MEASUREMENTS

Steady-State (Static) Sync Gain Distortion
• DEFINITION
Steady-state gain distortion of the sync signal is present when
horizontal sync amplitude is dependent on APL. This parameter
is evaluated by measuring sync amplitude at high and low APL
(typically 12.5% and 87.5%). The amount of distortion may
be expressed as a percentage of the amplitude at 50% APL,
or as a percentage of the maximum amplitude. This is an outof-service
test.
Steady-state gain distortion of the picture signal is also sometimes
measured. In this case APL dependencies of a 700 mV white
bar are evaluated.
Figure 94. A Staircase signal with variable APL.
• PICTURE EFFECTS
If only sync is affected, small amounts of static gain distortion
will not be noticeable in the picture. Large amounts of distortion
may affect the ability of some equipment to derive synchronisation
information and/or to clamp the signal. If the picture signal is
also affected, luminance levels will be APL-dependent if this
type of distortion is present.
• TEST SIGNAL
Any test signal with variable APL can be used to measure
steady-state sync gain. A 700 mV signal element such as a white
bar is required for steady-state picture gain measurements.
• MEASUREMENT METHODS
Figure 95. The sync pulse measures 300 mV at 50%
APL.
Waveform Monitor
To make a measurement, first select 50% APL and use the
waveform monitor variable gain to set the sync amplitude to
100%. Vary the APL of the signal to 12.5%, and then to
87.5%. At each APL level, record the amplitude of sync. The
peak-to-peak variation for the three APLs, expressed in percent,
is typically quoted as the steady-state sync gain distortion.
This measurement can be made with the 1781R voltage
cursors in the RELATIVE mode. Figures 95 and 96 illustrate the
measurement procedure.
Figure 96. At 87.5% APL, the sync pulse measures
260 mV. This indicates a steady-state distortion of
about 13%.
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