Radio frequency transmitters and receivers

摘要

A low power radio frequency transmitter, for use at 140 to 225 Mhz and output power of 1 to 20mW, includes a phase locked loop circuit 22 supplying a carrier which is modulated at 27 by data before application to a transmitting aerial via a muttipole filter 29 to remove unwanted signals. The use of a PLL enables the carrier frequency to be stable through wide variations in environment conditions without need for special crystals or SAW filters. The transmitter may use analog frequency shift keying(AFSK). A receiver (fig.5 not shown) suitable for use with the transmitter may include an input narrow band tuned filter, a BiFet pre-amplifier, a signal mixer and a pair of decoders to obtain an RF signal, an anlog output and a signal applied to a data slicer to recover data.A paging system responsive to data received from a computer for producing paging signals is also described.