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
 URL:      www.dsmith.org
 Email:    doug@dsmith.org

Transient Suppression Plane

Figure 1. Test Setup - Overall View
Figure 2. Test Setup - Close-up View

Figure 1 above shows a test setup in which an ESD current passes through two adjacent current probes positioned on the current carrying wire. A floating metal plate, held up by a book, is placed near but not contacting the wire. Figures 3 and 4 show the resulting output of the two current probes. The upper trace is from the current probe closest to the ESD simulator and the lower trace is the downstream probe (the one on the left above). The outputs are essentially identical except that the downstream probe shows slightly less high frequency detail (slightly longer risetime).

When the otherwise floating plate is making contact with the wire, the resultant current probe outputs are shown in Figures 5 and 6 below. Notice that the risetime of the downstream probe is significantly lengthened. This results from a current on the wire after the plate that has much lower di/dt and therefore reduced interference potential.

The free space capacitance of the plate represents a lower impedance to the high frequency components of the ESD current than the inductance of the wire. Therefore, the high frequency components of the current enter the plate and do not show up in the output of the downstream current probe. A metal plane used for filtering high frequency currents this way is often referred to as a Transient Suppression Plane.

Why does the output of the downstream probe in Figure 4 have a slightly longer risetime than the upstream probe? Click here for the answer. The effect is similar to the effect of the plate above.

Figure 3. Current Probe Outputs
(Plate Not Connected)

Figure 4. Expanded View of Figure 3
(Rising Edge)

Figure 5. Current Probe Outputs
(Plate Touching Wire)

Figure 6. Expanded View of Figure 5
(Rising Edge)

A pair of current probes with a typical transfer impedance accuracy of +/- 2 dB will likely have transfer impedances that are not close enough for comparing the outputs on a linear scale as has been done here. The data in Figures 3-6 was taken with a pair of Fischer F-33-1 current probes that have transfer impedances matched to within about 2%.

Top of page

Questions or suggestions? Contact me at doug@dsmith.org

Copyright © 1999 Douglas C. Smith