Duplex communication transceiver with modulation cancellation

Claim 1

A duplex communication transceiver comprising: transmitter means for providing for information transmission an output signal comprising a first carrier signal having a first predetermined frequency and modulated by a first information signal; receiver means, coupled to and simultaneously operable with said transmitter means, for receiving an input signal comprising a second carrier signal having a second predetermined frequency different from said first frequency and modulated by a second information signal different from said first information signal, said receiver means effectively demodulating said input signal and providing said second information signal in response thereto; said receiver means including; a first mixer means coupled to said transmitter means for receiving said modulated first and second carrier signals and for providing, in response thereto, a first IF signal modulated with said first and second information signals, said first IF signal having an IF frequency comprising an arithmetic combination of the frequencies of said first and second carrier signals, a first IF means fixed tuned to said IF frequency and coupled to said first mixer means for receiving and passing said first IF signal and rejecting signals at substantially different frequencies, a second mixer means coupled to said first IF means for receiving said passed modulated first IF signal therefrom and also receiving, from a local oscillator means, a local oscillator carrier signal having a predetermined local oscillator frequency and modulated by a signal related to said first information signal, and for providing in response thereto a modulated second IF signal having a second IF frequency comprising the arithmetic combination of said first IF frequency and said local oscillator frequency, signal adjustment means coupled to said transmitter means and said local oscillator means for receiving said first information signal from said transmitter means and adjusting it to provide said modulating related signal such that said second mixer means cancels said first information signal modulation contained in said first IF signal so that said second IF signal substantially comprises a carrier signal at said second IF frequency modulated by just said second information signal, and demodulator means coupled to said second mixer means for receiving said modulated second IF signal and providing said second information signal in response thereto, wherein the improvement comprises, said receiver means including a voltage controlled oscillator for providing said modulated local oscillator carrier signal wherein said oscillator includes a reactance element determining the frequency of said local oscillator carrier signal, and wherein said signal adjustment means comprises a selectable predetermined resistance element coupled between said first information signal and said reactance element wherein said resistance and reactance elements together provide a predetermined phase shift for said first information signal to provide said desired related signal.