User's Manual

Weehawken Tunnel VHF Repeater System
User/Maintenance Handbook
Handbook N.-Weehawken_VHF Issue No:-A Date:-05/08/05
Page:-
64 of 75
7.12 Channel Selective Modules (17-009143 & 17-009127)
7.12.1 Description
The channel selectivity module is employed when the Cell Enhancer requirement dictates that
very narrow bandwidths (single operating channels), must be selected from within the operating
passband. One channel selectivity module is required for each channel.
The Channel Selectivity Module is an Up/Down frequency converter that mixes the incoming
channel frequency with a synthesised local oscillator, so that it is down-converted to an
Intermediate Frequency (IF) in the upper HF range. An eight pole crystal filter in the IF
amplifier provides the required selectivity to define the operating passband of the Cell Enhancer
to a single PMR channel. The same local oscillator then converts the selected IF signal back to
the channel frequency.
Selectivity is obtained from a fixed bandwidth block filter operating at an intermediate
frequency (IF) in the low VHF range. This filter may be internal to the channel selectivity
module (Crystal or SAW filter) or an externally mounted bandpass filter, (LC or Helical
Resonator). Various IF bandwidths can therefore be accommodated. A synthesized Local
Oscillator is employed in conjunction with high performance frequency mixers, to translate
between the signal frequency and IF.
The operating frequency of each channel selectivity module is set by the programming of
channel selectivity module frequencies and is achieved digitally, via hard wired links, banks of
DIP switches, or via an onboard RS232 control module, providing the ability to remotely set
channel frequencies.
Automatic Level Control (ALC) is provided within each channel selectivity module such that
the output level is held constant for high level input signals. This feature prevents saturation of
the output mixer and of the associated amplifiers.
Alarms within the module inhibit the channel if the synthesised frequency is not locked. The
synthesiser will not usually go out of lock unless a frequency far out of band is programmed.
The channel selectivity module is extremely complex and, with the exception of channel
frequency programming within the design bandwidth, it cannot be adjusted or repaired without
extensive laboratory facilities and the necessary specialised personnel. If a fault is suspected
with any channel selectivity module it should be tested by substitution and the complete, suspect
module should then be returned to AFL for investigation. The channel selective modules fitted
to the VHF cell enhancers in the Weehawken system are all hard-wired and therefore not
adjustable, however, the modules fitted to the UHF and 800MHz enhancers have DIP switch
controller modules fitted, allowing the set frequency to be changed on site. There is no
functionality to change the frequencies remotely.