MEASUREMENTS

Depth of Modulation
Figure 107. Depth of modulation
levels.
Figure 108. The zero carrier reference pulse as it
appears in a baseband signal.
Figure 109. A signal which extends to 700 mV, such as
this Staircase signal, is used in conjunction with the
zero carrier pulse to verify modulation levels.
• DEFINITION
Depth of modulation (percentage of modulation) measurements
indicate whether or not video signal levels are properly represented
in the RF signal.
The PAL modulation scheme yields an RF signal that reaches its
maximum peak-to-peak amplitude at sync tip (100%). In a properly
adjusted signal, blanking level corresponds to 76% and white
to 20%. The zero carrier reference level corresponds to 0%, as
shown in Figure 107.
• PICTURE EFFECTS
Overmodulation often shows up as non-linear distortions such
as differential phase and gain, and picture effects therefore
correspond to those caused by the various distortions. ICPM or
white clipping may also result. Undermodulation often results in
degraded signal-to-noise performance.
• TEST SIGNAL
A signal with black and white levels is required for depth of
modulation measurements. This signal is used in conjunction
with the zero carrier reference pulse, which the demodulator
typically places on one line in the vertical interval. In the composite
signal the zero carrier pulse appears as a 0.95 volt
(above blanking) bar approximately 30 microseconds in
duration. See Figure 108.
• MEASUREMENT METHODS
Modulation depth is measured at the output of a precision
demodulator by verifying that the ratios between the parts of the
signal are correct. Overall amplitude is not critical, but it should
be adjusted in the system to be approximately 1.25 volts from
sync tip to zero carrier at 100% transmitter power. This will
minimise the effects of non-linearities in the measurement system.
Waveform Monitor
Depth of modulation can be measured with a waveform monitor
graticule. Use the variable gain if necessary, and position the
zero carrier reference pulse at 1.25 volts and sync tip at 0 volts.
Verify that blanking level and white level occur at the prescribed
points (0.3 and 1.0 volts respectively). The voltage cursors can
also be used for this measurement.
• NOTES
27. Envelope Detection Mode
Depth of modulation measurements should be made with the
1450 in the envelope detection mode to minimise effects of
ICPM. (Quadrature distortion will not affect modulation depth.)
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