Promax MC-377+, Инструкция по эксплуатации

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INSTRUCTION MANUAL MC-377+.
Page 28
May 2001
4.2.6.3 Measuring the C/N ratio of analogue and digital signals
Measuring the Carrier/Noise ratio provides us with information on the quality of
analogue signals and the sturdiness of digital signals. The Spectrum Analyser
operating mode, SPAN mode, enables this measurement to be made almost
automatically.
To measure C/N, the MC-377+ measures the maximum level or power (depending
on whether the analogue or digital measurement mode has been selected) present
inside the represented bandwidth (defined by the SPAN control [23]) and subtracts the
minimum noise level measured inside this bandwidth. Therefore, for the measurement
to be correct it is essential that the maximum and minimum level signals appearing on
the monitor are those for which you wish to evaluate the C/N ratio.
4.2.6.3.1 Measuring the C/N ratio of analogue channels
- Select the Spectrum Analyser operating mode, SPAN mode, and release the
audio control
[27] so that the C/N measurement bar does not get in the way
when choosing the right SPAN.
- Select the analogue signal measurement scale, LEVEL [33], the DIG key [11]
should be released and the ANALOG indicator [31] should remain lit.
- Tune the signal using the TUNING [24] and FINE TUNING controls [26] with the
help of the frequency counter display [22]. Turn the SPAN control [23] to select a
bandwidth so that the maximum signal on the monitor is the video carrier and the
minimum level signal is the noise. Remember that the noise can be measured, as
you wish, either inside or outside the channel.
- Press the audio control [27] to show the C/N measurement bar.
- Read the measurement on the monitor with the help of the scale calibrated in dB
(10-60 dB) C/N [34]. If the signal level is close to 80 dBµV or is greater than this
value the signal should be attenuated using the RF attenuators.
If on taking the measurement the Noise Limit indicator [2] should light up, this
means that the actual noise level is less than the noise level of the MC-377+ itself and,
therefore, the instrument can not measure under these conditions. In this event it can be
confirmed that the actual measurement is better than the one obtained (given that
the actual noise is less). One way to avoid this situation is to amplify the signal, though
then you will have to take into consideration the noise introduced by the amplifier.