5
FOCN
Instruction Manual
Operational Setup – Receiver (Forward Path)
The FOCN optical receiver as the last component in a specific optical link will provide carrier-to-noise performance and
an RF output level that is dependent on several parameters. The RF input level to the source optical transmitter and the
optical input level to the FOCN basically determine the link performance.
Both the laser modulation and the receiver output level are basically limited by total power. The RF input level to any
optical transmitter is dependent on the number of channels being transmitted. As the number of carried channels is
lowered, the RF input level to the transmitter can be raised. This results in increased RF levels at the FOCN receiver. This
improves the carrier-to-noise ratio over the link. Links should be designed and transmitters should be set up using values
that represent the maximum number of channels likely to be carried.
The following chart assumes approximately 77 channels from 55 MHz to 550 MHz and digital loading from 550 MHz to
1000 MHz at –6dB below analog. The RF levels are measured at 547.25 MHz.
Although the node will operate at optical input levels as high as +2 dBm, there is little improvement in the C/
Nperformance of the node at optical input levels above 0 dBm. For optimum distortion performance it is recommended
that the optical input to the node be kept at or below 0 dBm. The FOCN's RF output is directly proportionate to the
optical input level. If lower output is desired, the unit should be lowered by adding an in-line pad to the RF output
to reduce the output level. This will maintain optimum C/N performance.This will also reduce the OMI of the return
transmitter if so equipped.
The following chart shows the approximate C/N performance at various input levels and channel loading.
This performance can vary considerably depending upon fiber and laser performance so it is presented as
a design aid only.
Optical
Received
Approx. Forward
Approx. RF
Input Level
Power @ T.P
T.P. Level
Output Level
-8 dBm
.16V
-6 dBmV
14 dBmV
-6 dBm
.25V
-2 dBmV
18 dBmV
-4 dBm
.40V
2 dBmV
22 dBmV
-2 dBm
.63V
6 dBmV
26 dBmV
0 dBm
1.0V
10 dBmV
30 dBmV
+2 dBm
1.58V
14 dBmV
34 dBmV
