Chapter 4 - Electromagnetic Compatibility And Medical Devices: Susceptibility to Radiated Electromagnetic Interference

Susceptibility to Radiated Electromagnetic Interference

IEC-61000-4-3 specifies a modulated RFI test of 3 V/m as representative of the radiated
electromagnetic interference that may be caused on a medical device by wireless communication
equipment. For critical equipment such as life-support devices, 10 V/m is used for
testing. Currently, tests should be performed at frequencies of 26 MHz through 1 GHz with
1 kHz at 80% amplitude modulation, but there is serious talk about extending the upper limit
to 3 GHz. The frequency band is covered in steps of 1% of the fundamental frequency. For
frequencies of 26 to 200 MHz, a biconical transmit antenna is commonly used. For frequencies
above 200 MHz, a double-ridged horn transmit antenna is the popular choice.

As shown in Figure 4.21, the testing is usually performed in a shielded enclosure with
anechoic material placed throughout the enclosure to minimize reflections. The transmit
antenna is typically located 3 m from the device under test. An isotropic field strength
meter is placed inside the room at a location physically close to the device under test and
used as a secondary indication of the field strength. Testing is performed utilizing linearly
polarized antennas, with the device under test exposed to both vertically and horizontally
polarized fields on each of four sides. In addition to the frequency sweep, the device under

Figure 4.21 Setup for assessing the susceptibility of a medical device to radiated emissions. Testing is performed in a shielded enclosure with anechoic material placed throughout the enclosure to minimize reflections. The transmit antenna is typically located 3m from the device under test. An isotropic field strength meter is placed inside the room to yield an indication of the field strength.

test is also exposed to a 3-V/m field of 900 MHz modulated with a 200-Hz square wave
and modulated with 50% duty cycle in both vertical and horizontal polarizations.

Sometimes, additional test and monitoring equipment is needed to generate test signals
and to evaluate the performance of the device under test. Figure 4.22 shows the experimental
setup used to test the RFI susceptibility of a prototype implantable-device programmer.
The implantable device programmed by this device is meant to interact with the patient’s
heart. Although the implantable device itself was not the subject of this specific test, it had
to be in communication with the programmer so that the performance of the programmer
could be evaluated while being exposed to the 3-V/m RFI. In addition, since the programmer
has an ECG input, a patient simulator had to be connected to the programmer during the
tests. The patient simulator as well as the implantable device were placed under an aluminum
foil shield. A shielded closed-circuit TV camera relayed the image from the programmer’s
computer screen to those who were monitoring the device outside the shielded room.

Because of the amount of EMI generated, there is no easy legal way of conducting this test
outside a shielded room. As such, the common engineering practice is to apply good design
practice and then cross fingers when running the test at a qualified facility. Design-lab testing

will probably become more popular in the future. Since cellular telephones and handheld
transceivers can produce field strengths above 3 V/m, regulatory agencies are considering
increasing the EMI field level to 10 V/m for all medical electronic equipment. Passing 10 V/m
will be a very difficult 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 generator.
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 effectively 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, effectively 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
flyback. The original primary of the flyback transformer is not used. Instead, new primaries
are made by winding two sets of four turns each of insulated No.18 wire around the
exposed core of the flyback transformer. Feedback for the oscillator is obtained through an
additional coil of four turns of No.24 wire wound around the core.

Applied at the input of the flyback driver, 12 V should produce 15 to 20 kV dc at the
output of the flyback’s tripler. This high voltage is used to charge two transmission line
capacitors Z1 and Z2 which are etched on a double-sided 0.4-mm-thick copper-clad PCB
as shown in Figure 4.25. The TEM horn antenna is formed from two truncated triangular
pieces of single-sided PCB and edge-soldered to the Blumlein generator board. The spark
gap is simply a copper or bronze U shape with a bolt and nuts that permit the discharge
gap width to be adjusted.

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