I
-
Depth
of
Modulation
Test
Description
The depth of modulation test measures the percentage (50 to 93 percent
measurement range) of amplitude modulation (AM) on the visual carrier. The
analyzer measures the horizontal synchronization-pulse level and vertical
interval test signal (VITS) white level from which it calculates the percentage
Scrambled Channels
The visual carrier AM modulation depth test may not be valid for scrambled channels, depending on
the method used to scramble the channel’s signal. So, when testing a scrambled channel, You may
want to momentarily turn off scrambling for the channel while the channel is being tested for AM
modulation depth. You can use control line A of the analyzer auxiliary interface to determine when the
analyzer is testing a scrambled channel. When testing a scrambled channel, the control A line
fpin
11
of the analyzer auxiliary interface is set high in transistor-to-transistor logic
ITTL).
To perform the depth of modulation measurement, the analyzer does the
following:
1. Changes the center frequency and reference level of the analyzer.
2. Changes the amplitude scale and span of the analyzer.
3. Measures the minimum level and the maximum level and calculates the
percentage of AM modulation depth.
Changes the center The visual carrier is centered on the analyzer screen and a 1 MHz resolution
frequency and bandwidth is used to capture the entire modulation signal. For better
reference level of the accuracy, the reference level of the analyzer is changed to the level of the
analyzer visual carrier.
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