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4.6 CARRIER-TO-NOISE
It is a good CATV engineering practice to use a band pass filter on
the input of the SAM 4040D when making C/N measurements. This
ensures accuracy and extends measurement range. If a preamplifier
is used to boost test point levels prior to measurement, it should be
placed between the band pass filter and the SAM 4040D.
C/N This measurement is simply a comparison in amplitude between
the video carrier reference signal and the noise (FCC limit: > 43 dB).
The noise measurement must be made at a frequency offset
(displacement from the video center frequency) of least 2 to 2.5 MHz
from any other carrier on the system.
The C/N Measurement Mode displays the carrier-to-noise ratio of the
tuned channel or frequency. You can set C/N measurement
bandwidth and the frequency offset for the noise measurement from
the screen. A proprietary DSP technique allows C/N measurements
on modulated carriers (non-scrambled channels). Th C/N
measurement requires measuring the video carrier level and then
tuning to the offset frequency, searching for a quiet line. Once a
quiet line is found, the SAM 4040D measures four consecutive
frames
, then averages the values. This value is then corrected for
the selected bandwidth, and the C/N ratio is computed.
CAUTION: If you select C/N Mode before a carrier is available at the
input of the SAM 4040D, the readings will be wrong. If this occurs,
change to another mode or channel and then return to the channel
you want.
Information displayed in the C/N Mode is as follows:
•
Channel
number
•
Channel
label
•
Carrier
frequency
•
Noise
offset
frequency
•
Noise
frequency
•
Bandwidth
•
C/N ratio
•
Channel
plan
To make a C/N measurement, select a channel, and press the C/N
Measurement Mode key. For example, selecting channel 2, with a