case, the resonance is due to the sum of the inductance of the shields of the loop and the inductance around the split in the ground plane interacting with the capacitance between the shields and the ground plane of the board. As one would expect for a shielded loop, the plots in Figures 5 and 6 are not very sensitive to the normal and rotated positions of the loop.

Scope plot for signal on board from shielded loop

Figure 5.
Injected Signal for Shielded Loop

Scope plot of injected signal for Shielded Loop

Figure 6.
 Injected Signal for Reversed Shielded Loop

One can conclude from the above plots that the unshielded loop works better for injecting signals into a path crossing a ground plane split than does the shielded loop. I suspect this result holds in general for injecting signals into circuit boards with ground and power planes.

Summary:Capacitive coupling from an unshielded loop is not always a problem that requires the use of shielded loops to solve. On the contrary, sometimes unshielded loops work better than shielded loops. This series of four Technical Tidbits on square shielded loops has shown that unshielded loops are useful for injecting signals in many cases and into circuit boards specifically. Given the ease of constructing an unshielded loop and its low cost, this is an important result. Another conclusion that can be drawn is that shields are just thick wires with inductance and capacitance and are not a "magic" solution to prevent unwanted coupling in all cases.

Additional articles on this website related to this topic are:
  1. Signal and Noise Measurement Techniques Using Magnetic Field Probes (~600K)
    • (1999 IEEE EMC Symposium paper)
  2. December 2000, An Easy to Build Shielded Magnetic Loop Probe
  3. June 2006, Measuring Structural Resonances
  4. October 2007, Using Noise Injection for Troubleshooting Circuits
  5. November 2007, Measuring Structural Resonances in the Time Domain - Part 1
  6. February 2008, Using Resonant Frequency Measurements to Extract Circuit Parameters
    (Calculating the Capacitance of a BNC Barrel Adapter)
  7. May 2008, The Square Shielded Loop - Part 1
    (Construction Details)
  8. June 2008, The Square Shielded Loop - Part 2, Parasitic Coupling
  9. July 2008, The Square Shielded Loop - Part 3, Parasitic Coupling Between Unshielded Wire Loops
Additional references:
  1. Scott Roleson's original article in the January 1998 issue of Medical Device & Diagnostic Industry Magazine
    on measuring structural resonances
Equipment used in this Technical Tidbit:
  1. The spectrum analyzer used is an Agilent N1996A.
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 titled (now available online as a WebEx seminar):

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).

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Copyright © 2008 Douglas C. Smith