Handbook

Axell Wireless Limited
Technical Literature
WMATA CRCS Redundant Antenna System
General Overview
Document Number 60-228701HBK Issue No. 3 Date 28/05/2010 Page 79 of 114
5.12. RF Failover Switch Unit 60-228905
RF Failover Switch Unit 60-228905 is built into a 2U, 19” rack-mount case and receives both Primary
(Jackson Graham Building) and Redundant (Carmen Turner Facility) Downlink signals from the Dual Fiber
Optic Unit 60-228703; the RF level of the Primary Downlink is monitored by a power detector and if the
signal falls below a preset threshold the unit automatically switches from the Primary to the standby
Redundant path; the Uplink paths are switched at the same time.
The Primary Downlink signal from the Jackson Graham Building CRAS System by way of Dual Fiber Optic
Unit 60-228703, enters RF Failover Switch Unit 60-228905 via the N type port labeled PATH 1 D/L”
(annotated Ain section 5.12.5.) and the signal passes through an 8dB In-line Attenuator 93-000065 and
then through a 1W Low Power Amplifier 12-030301 which provides 30dB of signal gain, the amplifier is used
to overcome the subsequent power splitting losses of the CRCS fiber system. The Downlink path is then fed
out of the shelf via the SMA port labeled “PATH 1a” (annotated “B” in section 5.12.5.) which is connected to
the SMA port labeled “PATH 1b” (annotated “D” in section 5.12.5.) by a short length of UT141 cable
(annotated “C” in section 5.12.5.). This break in the signal path allows the amplifier to be isolated from the
rest of the path for testing purposes.
Upon re-entering the unit the Primary Downlink path passes through a 20dB Directional Coupler 90-852320
which couples off the signal and feeds it to an RF Detector Module 17-019805 which monitors the Downlink
signal strength and the main Primary Downlink path passes through the Downlink RF Transfer Relay 96-
900045. The detector module has a DC monitor jack labeled “PATH 1 MON” (annotated “W in section
5.12.5.) where the signal can be monitored with a multimeter with reference to the “0V” jack (annotated “Y”
in section 5.12.5.), the RF level at which the transfer relay operates is adjusted by the variable
potentiometer labeled “RX 1 ADJ” (annotated “U” in section 5.12.5.). There is a Green LED indicator labeled
“RX 1 ACTIVE” (annotated “BB” in section 5.12.5.) that illuminates when the RF signal level is above the
required threshold.
The Redundant Downlink signal enters RF Failover Switch Unit 60-228905 via the N type port labeled
“PATH 2 D/L” (annotated “G” in section 5.12.5.) and the signal passes through an 8dB In-line Attenuator 93-
000065 and then through a 1W Low Power Amplifier 12-030301 which provides 30dB of signal gain, the
amplifier is used to overcome the subsequent power splitting losses of the CRCS fiber system. The
Redundant Downlink path is then fed out of the shelf via the SMA port labeled “PATH 2a” (annotated “H” in
section 5.12.5.) which is connected to the SMA port labeled “PATH 2b” (annotated “J” in section 5.12.5.) by
a short length of UT141 cable (annotated “I” in section 5.12.5.). As with the Primary Downlink, this break in
the signal path allows the amplifier to be isolated from the rest of the path for testing purposes.
Upon re-entering the unit the Redundant Downlink path passes through a 20dB Directional Coupler 90-
852320 which couples off the signal and feed it to an RF Detector Module 17-019805 which monitors the
Redundant Downlink signal strength and the Redundant Downlink path also passes into the Downlink RF
Transfer Relay 96-900045 to provide the signal should the Primary Downlink signal fail. The detector
module has a DC monitor jack labeled “PATH 2 MON” (annotated “X” in section 5.12.5.) where the signal
can be monitored with reference to the “0V” jack, the RF level at which the Green LED indicator labeled “RX
2 ACTIVE” (annotated “CC” in section 5.12.5.) illuminates is adjusted by the variable potentiometer labeled
“RX 2 ADJ” (annotated “V” in section 5.12.5.).
After passing through the RF Relay the Downlink path passes through a 30dB Directional Coupler 90-
852330 which couples off the signal to provide a monitor/test port labeled “D/L MON -30dB” (annotated “F”
in section 5.12.5.) for the Downlink output. The Downlink path then exits the Unit for CRCS Donor Unit via
the N type port labeled “REMOTE D/L” (annotated “E” in section 5.12.5.).