4.2.3.3 M easurement of the C /N ratio (analogue channels)
This function allows the user to measure the ratio of the video carrier level to
the noise level present in the channel.
Follow the steps listed below to take this measurement.
1) Select the analogue channels measuring mode.
2) Tune the video carrier in the channel and wait a few moments until the read
level is stabilized.
3) Press the C /N [14] key
4) Wait a few seconds until the measurement is stabilized (maximum 10 seconds).
When the C /N [14] key is pressed, the instrument automatically takes a series
of measurements in order to arrive at the closest approximation to the noise level.
The time of approximation is a function of the content of the picture that is being
transmitted and of the C/N value itself.
When the noise level is lower than the sensitivity of the instrument, a limit value
">" will appear on the display, indicating that the C/N value the user is trying to
measure is greater than this limit value.
E xample 7.
Measuring the C/N ratio for the following signal
Video carrier level
=
82 dBµ V (22 dBmV)
Real C/N ratio
=
42 dB
C/N ratio measured = 42 dB ± error (see Appendix E)
Display shown after a 5-second wait.
Leave the C/N mode simply by pressing any key or turning the rotary switch.- 69 -
