FS71C-85 Series User Manual
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Copyright © 2015 Fiber-Span
Theory of Operation
The amplifi ers used in the RRU provide a set amount
of gain. Gain is defi ned as the ratio in the amount of RF
power of the output and input of the amplifi er.
If +20dBm of RF power is required at the RF Out port
of the RRU and 0dBm is input to the RF Input port at
the FTU, the gain must be set to 20dB.
When multiple RF carriers are input to an RF amplifi er,
the RF output power will be the input power plus the
gain.
Power Output = Power input + Gain (in decibels)
Power Output = Power input x Gain (in Watts)
As seen in the following fi gure, the output power tracks
the input power linearly. Eventually as the input power
increases, the amplifi er output power reaches the out-
put power limit. When this happens, intermodulation
products are generated at the amplifi er output.
The intermodulation products with the most likelihood
of generation are 2nd and 3rd order. For every 1dB
increase in input RF power, the 3rd order intermodula-
tion products increase 3dB.
If one were to plot the 3rd order output power on a
graph as in the preceding fi gure, the slope of the 3rd
order products would intersect the line of the output
power plotted vs. the input power. This point is defi ned
as the 3rd Order Intercept point and is used by ampli-
fi er manufacturers to characterize the maximum output
power of an RF amplifi er.
The RF output power of the amplifi ers is defi ned as
the total RF output power inside the bandwidth of the
duplexers. This RF output power is divided into the
number of dominant RF carriers at the output of the
amplifi er.
Figure 3. HRU Output Power vs Input Power (IP3 Plot)
