4.2.4.2 M easurement of the C /N ratio (digital channels)
The C/N function allows to measure automatically the ratio between the power
level of the digital channel and the noise level outside the channel being studied.
When pressing the C /N [14] key, the P R O M AX -6 automatically measures the
channel power level, the noise level outside the channel (at a lower frequency than
the main carrier and defined through the F 3 function) and shows the value of the
C/N ratio in the display.
Next figure shows that when measuring the C/N ratio it is necessary to check
if there is an adjacent channel at the lower frequencies, in order to do not confuse
the noise level with the adjacent channel signal. To select the optimum shift
frequency in each case, function F3 has been incorporated.
An easy way to check if there is an adjacent channel at the lower frequencies
consists of making a level measurement at a frequency 8 M H z lower to the main
frequency of the channel being studied and comparing if level is similar to the
power level of the channel under study (there is an adjacent channel) or the level
drops off significantly (there is no adjacent channel).
As a resume, and taking previous figure as an example, in the case there is no
adjacent channel at the lower frequencies (channels D24 or D27) noise level
measurement should be taken outside the channel at a frequency 4.5 M H z lower
than the studied channel main carrier. Otherwise, if there exists an adjacent
channel at the lower frequencies (D25 or D26) the best approximation consists of
making the noise level measurement in the separation between the studied channel
and its lower adjacent channel, where the modulation contents is minimum.
