Hameg Instruments HZ530 Manual Del Usuario página 38

Figure 2 shows the results when a 47 Ω resistor is used. In the
time domain a significant improvement occurs. The overshoot is
reduced and the risetimes are somewhat slower. The linear dyna-
mic 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 insertion 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 47 Ω resi-
stor. The load continues to be the printed circuit track and another
gate input. In the time domain, the difference appears negligible.
In the frequency domain, the middle and upper frequency range
is significantly improved. If a slower oscilloscope is used, any im-
provement would no longer be recognizable in the time domain.
The limitation of using an oscilloscope and using only time domain
measurements is easily recognizable: The EMC relevance of a
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Änderungen vorbehalten / Subject to change without notice