EMI standards establish that radiated-emissions test measurements should be performed at
a distance of 10 to 30 m, depending on the device’s classification. For compliance testing,
the device under test should be placed on a nonconductive table 0.8m above a ground
plane. The table is typically centered on a motorized turntable that allows 360° rotation. A
measurement antenna is positioned at a distance of 10 to 30 m as measured from the closest
point of the device under test. The radiated emissions are maximized by configuring
and rotating the device under test as well as by raising and lowering the antenna from 1 to
4 m. A spectrum analyzer with peak detection capabilities is used to find the maxima of the
radiated emissions during the testing. Then, final measurements are taken using a spectrum
analyzer with quasi-peak function with a measurement bandwidth of 120 kHz. The test
setup is shown in Figure 4.4. The limits for radiated emissions per EN-55011 for group 1
devices are presented in Table 4.2.
In reality, devices to be tested are not usually taken directly to the open-field test site.
Rather, they are first scanned for potentially offensive radiated emissions in a small shielded
room. The compliance testing is then conducted in the 10-m open-field test site, paying
special attention to peak emissions detected in the shielded room. This is almost a practical
necessity, because open-field test sites, even when located far from large metropolitan
areas, are still inundated by human-made RF signals. As an example, Table 4.3 shows
results obtained recently when testing an implantable-device programmer for radiated
emissions. The specific frequencies selected for testing were identified the night before
taking the device to an open-field test site in the middle of Texas’s hill country. Figure 4.5
shows the device being tested at the open-field site. The device sits atop a motorized
turntable. A biconical antenna can be seen placed 10 m away from the device under
test. At the 10-m distance specified for the tests, radiated emissions have their electric-field
E and magnetic-field H vectors orthogonal to each other but in the same plane. Under these
conditions, electromagnetic propagation occurs as a plane wave.
If the test probe is brought closer and closer to the device under test, however, the nature
of the electromagnetic field changes. Near the source of the radiation, the field produced
is mostly a function of the impedance of the source. If the field is generated by a circuit
carrying high current and low voltage, the field will be mostly magnetic in nature. If on the
other hand, the field is produced by an element placed at high voltage but carrying little or
no current, the field will be mostly electric in nature. This is the domain of the near field,
while the plane wave is in the domain of the far field.
The ideal generator for a magnetic field, or H-field as it is known, is thus a circular loop
of area S(m2) carrying an ac current I of wavelength λ. It should be noted that although a
static field is generated by a dc current and can be calculated with the method to follow,
static H-fields do not cause radiated emissions and are thus disregarded for EMI purposes.
If the loop size is smaller than the observation distance D, the magnitudes of the E and H
vectors can be found using the solutions derived from Maxwell’s equations. In the near
field, the simplified values for these magnitudes are
| |  |
and
where Z0 equals the impedance of free space, 120π or 377 Ω. Inspecting these equations,
we find that in the near field, H is independent of λ and decreases drastically with the
inverse of the cube of the distance. At the same time, the electric field increases as frequency
increases and falls off with the inverse of the square of distance.
The wave impedance may be defined as the division of E by H in an electromagnetic
version of Ohm’s law:
Thus,
where D < 48/f(MHz). In the far field [i.e., D > 48/f(MHz)], on the other hand, both E- and
H-fields decrease as the inverse of the observation distance as described by
which maintains a constant impedance equal to Z0, allowing direct calculation of radiated
power density in W/m2 simply by multiplying E and H. Notice that E and H, and thus
power, increase with the square of frequency. This shows, once again, that limiting the
bandwidth of radiated signals by a pulse train is of utmost importance in controlling EMI.
The region dividing the near field from the far field is called the transition region [i.e.,
at D ≈ 48/f(MHz)]. In it, abrupt transitions occur on the near-field characteristics until a
smooth blending leads to the far-field characteristics. Electromagnetic fields can also be
created by passing an alternating current through a straight wire dipole, just as in a radio
antenna. In this case, the near-field electric and magnetic vector amplitudes are
and
where l is the dipole length in meters. In contrast with the near-field H of a loop which falls
with the inverse of D3, the near-field H of a dipole falls off as 1/D2. Similarly, the near-field
E of a dipole falls off as 1/D3, in contrast to the near-field E of a loop that falls as 1/D2. The
wave impedance of emissions radiated by a dipole is also affected differently by frequency:
Compare this equation with the equation describing Zwave in the near field. The change in
wave impedance as a function of frequency in the case of a dipole is inverse to that of a loop.
In the far field, the behavior of the E- and H-fields is again similar to that of electromagnetic
radiation from a loop; that is, they decrease as the observation distance increases
as described by
Beyond the transitional point, the wave impedance again remains constant at the value of
Z0. The result of a constant impedance in the far field means that the ratio of E to H components
remains constant regardless of how the field was generated.
Of course, real-life circuits are neither ideal open wires nor perfect loops, but rather,
hybrids of these two. In a simplified form, as shown in Figure 4.6, a more realistic model
of a circuit which radiates electromagnetic emissions can assume that an ac voltage source
causes the flow of a current I in a rectangular loop enclosing an area S. The source impedance
is Zsource, and the impedance of the load is Zload, resulting in an overall equivalent
impedance of Zcircuit = Zsource + Zload.
In the near field, the electric- and magnetic-field vector magnitudes are given by
where Zcircuit ≥ 7.9D(m)×f(MHz), or
where Zcircuit ≤ 7.9D(m)×f(MHz), and
In the far field, the electric- and magnetic-field vector magnitudes are given by
and
The second lesson of controlling radiated emission leaps out from these equations-keep the
area enclosed by loops carrying strong time-varying currents to the minimum possible.
Similarly, traces carrying high voltages should be kept as short as possible and be properly
terminated.
Besides directing our attention to the parameters affecting radiated emissions, these
equations are very useful when designing for compliance with EMI requirements. As
exemplified by Figure 4.7, near- and far-field ballpark estimates of EMI can be obtained
from known circuit parameters for a large number of common circuit topologies.
EMI standards establish that radiated-emissions test measurements should be performed at
a distance of 10 to 30 m, depending on the device’s classification. For compliance testing,
the device under test should be placed on a nonconductive table 0.8m above a ground
plane. The table is typically centered on a motorized turntable that allows 360° rotation. A
measurement antenna is positioned at a distance of 10 to 30 m as measured from the closest
point of the device under test. The radiated emissions are maximized by configuring
and rotating the device under test as well as by raising and lowering the antenna from 1 to
4 m. A spectrum analyzer with peak detection capabilities is used to find the maxima of the
radiated emissions during the testing. Then, final measurements are taken using a spectrum
analyzer with quasi-peak function with a measurement bandwidth of 120 kHz. The test
setup is shown in Figure 4.4. The limits for radiated emissions per EN-55011 for group 1
devices are presented in Table 4.2.
In reality, devices to be tested are not usually...
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