Fiber optical channel monitoring – Grass Valley Telethon 3G User Manual
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Telethon 3G Operation
Fiber Optical Channel Monitoring
Fiber Optical Channel Monitoring
The Telethon 3G provides direct digital readout of the Fiber Optic Link signal strength for
signals received at the unit. This readout is presented in units of dBm. It is useful to
understand both the dB or decibel and the dBm or decibel referenced to one milliwatt.
The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a
physical quantity (usually power or intensity) relative to a specified or implied reference
level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless,
relative unit. A decibel is 1/10th of a bel, a seldom-used unit. Typically dB has been
employed in Audio Measurement and Fiber Optics among many uses.
Proper measurement of signal strength requires an absolute measurement and the dBm
provides this measurement. Since it is referenced to the milliwatt, it is an absolute unit,
used when measuring absolute power. By comparison, the decibel (dB) is used for
quantifying the ratio between two values, such as signal-to-noise ratio.
The Telethon 3G operates within a defined range of Fiber Optic Link signal strength. The
minimum recommended signal strength is -20 dBm or better. Typically the system should
operate at levels between -8 dBm and -20 dBm. The standard laser output strength is
-6 dBm. Cable length affects signal strength as does the number of connections between
the two Telethon 3G units. Any use of repeaters of cable bulkhead connector will produce a
minimal signal loss through each connection.
The optical output from each transmitter is generated by an infrared laser diode that is
coupled to a CWDM and onto the fiber. User connections on the Telethon 3G are made at
the bulkhead ST type connectors on the front and rear panels. Operation is intended for
use on single mode fiber. Since the CWDM output of the Python is the aggregate output of
all eight or 16 optical transmitters inside the frame, the total optical power output on a
single fiber optic cable will be in the +4 to +8 dBm range. Standard practice of NEVER
looking directly into a fiber should be followed at all times.
The maximum fiber distance is defined by the optical loss margin. The RX signal must be
-20 dBm or better. Losses on single mode fiber are approximately 0.5 dB/km or less.
CWDM’s account for about 5 dBm of loss per pair and must be considered when computing
your link loss budget.
The integrated optical power meter will show the received optical power for each receiver,
but note that this figure is post CWDM.
Read the Using Fiber Optics Guide for information on how to manage and deploy your
fiber optics cabling, safety precautions, tips & tricks, and recommendations for creating
complex fiber optic networks. You can find a copy of this document on the Support portal
(see