User manual

32
Subject to change without notice
Practical Selection of Signal-Line Filters
Figure 2 shows the results when a 47Ohms re-
sistorisused.Inthetimedomainasignicant
improvement occurs. The overshoot is reduced
and the risetimes are somewhat slower. The
linear dynamic range of an oscilloscope can not
demonstrate adequately the EMC characteristics
of the signal. The frequency spectrum shows only
a slight decrease of the upper frequencies. The
oscilloscope probe is partially responsible for this
error since the probe has a capacitance of 6pF.
The Hi-Z probe has only a load capacitance of 2pF.
ByselectingspecicvaluesofresistorstheEMC
characteristics may be slightly improved, but an
EMC success can not be scored with only the in-
sertion of a resistor. Another improvement can be
madebyinser tingacapacitortoformanRClter.
Figure 3 shows the results when 100pF is added
to the 47Ohms resistor. The load continues to be
the printed circuit track and another gate input.
In the time domain, the difference appears ne-
gligible. In the frequency domain, the middle and
upperfrequencyrangeissignicantlyimproved.
If a slower oscilloscope is used, any improvement
would no longer be recognizable in the time do-
main. The limitation of using an oscilloscope and
using only time domain measurements is easily
recognizable: The EMC relevance of a suppression
measure is not noticeable.
The next step is to insert a 47Ohms, 100 pF,
47OhmsT-lter.
Figure 4 shows that the change is quite noticeable
when compared to Figure 3. The frequency range
is now practically reduced to 200 MHz. At the same
timetherisetimeissignicantlysloweddown.The
approach may be questionable if this slow risetime
inuencesthedigitaloperation.Inthiscase,the
componentvaluesmaybevariedtondacom-
promise between desired EMC characteristics
and digital functionality. This suitable example
demonstrates the effectiveness of the measure-
ment procedures recommended here. Several
complete signal-line filters are commercially
available.Theeffectivenessoftheselterscanbe
evaluated using the same procedures.
Figure 5 shows the use of a three-pole capacitor
usedas asignal-line lterin thesamecircuit
as used in the previous examples. The results
are disappointing: Even though the risetime is
signicantlyreduced,thefrequencyspectrumis
only marginally reduced. This results from the
Practical Selection of Signal-Line Filters