MEASUREMENTS

Long Time Distortion
• DEFINITION
Long time distortions affect slowly varying aspects of the signal,
such as APL changes which occur at one second intervals. The
distortion usually appears as a very low frequency damped
oscillation. The affected signal components range in duration
from 20 milliseconds to 10s of seconds.
Figure 37. A Flat Field Bounce signal.
The peak overshoot which occurs as a result of an APL change,
expressed as a percentage of the nominal luminance amplitude,
is generally quoted as the amount of distortion. Settling time is
also sometimes given, and occasionally the slope at the beginning
of the phenomenon, in % per second.
• PICTURE EFFECTS
Long time distortions are slow enough that they are often
perceived as flicker in the picture.
• TEST SIGNALS
A flat field test signal with normal sync signals and variable
APL is used to measure long time distortion. The signal should
be "bounced", or switched between 10% and 90% APL, at
intervals no shorter than five times the settling time. See Figure 37.
Figure 38. Long time distortion measurement
parameters.
• MEASUREMENT METHODS
Long time distortions are measured by examining the damped
low-frequency oscillation resulting from a change in APL. The
peak overshoot and settling time are generally measured, as
shown in Figure 38.
Waveform Monitor
It is usually necessary to use a storage oscilloscope or a waveform
monitor in the SLOW SWEEP mode to measure long time
distortion. A waveform photograph can be helpful in quantifying
the distortion. Once you have obtained a stable display or a
photograph, measure overshoot and settling time as shown in
Figure 38.
Figure 39. The VM700A Bounce display.
VM700A Automatic Measurement
Select Bounce in the VM700A MEASURE mode to obtain a
display of long time distortion. This display is shown in Figure 39.
Peak deviation and settling time are given at the bottom of the
screen.
36