A square wire loop excited by a signal generator can be used to
inject pulses or RF signals precisely into a circuit without disturbing
circuit conditions. Used in this way, such a loop can be a
significant aid in design troubleshooting. Suggestions for such uses
: Many other articles on this website discuss using
loops to measure signals and noise in a circuit. In this article, just
the opposite is discussed, injecting signals and noise into circuits.
Figure 1 shows two similar loops constructed from stiff wire and held
adjacent to each other. If one loop is driven from a signal generator,
a significant signal can be coupled into the other loop. Either a pulse
continuous wave RF signal generator may be used. The loop to loop
transmission, such as in Figure 1, gives an estimate of what is
possible to inject into any nearby circuit. Injection into printed
boards using ground and power planes will be smaller than the loop to
loop transmission because of the planes in the board. In that case, the
injected noise can be measured with an oscilloscope connected to the
If an energized
loop is held close to a circuit, the pulses or RF signal can be injected
precisely into parts of the circuit for troubleshooting purposes.
Injecting noise into a circuit this way does not affect the DC
operating conditions of the circuit and has minimal effect on AC
conditions except for the noise induction.
For the case shown in Figure 1, the loops are made of 16 gauge brass wire
purchased from a hardware store and covered with heat shrink tubing.
One end of each loop is soldered to the side of a BNC barrel adapter
and the other end of the wire is inserted into the center of the BNC adapter. A
convenient size for such loops is about two cm or one inch. The loop
size should be such as to allow access to the circuit to be tested.
Figure 2 shows the output from one loop of Figure 1, about 2.5 cm, when the other loop is driven from an IEC 61000-4-4
fast transient (EFT) generator. The 50 Ohm IEC generator is set to 250
Volts and the receiving loop is connected to the 50 Ohm input of an
oscilloscope. 50 Ohm
coaxial cable was used to connect both loops. It is important to make
sure the brass wire is securely soldered to the shield of the coaxial
cable from the generator, the BNC barrel adapter in this case. If the
ground side of the loop comes loose, the full open circuit output of
the EFT generator, as much as 4000 Volts,
will be present on the loop and an RF burn to the skin or circuit
damage is possible.
The EFT generator has a 50 Ohm output, so the 250 Volt setting
gives a five Ampere short circuit current through the loop neglecting
the drop across the loop itself to a first order approximation. For
reasonable assumptions on wire size and a 2.5 cm loop, the inductance
of the loop is about 80 nH and the
voltage across the inductance the loop is approximately given by:
E = Ldi/dt ~ 80nH(5A/5ns) ~ 80V
(this is small compared to the drop across the 50 Ohm source impedance of the generator)
The voltage drop across one side of the loop would be about one fourth of
80 Volts or about 20 Volts. Since the output of the receiving loop reached
almost 4 volts, about 1/5 of the 20 Volt drop in the adjacent side of the
energized loop was coupled into the receiving loop, a typical number.
250 Volt EFT Coupled Signal Between 2.5 cm Square Loops
Imagine now, holding an energized loop next to a wire whose insulation thickness was similar to that of
the receiving loop (thicker insulation would reduce coupling). A
similar voltage to that shown in Figure 2 would be injected in series
with the wire since the coupling is mostly inductive
. The exact value of coupled noise would be affected by nearby ground/power planes or metal and can be determined
with an oscilloscope. Using this technique, one can inject pulses of known amplitude into
precise locations in a circuit to see how the circuit reacts for
Continuous RF signals can also be injected if an RF signal
generator is used to excite a loop. In many instances, an output of
+10 to +13 dBm is adequate to elicit problems to show themselves in a
Using nothing more than a wire loop, pulses and RF signals can be precisely
injected into circuits for troubleshooting purposes. Significant noise
can be injected with amplitudes of several Volts or more possible.
tutorial on this subject, covering background as well as more technical details, is available at:
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