Figure 1 shows an overall view of the test setup comprised of a pair of square wire (unshielded) loops connected to an Agilent N1996A spectrum analyzer
up to perform a two port insertion loss measurement. The two loops are
positioned end-to-end. A close-up of the two loops is shown in Figure 2. The loops are about one inch, a few cm, on a side.
In Figure 2, the loops are reversed in position, that is
the + side of one loop (center conductor of BNC) is opposite the - of
the other loop (ground side of BNC). This will be referred to as the
"reversed" direction. By inverting one of the loops in Figure 2 the
"normal" direction is obtained (+ side to + side of the loops).
Data is presented for both directions. Capacitive coupling between the
loops would cause the normal and reversed positions of the loops to
yield different responses or cause resonant responses such as with the
shielded loops discussed in the June 2008 Technical Tidbit
If only magnetic field coupling existed between the loops, the change
resulting when one of the loops is reversed would be a 180 degree phase
shift in the output, which would not change the spectrum analyzer plot.
If significant capacitive coupling existed between the loops, the
output of the receiving loop would be the combination of the inductive
and capacitive components. Since the phase of the inductive component
is reversed when one loop is reversed but the phase of the capacitive
component is not, the spectrum analyzer plot will be different.
Figure 2. Close-up of Loops for Coupling Measurement
Figures 3 and 4 show the loop-to-loop transmission from 10 MHz to 1 GHz
for the reversed case and normal cases of the two loops as explained
above. In last month's June 2008 Technical Tidbit
, it was shown that
the loop-to-loop transmission for a pair of shielded loops in the end-to-end configuration contained a
deep resonance of about 30 dB due to inter-loop capacitance. Neither
Figure 3 or Figure 4 shows such a resonance, with the frequency
response being relatively flat by comparison. The differences between
Figure 3 and Figure 4 are about 6 dB maximum at a few frequencies and
the traces are not as smooth as the shielded loops. However the "lumps"
in the frequency response in Figures 3 and 4 are only a few dB in peak amplitude, much
smaller than the 30 dB resonant dip seen in the shielded loop case
discussed in the June 2008 Technical Tidbit
Response of End-to-End Loops (reversed directions)
Response of End-to-End Loops (normal directions)
The relatively flat frequency response in Figures 3 and 4 are
indicative that simple wire loops may be useful for injecting signals
into circuit board paths and cables by inductive coupling. Using loops for this purpose will
be covered in Part 4 of this series of Technical Tidbits.
Figure 5 shows the overview of a related case where the loops are
overlapped to insure maximum coupling. Figure 6 shows a close-up of the
loops. Notice in Figure 6 that the loops are arranged in the normal
position as opposed to reversed in Figure 2, that is the side of the
loops connected to the BNC center pin (or shield connection) are both
on the same side.
Figure 5. Measuring Loop to Loop Coupling for a Pair of Square Shielded Loops
Figure 6. Close-up of Loops for Coupling Measurement
Figures 7 and 8 show the coupling from 10 MHz to 1 GHz for the setups of
Figures 5 and 6 in both the normal and reversed orientations. The
difference between the two traces is only a few dB, and without
shielding! The response curves look very close to the response of
shielded loops that are overlapped, within a few dB.
Response of Overlapped Loops (normal direction)
Figure 8. Response of Overlapped Loops (reversed direction)