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4.7 HOW TO MEASURE C/N
It is a good engineering practice to use a bandpass filter on the input
of the receiver when making C/N measurements. This is to ensure
accuracy and extend measurement range. If a preamplifier is used to
boost test point levels prior to measurement, it should be placed
between the bandpass filter and the receiver. This measurement is
simply a comparison in amplitude between the video carrier refer-
ence signal and the noise (FCC limit: > 43 dB). The noise measure-
ment must be made at least 2 to 2.5 MHz from any other carrier on
the system.
In the C/N mode the carrier to noise ratio of the tuned channel or
frequency is displayed. The C/N measurement bandwidth and the
frequency offset for the noise measurement may be controlled from
the screen. A proprietary DSP technique allows C/N measurements
on modulated carriers (non-scrambled channels). The measurement
is made by measuring the video carrier level and then tuning to the
offset frequency. Once a line is found, the unit will then measure
four consecutive frames and average the values together. The value
is then corrected for the selected bandwidth, and the C/N ratio is
computed. If the mode is selected before a carrier is available on the
input, it will cause the readings to be wrong. If this occurs, change to
another mode or channel and then return to the desired channel.
