User manual

31
Subject to change without notice
sure directly in digital circuits because of this im-
pedancewhichwillinuencethecircuitbehavior.
As a minimum the measurement results are false.
Consequently, for the measurement in digital
circuits a high impedance probe is required which
does not load the circuit and convert the signal to
a 50 Ohms system over a wide frequency range.
The following measurement results were mea-
sured with the High Impedance Probe connected
to a Spectrum Analyzer and with a digital scope.
In principle, it is easy to assume that it is possible
toselectsignal-lineltersfromcatalogvalues.
Well-known manufacturers offer filters with
measurement data in the time- and frequency-
domain.Unfor tunately,thelterdataisperformed
with an entirely resistive load and therefore the
data looks very good. However, in practice an
entirely resistive circuit seldom exists. Therefore,
theltersmustbeevaluatedwheninstalledina
practical circuit. It is then observed that the per-
formanceoftheltersisnotaspromisedinthe
catalog. This shall be demonstrated with a series
of illustrative examples which are measured in
circuits of the 74 ACT family. The gates are always
operated with a 5MHz frequency.
Figure 1 shows the time and frequency domain
outputs of such a gate which is mounted on a
printed circuit and is not loaded. The frequency
spectrum is measurable to 1,000MHz. In fact, it
extends even above 1,000MHz, but for compari-
son purposes all measurements are scaled only
1,000MHz. In the time domain relatively strong
over and under shoot and fast risetimes are obser-
vable. This signal is very poor relative to the EMC
characteristics. The excessive bandwidth permits
radiation to take place on relatively small printed
circuit boards. When this signal is conducted to
other parts, it is especially important to limit the
spectrum to avoid excessive shielding structures.
Asarstmeasuretolimitthespectrum,aresi-
stor is recommended between the gate output
and the conductor connection. The conductor is
simulated by an individual gate input to obtain a
realistic circuit. The connection and the conductor
length must correspond to the actual relationship
tomake the measurements of signal line lter
evaluation meaningful. The effectiveness of line
ltersisstronglyinuencedbytheirtermination.
Practical Selection of Signal-Line Filters