R-F Hybrids Used for Paralleling Terminal Equipment for Closely Spaced Carrier-Current Channels [includes discussion

Abstract

Itwouldbepossible touseafilter ofthe proper bandwidth intheoutput ofeach transmitter toafford thedesired isolation;however, wherethechannels are spaced only1/2percentinfrequency, as arecrystal-controlled frequency-shift controlchannels, these filters becomevery expensive andhaveahighinsertion loss whichwoulduseupalarge portion ofthe useful operating rangeofthechannel equipment. Radio-frequency (r-f) hybrid units have beendesigned asamoreeconomical, less bulkysolution tothisproblem.The principal ofoperation issimilar tothatof themorefamiliar audiohybrids4 which areusedtoform4-wire to2-wire audio connections. Inthislater casevarious bridge orbalance circuits havebeenused topermit tying a2-wire telephone line to anincoming andoutgoing amplifier withoutcoupling theamplifiers together and causing singing. Asimilar balance circuit applied tocarrier-current linecoupling permits coupling twocircuits tothepower line withaminimumofcoupling between thetwocircuits. Whentransmitters are coupled totheline through anetwork of these hybrid units apowerloss ofabout l/nresults (nagainisthenumberof transmitters). Thismeanstheloss indecibels duetocoupling ofntransmitters is onlyhalf that imposed bystraight parallel operation. Hybrid-A Multiterminal Network Theideal circuit solution tothis problemwouldbeacomplex hybrid withmany pairs ofinputterminals' andonlyone balance andoneloadpair. Suchanideal network wouldhaveonlya3-decibel loss perchannel because oftheequal division ofenergy between theloadandbalance.