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Technical Considerations
Meeting Emissions Standards
The emissions limits that a piece of equipment must
meet will depend on the intended market for that piece
of equipment. If there is more than one market, more
than one emission standard may have to be met. This
can have a substantial effect on the circuit, size, and cost
of a filter. Standards like the CISPR’s or the FCC Rules
Part 15 have frequency limits of 150 kHz to 30 MHz.
FCC 15 AND CISPR CONDUCTED EMISSION LIMITS
DIGITAL EQUIPMENT
(Line Impedance Stabilization Network) and using
signal generators to inject RF of varying amplitude and
frequency, some insight can be gained as to the nature
of the problem. However, the criteria for acceptable
performance will have to be decided upon so that a
filter yielding this level of performance can be obtained
from the test procedure. Unfortunately, this still does not
eliminate the need for final testing in the actual operating
environment which, in many cases, occurs in the field.
Selection of a suitable filter can best be based on
the type of power supply or input impedance of the
equipment and on the mode of the offending RFI noise.
80 –
78 –
Quasi-peak
Noise Modes
dBμV
76 –
74 –
72 –
70 –
68 –
66 –
64 –
62 –
60 –
58 –
Average
Quasi-peak
Class A Industrial
Class A Industrial
Class B
Residential
Power line filters attenuate noise in two different modes.
Common Mode: Also known as line-to-ground noise
measured between the power line and ground
potential.
Differential Mode: Also known as line-to-line noise
measured between the lines of power.
Power line filters are designed to attenuate either
one or both modes of noise. The need for one design
over another will depend on the magnitude of each
56 –
noise type present. The attenuation is measured in dB
54 –
52 –
50 –
48 –
46 –
Average
Class B
Residential
(decibels) at various frequencies of signal.
Circuit Con ? guration
Power line RFI filters are generally built with two or
0,15
0,5
Frequency (MHz)
5
30
three-pole filter networks. As the number of poles and
the corresponding component count increases, the
EMI measurements are generally made using
Spectrum Analyzers with Average or Quasi-Peak
detectors in accordance with methods described
in CISPR 16. Quasi-Peak differs from Average
measurements by weight-averaging the peaks into
the total.
Equipment meeting these specifications can
utilize a filter with a fairly high cutoff frequency. Other
standards like FCC 18 with a low frequency limit of 10
kHz will result in the equipment using lower cutoff filters.
As might be expected, the lower the cutoff frequency,
the larger the physical size and the higher the cost of
the filter.
Conducted RFI Susceptibility
The problem of susceptibility can be extremely difficult
to deal with because the amplitude and frequency of
the offending RF noise are seldom known and are often
intermittent. If the malfunction can be duplicated by
isolating the equipment from the power line with LISN’s
cost will increase also. Trying to typify an equipment’s
impedance as either high or low for purposes of filter
selection may not be successful. If it is a complex
impedance, it could probably be low at some
frequencies, high at others, and some intermediate
value at still other frequencies.
Although we have been generally successful in
recommending a two-pole network for linear power
supplies and three-pole networks for switching power
supplies and synchronous motors, you should not limit
your testing to just one circuit type if either additional
circuit performance or lower cost is desired. Consider
the following: If the equipment looked strictly capacitive,
the performance of a two-pole network would be
reduced to that of a single-pole filter.
Figure 1a.
A signal source (E) with
its internal impedance
driving a capacitive load.
86
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Curtis Industries
A Division of Powers Holdings, Inc.
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