frequency synthesisers with internal u.h.f. prescalers are commonly used, whereas previously these were comprised of a separate ECL prescalar, programmable divider, reference divider, and phase comparator/loop filter. The use of a relatively high first IF (Intermediate Frequency) with advanced filtering methods UNIT 1 UNIT 2 allows the use of lesser filtering stages in the r.f. front end stages to obtain the required band selectivity, thus minimising the physical size of the receiver circuits. Digital filtering at the second IF, Figure 46.1 Simplex operation typically 455kHz, may be used to allow the selection of IF selectiv- ity under software control, to allow interchangeability of equipment between 25kHz, 20kHz, 15kHz and 12.5kHz r.f. channel spacings. 46.2.2 Half duplex Digital control of transmitter processing and resultant frequency deviation may be performed using a single IC as opposed to manual Progressing from this, with transmit/receive switching in use at both potentiometer adjustments. stations, a 'half duplex' mode occurs where personal or mobile units communicate using a different transmission frequency to that used for reception. The central radio base station uses the reverse pair of 46.1.2.2 Tuned circuits these frequencies to thus enable communication between itself and all mobile outstations. Although at first this may seem inefficient Varicap controlled tuned filters under software control may be used use of the spectrum, by suitable frequency management linked with in combination with D/A converters in the r.f. stages, to allow geographical base station planning, more efficient use in fact may automatic factory alignment to be performed by digital means. The be made due to planned frequency re-use in adjacent areas. parameters for each channel or frequency range are stored in the transceiver's memory circuitry and are retrieved according to the 46.2.3 Full duplex channel selected. This may be coupled with active tuning of trans- mit stages to provide efficient operation with minimal generati