This part of IEC 62604 concerns duplexers which can separate receiving signals from transmitting signals and are key components for two-way radio communications@ and which are generally used in mobile phone systems compliant with CDMA systems such as N-CDMA in second generation mobile telecommunication systems (2G)@ W-CDMA / UMTS (3G) or LTE (4G). While in 2G systems mainly dielectric duplexers have been used@ the ongoing miniaturization in 3G and 4G mobile communication systems promoted the development and application of acoustic wave duplexers due to their small size@ light weight and good electrical performance. While standard surface acoustic wave (SAW) duplexers have been employed for applications with moderate requirements regarding the steepness of individual filters@ applications with narrow duplex gap (e.g. Bands 2@ 3@ 8@ 25)@ i.e. the frequency gap between receiving and transmitting bands@ require the application of temperature-compensated (TC) SAW or bulk acoustic wave (BAW) technology@ because of their better temperature characteristics and resonator Q-factors. It is neither the aim of these guidelines to explain theory@ nor to attempt to cover all the eventualities which may arise in practical circumstances. These guidelines draw attention to some of the more fundamental questions@ which should be considered by the user before he places an order for SAW and BAW duplexers for a new application. Such a procedure will be the user??s insurance against unsatisfactory performance. Because SAW and BAW duplexers have very similar performance for the usage@ it is useful and convenient for users that both duplexers are described in one standard. Standard specifications@ such as those of IEC@ of which these guidelines form a part@ and national specifications or detail specifications issued by manufacturers will define the available combinations of centre frequency@ pass bandwidth and insertion attenuation for each sort of transmitting and receiving filters and the isolation level between transmitter and receiver ports@ etc. These specifications are compiled to include a wide range of SAW and BAW duplexers with standardized performances. It cannot be over-emphasized that the user should@ wherever possible@ select his duplexers from these specifications@ when available@ even if it may lead to making small modifications to his circuit to enable the use of standard duplexers. This applies particularly to the selection of the nominal frequency band.