Hameg HZ540 Benutzerhandbuch Seite 32

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
Figure 2 shows the results when a 47Ohms re-
sistor is used. In the time domain a significant
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 specific 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 inserting a capacitor to form an RC filter.
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 significantly 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
32
Subject to change without notice
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-filter.
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 significantly slowed down. The
approach may be questionable if this slow risetime
influences the digital operation. In this case, the
component values may be varied to find 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 filters can be
evaluated using the same procedures.
Figure 5 shows the use of a three-pole capacitor
used as a signal-line filter in the same circuit
as used in the previous examples. The results
are disappointing: Even though the risetime is
significantly reduced, the frequency spectrum is
only marginally reduced. This results from the