PAL SYSTEM TELEVISION MEASUREMENT : ICPM

ICPM

DEFINITION
ICPM (Incidental Carrier Phase Modulation) is present when picture carrier phase is affected by video signal level. ICPM distortion is expressed in degrees using the following definition:
ICPM = arctan (quadrature amplitude/video amplitude)
PICTURE EFFECTS
The effects of ICPM will depend on the type of demodulation used to recover the baseband signal from the transmitted signal.
ICPM shows up in synchronously demodulated signals as differential phase and many other types of distortions. With envelope demodulation, the demodulation typically used in home receivers, the baseband signal is generally not as seriously affected and the effects of ICPM are rarely seen in the picture. The sound, however, is another matter.
ICPM may manifest itself as audio buzz in the home receiver.
In the intercarrier sound system, the picture carrier is mixed with the FM sound carrier to form a sound IF. Audio rate phase modulation in the picture carrier can therefore be transferred into the audio system and heard as a buzzing noise.
TEST SIGNAL
ICPM is measured with an unmodulated linearity signal. A staircase is generally used but a ramp signal may also be used.
MEASUREMENT METHODS
ICPM is measured by examining an XY plot of VIDEO OUT versus QUADRATURE OUT with the demodulator operating in the synchronous detection mode. A phase error will produce an output from the quadrature detector.
If this phase error varies with amplitude, the result is a tilted display. The demodulator zero carrier reference pulse must be turned on and the detection mode set to synchronous. Select the SLOW time constant when
using the 1450.
Waveform Monitor.
To obtain an ICPM display with a waveform monitor, connect the demodulator outputs to the waveform
monitor inputs as shown in Figure 101. Select ICPM in the 1781R MEASURE menu or EXT HORIZ on the 1481 front panel.
Although it is not strictly necessary, it is generally recommended that the signals be lowpassfiltered to make the display easier to interpret. With either the 1781R or the 1481, this can be accomplished in the vertical
channel by selecting the LOWPASS filter. Use an external 250 kHz lowpass filter for the horizontal. Figure 101 shows a typical measurement setup.

Figure 101. How to set up the 1781R for ICPM measurements.

Figure 102. 1781R ICPM display with no distortion present.

The display resulting from this configuration, which appears on the right-hand screen in the 1781R, is shown in Figure 102. The amount of tilt (deviation from the vertical) is an indication of ICPM. There is no ICPM
in the signal shown in Figure 102, while distortion is present in Figure 103. To adjust for minimum ICPM, make the line as nearly vertical as possible.
The 1781R has an electronic graticule which can be used to quantify the amount of tilt. The waveform should be positioned so the small dot corresponding to the zero carrier reference pulse is set on the cross at the
top of the screen. The horizontal magnification will automatically be set to X25 when this mode is selected. X50 magnification can be used for greater resolution.
Start with the two graticule lines widely separated and use the l a rge knob to move them together to the point where a graticule line first contacts one of the dots. Disregard the “loops” in the display. These correspond to the level transitions and are not indicative of distortion. The amount of ICPM distortion is
indicated on the screen (see Figure 103).
An external ICPM graticule is available for the 1481. Position the zero carrier reference pulse, which shows up as a small dot, on the cross at the top of the graticule. The graticule is calibrated for 2 degrees per radial
division when the horizontal magnifier is set to X25 or 1 degree per division with 50X horizontal magnification. Read the amount of ICPM from the graticule at the point of maximum distortion.

Figure 103. The 1781R electronic graticule indicating an ICPM distortion of 6 degrees.

VM700T Automatic Measurement.
The ICPM selection in the VM700T MEASURE mode provides an ICPM display and numeric results. An ICPM measurement is also provided in the AUTO mode. The quadrature output must be connected to
VM700T “C” input.

Figure 104. The 1481 ICPM graticule.

Figure 105. The VM700T ICPM display.

NOTES
25. Configuring the 1481.
1481 instruments are shipped with unblanking disabled in the EXTERNAL HORIZONTAL mode to prevent damage to the CRT. ICPM measurements can be made in line select with the instrument in this mode. For full-field measurements, the unblanking must be enabled.
Instructions on how to accomplish this can be found in the OPERATING CHANGES section of the 1481 manual.
26. Other XY Displays.
Any XY display can be used to measure ICPM. Connect QUADRATURE OUT to the horizontal and
VIDEO OUT to the vertical and use the formula given on page 61 to calculate the amount of distortion. For small errors, some amount of gain will be needed to improve the measurement resolution.
Lowpass filters in both channels are recommended.
27. Phase Noise.
Some demodulators have large amounts of phase noise which makes it difficult to make ICPM measurements on w a v e f o rm monitors. The VM700T AVERAGE mode can eliminate this effect. The Tektronix 1450 has sufficiently low phase noise for measurements with a waveform monitor, as do all TV1350 units shipped after July, 1998.
Older TV1350 units can be retrofitted to improve phase noise performance. Contact your local Tek ronix service depart m e n t for information on how to update older instruments.


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Table of Contents

Preface	3
Good Measurement Practices	4
EQUIPMENT REQUIREMENTS	4
CALIBRATION	6
INSTRUMENT CONFIGURATION	6
DEMODULATED RF SIGNALS	8
TERMINATION	8
DEFINITION OF THE PAL TELEVISION STANDARD	8
PERFORMANCE GOALS	8
Waveform Distortions and Measurement Methods	9
I VIDEO AMPLITUDE AND TIME MEASUREMENTS	9
Amplitude Measurements	10
Time Measurements	12
SCH Phase	15
II LINEAR DISTORTIONS	18
Chrominance-to-Luminance Gain and Delay	19
Short Time Distortion	24
Line Time Distortion	26
Field Time Distortion	28
Long Time Distortion	30
Frequency Response	31
Group Delay	36
K Factor Ratings	38
III NONLINEAR DISTORTIONS	41
Differential Phase	42
Differential Gain	46
Luminance Nonlinearity	50
Chrominance Nonlinear P h a s e	5 2
Chrominance Nonlinear Gain	5 3
Chrominance-to-Luminance Intermodulation	54
Transient Sync Gain Distortion	55
Steady-State (Static) Sync Gain Distortion	56
IV NOISE MEASUREMENTS	57
Signal-to-Noise Ratio	58
V TRANSMITTER MEASUREMENTS	6 0
ICPM	61
Depth of Modulation	63
GLOSSARY OF TELEVISION TERMS	64
APPENDICES
APPENDIX A - PAL COLOUR BARS	67
APPENDIX B - SINE-SQUARED PULSES