User manual

33
Subject to change without notice
Practical Selection of Signal-Line Filters
generally poor ground connection of a three-pole
capacitor which is relatively high in inductance
compared to a R-C-R combination in surface
mount technology (SMT). Some offered three-pole
capacitorsarepoorhighfrequencylters.
Another example is a wideband choke used as a
signallinelter.
Figure 6 shows the results. The frequency spec-
trum is poorly suppressed, but the risetimes are
signicantly slowed down. Itshould benoticed
here that a time domain analysis only will lead
to poor EMC performance and the wrong con-
clusions. This is an expensive measure that will
inuencethedigitalfunctionwithdisappointing
EMC suppression.
AsanalexampleamodernSMTchipltercon-
sisting of two ferrite beads and a feed-through
capacitor is shown.
Figure 7 shows the results which are relatively
good. The spectrum is limited and the risetime
is surprisingly fast. The over- and under-shoot
issomewhatdisappointing.Thisoccursinlters
Figure 6
Figure 7
which consist of only inductance and capacitance.
In conclusion, it is observed that the digital cir-
cuit designer who is aware of EMC problems,
must look at the frequency domain and not only
at the time domain or a false picture may result.
Theoretically, everything is contained in the time
domain which is only differently presented in
the frequency domain. The problem rests with
the linear presentation and the resolution of the
oscilloscope. Using a generally poor oscilloscope
will not lead to a theoretically optimal solution.