Duplexers

The simultaneous operation of the receiver and the transmitter raises questions of the electromagnetic compatibility of these nodes. If the output signal from the transmitter output goes to the input of its receiver, then it may not only make it impossible to receive any stations, but also disable the receiver input cascades.

For this reason, there are issues of suppressing the signal of the own transmitter at the input of the receiving device. This is done using a duplex filter. A duplex filter (duplexer or diplexer) consists of two bandpass filters tuned to different frequencies interconnected. The band-pass filter included at the output of the transmitter is configured to pass the entire range of acceptable frequencies of the transmitter. The band-pass filter included at the receiver input is configured to pass the entire allowable frequency range of the receiver. As a result, the signal from the transmitter output can freely pass to the antenna, but at the same time it will be suppressed at the receiver input. The block diagram of a duplex radio station with digital modulation types is shown in Figure 1.

Figure 1. Block diagram of a duplex digital radio station

Given that the band-pass filter cannot be implemented with a strictly rectangular amplitude-frequency characteristic, a guard interval is introduced between the transmitter frequency range and the receiver frequency range, which is called duplex frequency separation. The higher the frequency range of the mobile communication system, the more duplex frequency spacing is applied. For example, for cellular communication systems GSM900 or CDMA, the duplex frequency separation of 45 MHz is used, and for the GSM1800 system - already 140 MHz.

The amplitude-frequency characteristics of the duplexer are shown in Figure 2.

Figure 2 Amplitude-frequency characteristics of the duplexer