User Manual

MN021-01
Page 16 of 28
10. Functional description
10.1. Block diagram
Figure 11 Remote Unit TFAF Block Diagram
10.2. Up link operations
Low noise amplification: the low level signal coming from the antenna and duplexer is amplified
and filtered.
E/O Conversion: The RF signal modulates the intensity of an optical carrier through an electro-
optic device (laser).
10.3. Down link operations
O/E Conversion: The modulated optical signal coming from the local units is detected and
demodulated back to electrical signal.
Amplification & AGC: Variable amplification is needed to compensate for the variable optical
fibre loss, maintaining a good signal to noise ratio, to maintain a constant gain for each link. The
resultant signal is amplified, filtered and is duplexed to feed the antenna.
10.4. Remote supply
The remote unit is powered with –48VDC supply, positive to GROUND, by means of composite
copper wire and fibre cable.
10.5. Automatic Gain Control
Optical link losses are typically related to:
• Fibre length (0.2-0.4dB/Km @ 1300nm)
• Splices loss
• Optical connectors tolerances and aging
BriteCell™ Fast implements an automatic gain control (AGC) mechanism to maintain constant
RF link gain, regardless of the overall optical link losses. The allowed optical loss per link must
be in the range of 3dB. Losses outside this window will trigger alarms and warnings according
to the following table:
DC DC -48VDC
PHOTODETECTO
Downlink
AGC
VARIABLE
GAIN AMP.
D
A
C
A
DD
C
µPROC.
Diff. AMP
LASER module
MONITOR PHOTODET.
LASER DIODE
VOLTAGE
CONTROLLED
ATTENUATOR
MATCHING
NETWORK
UL ATTENUATOR CONTROL
SWITCHES
attenuator
control
voltage
DL final amplifier stage
current monitor
RF
SMA
RF SMA
50 Ω
Data link
DC/DC
converter
Voltage
Regulator
RX optical power monitor
Shut down
control
CURRENT
GENERATOR
LASER control
module
+12 V
-8 V
+5 V
Auxilliary External
alarms
DC loop