Hameg Instruments HZ530 Manual Del Usuario página 36

actual risetime in CMOS circuits is not easily measurable in most
digital labs. To measure the actual risetimes, oscilloscopes with the
ability to measure 100ps (10E-10s) must be used. Such oscilloscopes
are available but at a significant price.
Such fast oscilloscopes are not really necessary to observe the digital
system operation. This is the reason that these fast oscilloscopes
are not used in digital laboratories and slower oscilloscopes are
used. However, slow scopes simulate a risetime which in reality
do not exist because they measure only the internal risetimes of
the oscilloscope.
This exposes a measurement problem: The relevant EMC characte-
ristics cannot be measured with existing equipment in many cases
and the necessary oscilloscope is very expensive.
A practical solution is to perform the measurements in the fre-
quency domain: The digital function is observed with a slower and
economical oscilloscope and the relevant EMC characteristics are
measured with a spectrum analyzer. Since the spectrum analysis
of corresponding frequency ranges is technically simpler than the
measurement of the equivalent risetimes, basic spectrum equipment
can be obtained which is relatively more economical. Spectrum
analyzers with a bandwidth of 1,000MHz are already suitable for
analyzing CMOS circuits. The corresponding oscilloscopes are still
very expensive.
Spectrum analyzers are high frequency equipment and have there-
fore an input impedance of 50 Ω. They are therefore not suitable
to measure directly in digital circuits because of this impedance
which will influence the circuit behavior. As a minimum the measu-
rement 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 Ω system over a wide
frequency range.
The following measurement results were measured with the High
Impedance Probe HZ530 connected to a Spectrum Analyzer and
with a digital scope.
In principle, it is easy to assume that it is possible to select signal-
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Änderungen vorbehalten / Subject to change without notice