This part of IEC 62604 concerns the duplexers, which can separate receiving signal from transmitting signal and are key components for two-way radio communications. They are generally used in mobile phones using CDMA systems such as N-CDMA, W-CDMA / Universal Mobile Telecommunication System (UMTS). So far, dielectric duplexers have been mainly used. However, recently SAW duplexers, which are utilized surface acoustic wave (SAW), are becoming popular and replacing the dielectric duplexers year by year in recent mobile phones, because of their advantage of small size, light weight and good electrical performances. In addition to SAW duplexers, BAW duplexers, which are utilized bulk acoustic wave (BAW), are also becoming in the spotlight and popular because of their higher Q property and better performances especially in PCS band. 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 of transmitting and receiving filters and isolation level between transmitting and receiving 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 normal frequency.