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 99 of 114
5.17. 16 Ch. DSP U/L Squelch Gate 60-228904
16 Ch. DSP U/L Squelch Gate 60-228904 is built onto a 4U, 19” rack-mount case.
There are 26 DSP U/L Squelch Gate units in the Gallery Place System each one receives an Uplink input
from the existing CRCS donor unit. The purpose of the Squelch gate is to selectively filter the required
frequencies from the input broadband noise and intermodulation and to incorporate a digit squelch to the
signal. This ensures that only uplink paths with radio traffic are fed to the base station which in turn gives
over 13dB improvement in the carrier to signal noise floor.
The 60-228904 utilizes Software Defined Radio (SDR) technology to digitally process the signal to define
the required channel frequencies and bandwidth (eight channels in the Uplink band 1 path and eight in the
Uplink Band 2 path). As part of the digital signal processing function the signal amplitude is monitored to
apply squelch functionality to the signal path to attenuate unwanted noise when the input signal level is low.
The unit is pre-programmed with the 16 required uplink frequencies and channel bandwidths as indicated in
section 5.17.2., these can be modified by the use of RMC software.
The Uplink signals from the CRCS donor unit fiber optic receivers enter the Squelch Gate unit via the N type
port labeled “UPLINK IN” (annotated “A” in section 5.17.4.), the signal passes through a 3dB
Splitter/Combiner 05-002603 which splits the signal into two equal paths, each path then enters the UHF
SDR Module E921030. Path A handles frequencies in the range of 492.5MHz to 494.0MHz While Path B
handles frequencies in the range of 499.3MHz to 499.7MHz. The SDR module selectively amplifies and
down converts each band of frequencies to a VHF Intermediate Frequency which is converted from
Analogue to Digital before Digital Signal Processing to the required pass bandwidth and frequency (16
Uplink channels per Squelch Gate unit). During the digital processing, the amplitude of the required channel
is compared with a preset reference and if the signal exceeds the threshold setting, the signal is enabled to
the output. After signal processing has taken place, the signals pass through a Digital to Analogue process
before being up converted back to the original to frequency bands. The two Uplink signal paths (A & B) are
then recombined by a second 3dB Splitter/Combiner 05-002603 and the Uplink signal exits the Squelch
Gate via the N type port labeled “UPLINK OUT” (annotated “B” in section 5.17.4.)
16 Ch. DSP U/L Squelch Gate 60-228904 is powered by a 24V DC supply from the Redundant PSU
Modules 96-300091 in PSU Housing Shelf 96-300090, The 24V DC supply feeds an on-board DC/DC
converter J1421001 which steps down the 24V supply to provide 6.45v and 15v DC to power the SDR,
Controller and Ext. Alarm/battery modules within the Squelch Gate shelf.
An alarm system is fitted, the Squelch Gate shelf has an alarm output which is accessible from the terminals
on the front of the External Alarm and Battery Module J1161030 (annotated “E” in section 5.17.4.), from
where the alarm data is fed to the Krone terminal block located near the top of the rack (see section 5.10.).
The SDR controller module H561003 is used to control and supervise the SDR functionality, two ports are
provided where an operator may connect into the system, a local serial port, (a 9 way D” panel socket
annotated “H” in section 5.17.4.) is used with a laptop PC running suitable terminal-emulation software to
interrogate and control the equipment using GET and SET commands. Alternatively the equipment can be
configured using RMC software via the RJ45 ethernet port (annotated “I” in section 5.17.4.) To ensure
synchronous communications between the SDR and the H56 controller, a high stability 10MHz reference
generator, R031002 is incorporated in the equipment backplane.