Figure 1 shows the test setup used for this experiment. A Fischer Custom Communications
TG-EFT high voltage pulse generator is connected through about 10
meters (~30 feet) of 50 Ohm coaxial cable to a square wire loop covered
with heat shrink tubing. Figure 2 shows construction of the loop.
A similar second loop is connected through one meter of coax cable to an Agilent
Close-up of Loop Construction
The resulting waveform
recorded on the scope is shown in Figure 3. The waveform is Mdi/dt of
the current flowing in the loop driven by the TG-EFT. For a
setting of 100 Volts peak of the TG-EFT, the resulting initial pulse
picked up by the second loop connected to the scope is about 4 Volts.
This property is useful for injecting controlled pulses into a PCB as
described in the August 2008 Technical Tidbit
However, since the output of the TG-EFT is a relatively low impedance
(<50 Ohms) during and for a relatively long time after the pulse, the reflection from the loop is reflected
again off of the TG-EFT and appears about 120 ns later, attenuated to about one half of
its original value in Figure 3. The reflections continue until they die out.
of the attentenuation of the pulse is due to loss in the cable. For
shorter cables, the round trip delay and the loss in the cable will be
less resulting in closer spacing of the pulses and less attenuation
between the pulses. A train of pulses such as shown in Figure 3,
especially if a shorter cable is used, can be used to excite a failure
mechanism which has a low probability of occurrence for each pulse by
supplying multiple pulses. But, often just one pulse is desired. This
can be achieved by adding a 50 Ohm series termination at the output of
Figure 3. Waveform Induced into Second Loop
Figure 4 shows the 50 Ohm series termination used for this experiment. Its construction is described in the April 2008 Technical Tidbit
on this site. Mounting of the termination at the output of the TG-EFT is shown in Figure 5.
Figure 4. 50 Ohm Shielded Series Termination
Figure 5. Termination Connected to the TG-EFT Pulse Generator
Figure 6 shows the resulting output from the second loop when the termination is used. Note that the
reflections are attenuated significantly with respect to the original
pulse allowing the circuit to receive a single pulse for
troubleshooting. Usually the TG-EFT output is gradually increased until a
response from the circuit under test occurs so the reflections in Figure
6 do not significantly affect the circuit troubleshooting.
Figure 6. Waveform Induced into Second Loop With Series Termination at the Pulse Generator