Overdrive and Pulse
Rounding
Additional timing errors may be caused by
triggering with insufficient overdrive, see Fig
-
ure 4-17. When triggering occurs too close to
the maximum voltage of a pulse, two phenom
-
ena may influence your measurement uncer
-
tainty: overdrive and rounding.
Overdrive: When the input signal crosses
the hysteresis band with only a mar-
ginal overdrive, triggering may take
some 100 ps longer than usual. The
specified worst case 500 ps system-
atic trigger error includes this error, but
you can avoid it by having adequate
overdrive.
Rounding: Very fast pulses may suffer
from pulse rounding, overshoot, or
other aberrations. Pulse rounding can
cause significant trigger errors, partic
-
ularly when measuring on fast cir
-
cuitry.
Auto Trigger
Auto Trigger is a great help especially when
you measure on unknown signals. However,
overshoot and ringing may cause Auto to
choose slightly wrong MIN and MAX signal
levels. This does not affect measurements like
frequency, but transition time measurements
may be affected.
Therefore, when working with known signals
such as logic circuitry, set the trigger levels
manually.
Always use manual trigger levels if the signal
repetition rate drops below 100 Hz (default),
or below the low frequency limit set by enter
-
ing a value between 1 Hz and 50 kHz in the
menu Auto Trig Low Freq. You can reach it
by pressing SETTINGS ® Misc.
Measuring Functions
4-16 Measurement Errors
T r i g g e r l e v e l
C r o s s i n g t r i g g e r l e v e l
A c t u a l t r i g g e r i n g
Fig. 4-17 Insufficient overdrive causes
Trigger Error.
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