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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.
Summary of Contents for 3HRV
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