User's Manual

Maintenance Handbook
Baltimore Radio Repeater System
Handbook No. 60-166701HBKM Page 8 of 24
Photographs continued………
Inside Case Floor showing Alarm Krone Blocks & Relay PCB’s
3.1.1 800MHz Remote Site BDA Description
The 800MHz BDA is a wall mount unit, 24” x 24” x 12” in size. It provides the required amplification at
a Remote Site for the wanted 800MHz signals.
The 800MHz BDA operates primarily from an 110V AC supply but it also has a 24V DC supply port
for connection to a Battery Backup Unit if required.
In more detail, the 800MHz BDA Downlink (Base Transmit) input the 800MHz Downlink port. From
this port a 20dB coupler provides a Downlink Input Monitor port for test purposes and this is followed
by a bandpass filter and then an electronically switchable attenuator which allows for up to 15dB of
gain adjustment in 1dB steps. The attenuation control is made via the NFM unit located in the UHF
BDA. A 15dB gain LPA then provides a first stage of amplification and this is followed by an AGC
Attenuator and then a 36dB gain 20Watt Power Amplifier. After the 20W PA there is a 30dB coupler
which is used to drive an AGC Detector which then controls the previously mentioned AGC
Attenuator. A second bandpass filter, this time a duplexed filter, then connects the Downlink path to
the Mobile Antenna port after passing through a 20dB coupler which provides a Downlink Output
Monitor Port for test purposes. The Mobile Antenna port is then connected to the UHF BDA 800MHz
Antenna Interconnect port.
In the Uplink (Base Receive) path, the Mobile Antenna port is the Uplink input and the Uplink signals
are first passed back through the Downlink Output coupler and are then connected to a duplexed
bandpass filter. The filter is followed by a 30dB gain LNA and then an electronically switchable
attenuator which allows up to 15dB of gain adjustment in 1dB steps. As with the Downlink attenuator,
the Uplink attenuator is controlled remotely via the NFM unit in the UHF BDA. An AGC Attenuator is
then fitted before a second bandpass filter which leads to a second LNA, this time a 20dB unit. The
LNA is followed by an AGC Detector which controls the previously mentioned AGC Attenuator.
Finally a 20dB coupler provides an Uplink Output Monitor port for test purposes before the 800MHz
Uplink port which connects to the UHF BDA 800MHz Uplink port.
Alarm outputs are taken to the UHF BDA for connection to the NFM unit there which is fitted to the
inside of the UHF BDA cabinet door.