The operating frequency range of the RF duplex should not be narrower than the operating frequency range of the wireless telephone itself. The bandwidth we usually refer to refers to the input bandwidth of the receiver and the output bandwidth of the transmitter after the wireless telephone is equipped with an RF duplex. For RF duplexers, it refers to the stopband bandwidth of two equivalent bandstop filters, rather than being determined by the passband bandwidth. From its frequency response curve, it can be seen that it is the frequency range of two stopbands at a certain attenuation amount. As we all know, the high-frequency input bandwidth of VHF and UHF wireless telephones can generally be above 5MHz, and the high-frequency output bandwidth of transmitters can be above 10MHz. That is to say, the high-frequency input and output bandwidth of the wireless telephone itself is relatively wide, so it is required that the bandwidth of the RF duplex should also have a certain width to overcome the trouble caused by bandwidth issues when users declare frequencies. However, according to the regulations of radio frequency management departments in China and international countries, duplex wireless telephones used for dual frequency duplex networks have a transmission and reception frequency difference of 5.7MHz at 150MHz and 10MHz at 450MHz. Therefore, the ideal RF duplex can only be half of the frequency difference specified for the corresponding frequency band of wireless telephones above, which is 2.85MHz for 150MHz and 5MHz for 450MHz

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