Biomedical Engineering Reference
In-Depth Information
Figure 4.22 A prototype implantable-device programmer is being tested to assess its susceptibility to
radiated EMI. An implantable device and a simulator need to interact with the device under test to assess
its behavior. These test accessories are placed under the aluminum foil shield. A TV camera relays the
image from the programmer's computer screen to the control station outside the shielded room.
will probably become more popular in the future. Since cellular telephones and handheld
transceivers can produce
fi
field strengths above 3 V/m, regulatory agencies are considering
increasing the EMI
fi
field level to 10 V/m for all medical electronic equipment. Passing 10 V/m
will be a very di
cult challenge for the designers of sensitive patient-connected devices!
A beefed-up indirect-injection ESD test can serve as the basis for a test to give a rough
indication of a device's susceptibility to radiated EMI. This is the way in which the military
test equipment hardened against electromagnetic pulses (EMP) generated either by nuclear
explosions. A cheap wideband EMI generator, albeit not nearly as powerful as that used to test
for EMP susceptibility, can be built using a high-voltage generator that charges a capacitor and
releases its energy into an antenna. The trick is to produce a very fast rise time (less than 1 ns,
if possible) and a relatively long total duration (100 ns or more). One way of doing this is
shown in Figure 4.23. The core of this wideband EMI generator is Blumlein's pulse genera-
tor. The capacitances of two transmission lines are charged by a high-voltage power supply
via a series charging resistor R charge . When charging, the transmission lines are e
ectively in
parallel because inductor L bypass does not present any substantial impedance to low-frequency
signals. When a certain voltage is developed across the transmission line, the spark gap breaks
down, e
ff
ectively shorting one end of transmission line 1. This causes a very fast pulse to
appear across the wideband antenna. Blumlein generators are often used to power nitrogen
lasers, ground-penetrating radar, and other instruments that require sharp, high-voltage pulses.
A traveling-wave TEM horn antenna can be used to radiate the pulse generated by the
Blumlein generator toward the device under test. A traveling-wave TEM horn consists of
a pair of triangular conductors forming a V structure in which a spherical TEM-like mode
wave propagates along the axis of the V. The schematic diagram for an experimental wide-
band generator circuit is shown in Figure 4.24. Here, a push-pull oscillator drives a TV
fl
ff
flyback transformer is not used. Instead, new primar-
ies are made by winding two sets of four turns each of insulated No.18 wire around the
exposed core of the
flyback. The original primary of the
fl
flyback transformer. Feedback for the oscillator is obtained through an
additional coil of four turns of No.24 wire wound around the core.
fl
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