2.13.2
Flux Budget/Gain Margin
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NOTE: The flux budget calculations for each FO link
are used to determine if the gain margin designed
into the link is large enough to allow for splicing
of broken fibers and aging effects. The flux budget
is the power difference between the transmitter
output power and the receiver input power for a
given bit error rate.
This power is usually measured in dBm (i.e.
referenced to 1 milliwatt) and is an absolute
measurement. LED transmitter output power is
typically between -10 and -18 dBm. Positive
Intrinsic Negative (PIN) receivers with
pre-amplifiers have a power input typically between
-24 and -37 dBm. Using the flux budget, the link
designer can determine the total losses the system
links can have and still work properly.
The flux budget is divided into components. The
components are the real losses in the system and the
gain margin. The real losses consist of all the
system losses such as cable attenuation, coupling
and splicing losses. The gain margin is a reserve
for future losses, such as aging and future splices.
System losses and the gain margin are usually
measured in dB and are a relative measurement.
The receiver dynamic range is the range of input
power that can be successfully detected by the
receiver (also referred to as the maximum and
minimum optical input power). The variables that
can influence the flux budget include changing the
transmitter power output, the receiver sensitivity,
the imposed signal wavelength and system losses.
Allow 3 dB for aging and 3 dB for each cable repair
for a total margin of not less than 6 dB. Different
wavelengths exhibit different cable attenuation;
thus for links that use DWDM or DWDM, the flux
budget should be determined using the cable
attenuation and connector losses for the wavelength
with greater loss (which is usually the shorter
wavelength).
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FO links shall have a minimum gain margin of 6 dB. The flux budget is the
difference between the transmitter output power and the receiver input
power required for signal discrimination when both are expressed in dBm.
The flux budget shall be equal to the sum of losses (such as insertion
losses, connector and splice losses, and transmission losses) plus the gain
margin. When a repeater or other signal regenerating device is inserted to
extend the length of an FO circuit, both the circuit between the
transmitter and the repeater-receiver, and the circuit between the
repeater-transmitter and the receiver are considered independent FO links
for gain margin calculations.
SECTION 16768A
Page 24
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