User Manual

Axell Wireless Limited
Technical Literature
UHF Dual Band Line Amplifier
Document Number 55-227901HBK
Issue No. 4
Date 23/06/2014
Page 11 of 30
3. WMATA Dual Band UHF Line Amplifier 55-227901
3.1. Equipment Overview
WMATA Dual Band UHF Line Amplifier 55-227901 is a Bi-directional Line Amplifier designed for
Radiating Cable signal extension. The equipment consists of four signal paths (2 downlink and 2
uplink) to provide dual band operation.
The two Downlink bands are: 489.5 MHz – 491.0 MHz & 496.0 MHz – 496.9 MHz.
The two Uplink bands are: 492.5 MHz – 494.0 MHz & 499.0 MHz – 499.9 MHz.
The equipment provides 17dB to 47dB of gain, via a number of amplifiers in each direction. An ALC
system is fitted to each amplifier path to provide Automatic Level Control to prevent signal overload
and interference. High selectivity band pass duplexers are used on both the downlink and uplink
paths to provide isolation between the interleaved frequency bands.
The equipment is built into a wall-mounted, environmentally protected NEMA lockable steel case, the
RF ports and connectors are also NEMA rated to ensure a weatherproof product. A supply isolator
switch is fitted inside the unit and there are Power On and Alarm indicators on the outside of the door.
3.2. Theory of operation
Please refer to system Diagram para 3.3 which identify the component positions thus (x).
The downlink input signal from the proceeding base station feeding radiating cable enters the donor
input port and is fed to a circulator (1) which directs the transmission into the downlink duplexer (2).
The downlink duplexer filters the signal to provide 2 downlink outputs to the amplifier chains. Each
amplifier chain consists of a similar line up of components. The filtered downlink is fed via an
adjustable attenuator (3 or 9) and into a low noise amplifier (4 or 10) which provides 28dB gain. The
amplified signal is then fed via a diode attenuator module (5 or 11) which forms part of the ALC
feedback control loop. The ALC attenuator then feeds the signal into an output amplifier (6 or 12)
which provides 30dB gain but is restricted to 0.4W output power. The downlink signal then passes
through the ALC detector (7 or 13), the ALC detector ensures that should the output power level
become equal to or exceed the 0.4W threshold, a control voltage is fed back to the ALC attenuator to
reduce the input drive level to prevent overload of the output amplifier. Following the detector the two
downlink paths are recombined in the output duplexer (8) before passing through the server port
circulator (14) and the 20dB signal monitor tapper (15) and out to the server radiating cable.
The uplink input signal from the mobile feeding radiating cable enters the server input port and is fed
via a 20dB signal monitor tapper (15) to a circulator (14) which directs the transmission into the uplink
duplexer (16). The uplink duplexer filters the signal to provide 2 uplink outputs to the amplifier chains.
Each amplifier chain consists of a similar line up of components. The filtered uplink is fed via a low
noise amplifier (17 or 23) which provides 30dB gain and into an adjustable attenuator (18 or 24). The
signal is then fed via a diode attenuator module (19 or 25) which forms part of the ALC feedback
control loop. The ALC attenuator then feeds the signal into an output amplifier (20 or 26) which
provides 32dB gain being restricted to 0.1W output power. The uplink signal then passes through the
ALC detector (21 or 27), the ALC detector ensures that should the output power level become equal
to or exceed the 0.1W threshold, a control voltage is fed back to the ALC attenuator to reduce the
input drive level to prevent overload of the output amplifier. Following the detector the two downlink
paths are recombined in the output duplexer (8) before passing through the donor port circulator (1)
and out to the base station fed radiating cable.