Full duplex two=wire transmission unit circuit - has two bandpass filters whose centre frequencies can be switched by control criterion between carriers

摘要

The transmission unit, esp. modern, operates in two different frequency channels in the speech band. It has a modulator, a demodulator, two band-pass filters, each tuned to the corresponding channel carrier frequency, and a hybrid circuit connected to both filters and to the line. The two band-pass filters (F1, F2) are structurally similar to each other, and their centre frequencies can be switched by a control criterion between the carrier frequencies of the two channels. One filter (F1) is directly connected to the modulator (M), while the other filter (F2) is permanently connected to the domodulator (D) input.