Chapter 13 - Optical Receiver
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13.5 Measuring the optical modulation index (OMI)
The optical modulation index (OMI) is comparable with the modulation index for an amplitude
modulation. The amplitude (intensity) of a carrier – here, the light – is modulated. The greater the
difference between the maximum intensity and the minimum intensity, the greater the OMI and the
RF voltage (level) after the optical receiver. The OMI is specified in %. With this instrument, the OMI
is selectively measured for a certain frequency, meaning that only the RF power within the signal
bandwidth is taken into account. It is designated as the individual OMI or the channel OMI. The
attainable signal-to-noise ratio depends on the individual OMI. A value of around 4% is ideal.
Note!
The level specified after the OMI value corresponds to the internal RF level after the
optical/electrical converter. This information is only relative. This specification is
primarily used to determine the relationships between the levels of the individual
channels.
A brief overview of the relationships between optical power, RF level and OMI is provided below:
If the optical power is increased by 1 dB for optical transmission, the RF voltage increases by 2dB
after the optical receiver, while the optical modulation index (OMI) remains unchanged. The RF
voltage is proportional to the square of the optical power.
If the optical modulation index is doubled (e.g. increased from 2% to 4%) with the same optical
power, the RF voltage after the optical receiver increases by 6dB. This means that the OMI and RF
voltage are linearly proportional to one another.
13.6 Remote supply
An optical LNB can be supplied with a voltage of 14 V and max. 500 mA using the F connector. For
Invarcom FibreMDU and equivalent products, you can use a 14 V supply.
13.7
Cleaning the fibre optic plug connection
The weak point of every optical transmission system lies in the splice and plug connections. For plug
connections, it is important to ensure that the contact surfaces are very clean.
But the ferrules of a fibre optic connection must also remain free of dust so that no contamination
reaches the connectors’ interfaces when they are plugged in. Industry-standard cleaning sets are
available for this purpose. Immediately after cleaning the plugs and connections, you should put dust
covers on them unless you are going to use them again right away.
The measuring instrument’s fibre optic connection is equipped with a hinged lid that seals the
connection as soon as the plug is removed. However, you must still ensure that the area around the
lid remains free of contamination.
13.8 USB Microskope
A USB microscope can be connected to the device for plug connector inspection. The device
supports DI-1000 type USB microscopes from Lightel Technologies.
Contamination in optical plug connections can impair the signal quality. Typical types of
contamination include dust particles, hand lotion, skin and alcohol residues.
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