Discussion: Figure 1 shows a
square unshielded wire loop held next 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 from a
Fischer Custom Communications TG-EFT pulse generator connected to the loop was measured at the BNC connector on the
board (left side) using an oscilloscope for cases where the loop is positioned as shown and for a 180 degree
rotation of the loop. A similar test was done using a square shielded loop 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. Bandwidth of the oscilloscope used was 500 MHz and the TG-EFT was
set to generate open circuit pulses of 100 Volts with a risetime of
about 2 ns and a pulse duration of about 100 ns. Figure 3 shows the
TG-EFT pulse generator.
Figure 2. Coupling a Signal into a Circuit Path Using a Square Shielded Loop
(embedded in a plastic housing for strength)
Figure 3. Fischer Custom Communications TG-EFT Pulse Generator
Figures 4 and 5 show the results as displayed on the oscilloscope using
the unshielded loop oriented in the both positions parallel to the path
over the break in the ground plane, 180 degrees rotated from each
other. Both plots have about the same pulse shape, amplitude, and
width. Any change between the plots might be attributable capacitive
coupling, however the only difference of any note is that the risetime
in Figure 1 is a little faster than in Figure 2. The overall difference
in the plots is not significant enough to make much difference when
using pulse injection for troubleshooting designs.
Figure 4. Injected Signal for Unshielded Loop
(Vertical scale = 500 mV/div, Horizontal scale = 5 ns/div)
Figure 5. Injected Signal for Reversed Unshielded Loop
(Vertical scale = 500 mV/div, Horizontal scale = 5 ns/div)
The plots in Figures 6 and 7 for the shielded square loop are also very
similar as well as having about the same risetime for both plots. The
amplitude of the injected pulse is about 20% less because the distance
between the center conductor of the semi-rigid coax forming the loop is
further from the path on the circuit board due to the diameter of the
coax and the thickness of the plastic housing. The slight
improvement in
matching of risetimes is not significant enough to warrant the extra
complication and cost of shielded loops. In addition, if the scope had
greater bandwidth, the resonance at about 600 MHz between the shielded
loop and the board demonstrated in the
August 2008 Technical Tidbit, The Square Shielded Loop - Part 4, Coupling to a PCB would likely cause distortions in the pulses displayed in Figures 6 and 7.
Figure 6. Injected Signal for Shielded Loop
(Vertical scale = 500 mV/div, Horizontal scale = 5 ns/div)
Figure 7. Injected Signal for Reversed Shielded Loop
(Vertical scale = 500 mV/div, Horizontal scale = 5 ns/div)