Hz530 Near-Field Probe Applications Practical Selection Of Signal-Line Filters - Hameg Instruments HZ530 Manual Del Usuario

HZ530 Near-Field Probe Applications
Practical Selection of Signal-Line Filters
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The steadily increasing operating speed of modern digital logic
causes significantly greater concerns with EMC problems. This has
become more noticed by all manufacturers of electrical and electronic
devices since 1 January l996, the effective compliance date for the
European Union EMC Directive. The EMC Directive does not cause
the radiated interference problems, but it causes conflict with the
requirements of compliance for each manufacturer.
The times are long gone when the EMC problems could be left to
the EMC department or a non-compliant product was not noticed
and could be sold anyhow. Every circuit designer must at the be-
ginning of a development be aware of potential EMC problems to
even allow the successful certification of a product. Printed circuit
boards must be built differently than was possible several years
ago. A reasonable broadband decoupling of the supply voltages is
the present state-of-the-art.
But also the design of signal lines must be considered and can not
be left to chance. Digital signals have a spectrum with a bandwidth,
B, that is related by:
B = 1 / (3.14 • tr),
Consequently, the risetime of a digital signal transition is the deter-
minant. The shorter the risetime, the wider the frequency range.
However, the calculated bandwidth is not as important as the one
that actually exists which can be significantly different than the
calculated one. The reason for this is that the calculated value is
referenced to a capacitive total load. For most practical cases this
does not occur. An approximate calculation shows that one half of
the capacitive load means a twice faster risetime; e.g. a micropro-
cessor has a specified risetime of 2 x 10
load is supposed to be 150 pF. If a signal from this processor is
loaded only with a CMOS gate of 12.5 pF, the risetime will be 12
times faster and a value of 200 x 10E-12 s (200ps) must be expec-
ted. In the frequency domain, 200ps is equivalent to a bandwidth
of 1.6GHz. Even in practical circuits, where additional capacitance
can be expected, actual bandwidths of over 1GHz are measurable.
From an EMC point of view, this is naturally very damaging. The
where tr is the risetime.
s (2ns). The capacitive
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