Q. What is the requirement of a hybrid in telephone networks? How does it work?

Ans:

Digital exchanges require receive and transmit signals on separate two-wire circuits. This  calls  for  2-wire  to  4-wire  conversion.  Such a  conversion  is  generally required for trunk transmissions in analog exchanges. Circuit which performs 2-wire to 4-wire conversion  isknown as Hybrid.  A transformer based  hybrid circuit is displayed in Figure. Main function of a hybrid is to make sure that there is no coupling of signal from the input to output in the 4-wire circuit. Operation of the circuit is as follows: Input signal is coupled to Band F windings equally. Through the C winding, input is coupled to the 2-wire circuit. The same signal when it flows through balanced 2-wire couples the signal to winding D through winding C. Signal induced in B flows via E and induces a current in D which opposes the current induced by F. If impedance Z_B exactly matches that of the 2-wire circuit, effect of input signal on the output winding D is entirely nullified. Similarly, input signal from the subscriber end is entirely nullified from coupling into winding A.