Handbook

PBL UHF1/1A 1 & 2 Air Interface
Maintenance Handbook
H/book Number:-60-055900HBKM
Issue No:-2
Date:-20/10/2003
Page:-
57 of 85
5.1.8 Channel Selective & Channel Control Modules (17-003012 & 17-002101)
5.1.8.1 Channel Selective Module 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.