cover- 45.6.1 On site transmitters age. Sequential polling is the obvious solution for those on site systems where speech is not used. When both paging calling and On small systems 0.5W at u.h.f. and 1.0W at h.f., are sufficient speech is used either quasi synchronisation or full synchronisation transmitter r.f. powers to satisfy the small site application, and these may be used. The former requires transmitters of high inherent can be integral with the encoder. For larger sites, higher powers are stability, +2 X 108 over-10°C to +50°C, and hence is expensive for required with appropriately sited antennas. A limit of 5W ERP is on site use. Full synchronisation can give poorer radio performance generally imposed by regulation in Europe, (limit is 2W in Belgium) in that a fixed standing wave pattern is set up, leading to fixed null SO it often becomes necessary to install more than one transmitter areas. The cost is however less for small systems and thus line on the large area sites and campus (100 sq. km). Table 45.4 shows synchronisation units are used as one method of fully synchronising typical on site transmitter specifications. Greater r.f. powers, up to transmitters. 25W or 100W depending on the system, are allowed in North Transmitters in both the h.f. and u.h.f. bands will probably use America. phase locked techniques with a crystal reference oscillator as the Since a variety of proprietary analogue and digital code formats, frequency determining element. An audio signal in the 2.8kHz to as well as speech, have to be transmitted, modulation techniques 3.0kHZ range, derived by dividing this oscillator, may be sent down differ. FM is suitable for many applications but for non return to a telephone line. At the slave transmitter, the crystal oscillator, zero codes (Figure 45.3) such as CCIR RPC1 (POCSAG), FSK divided to the same frequency, can be fed to a phase detector with modulation is also required. Although POCSAG is a large capacity the filtered signal from the line. The resultant error signal then cod