User manual


Subject to change without notice
P r a c t i c a l S e l e c t i o n o f S i g n a l - L i n e F i l t e r s
make the measurements of signal line lter eva-
luation meaningful. The effectiveness of line lters
is strongly inuenced by their termination.
Figure  shows the results when a 7Ohms 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 pF. 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 insertion of a resistor. Another improvement
can be made by inserting a capacitor to form an
RC lter.
Figure  shows the results when 100pF is added
to the 7Ohms resistor. The load continues to be
the printed circuit track and another gate input.
In the time domain, the difference appears neg-
ligible. 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 7Ohms, 100 pF,
7Ohms T-lter.
Figure  shows that the change is quite noticeable
when compared to Figure . The frequency range
is now practically reduced to 00 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
Pr a c t ica l S ele c t i on o f Sig n a l -Li n e Fil t e r s
Figure 4
Figure 5