A little bit of information.
A pair of low band Philips FX5000 repeaters have been modified (so we have a spare) giving
a receive sensitivity better than 0.18uV for 12db SINAD and a programmable output power from ten to fifty
watts. Our output power is currently set at 14dbW (25W). The logic board has
been developed to fit in place of the original RCM PCB inside the control unit,
consequently no external logic is required. All logging, configuration, status and
functional commands are done via the serial RS232 connector on the rear of the repeater
chassis. The microprocessor is a Microchip 16F1947 running at 32MHz. This has
sufficient power for a multitasking environment and concurrently serves the repeater
logic, IIC clock module, two bi-directional serial ports, DTMF decoder, two independent
software Morse code generators, 1750Hz tone decoder and a host of internal time-slice and
timed functions. Fittings and connectors are available for future expansion.
The receiver and transmitter driver modules each have their own processor controlling the
synthesisers. Operation of the repeater is completely stand-alone but an attached PC
gives logging, control and other functionality. All add-on boards etc. use the
existing connectors so few modifications need to be done to the raw units to produce a
fully functional repeater. The modifying of a low band (E0 66 - 88MHz) unit down to
50MHz is another story (definitely not for the faint of heart).
The duplexer is constructed using six quarter wave cavities giving an isolation in excess
of 100db for a forward loss of ~ 0.5db. The cavities have been designed and built
from scratch using 150mm construction grade aluminium for the outer tube with 45mm
diameter resonators and 25mm thick end caps making a very rigid construction. Tuning
rods are invar steel alloy for temperature stability and a unique plunger design prevents
noise even after prolonged usage in a damp, hostile environment. The coupling loops, made from 4mm silver plated copper tube,
are tuned and designed for temperature stability.