DOUBLE SIDEBAND MODEM WITH EITHER SUPPRESSED OR TRANSMITTED CARRIER

• US 3,719,903 A
• Filed: 06/25/1971
• Issued: 03/06/1973
• Est. Priority Date: 06/25/1971
• Status: Expired due to Term

• Current Assignee: William Eugene Goodson
• Original Assignee: William Eugene Goodson
• Inventors: Goodson, William Eugene
• Primary Examiner(s): Brody, Alfred L.
• Application Number:
  US05/156,843
• Time in Patent Office: 620 Days
• Field of Search: 329/50,101,192 332/31T,37,43B,44,45 325/137,138,329,330 307/235R
• US Class Current: 332/168
• CPC Class Codes: H03C 1/36 by means of semiconductor device having at least three electrodes H03C1/34, H03C1/50, H03C1/52, H03C1/62 take precedence

Abstract

An inexpensive, two-transistor modem wherein the first transistor is a high gain amplifier connected to receive the modulating signal, and the second transistor is connected as a shunt modulator driven by a carrier signal source. Individual resistors are connected with the collector and emitter electrodes of the first transistor and so proportioned that the input baseband (for modulation) and carrier (for demodulation) signals are cancelled at a common summing node. The circuit is useful for double sideband suppressed or transmitted carrier applications, especially where the desired sidebands are very near, or go down to, the baseband frequencies.

Claims

1. An unbalanced modem circuit having a pair of input and a pair of output terminals comprising an amplifier having an input connected to one of said pair of modem input terminals, the other of said modem input terminals being connected to one of said pair of output terminals to form a common input-output terminal, said amplifier having an inverted output and a non-inverted output, non-linear means connected to and controlled by a source of carrier frequency to regulate conduction through said non-linear means in accordance with the frequency of the carrier signal, means connecting said non-linear means between the inverted output of said amplifier and said common input-output terminal, and means interconnecting said inverted and non-inverted outputs of said amplifier in a common summing node which is connected to the other of said pair of output terminals, whereby the input signal to said modem is substantially suppressed at said output terminals without the need for balanced and matched circuit parameters.

2. An unbalanced modem circuit in accordance with claim 1 wherein said means interconnecting said inverted and non-inverted outputs of said amplifier in a common summing node comprise individual resistors connected with each of said outputs, said resistors being proportioned so as to cause said baseband and carrier sideband signals appearing at said amplifier inverted and non-inverted outputs to be substantially cancelled during modulation and demodulation, respectively.

3. An unbalanced modem circuit having a pair of input and a pair of output terminals comprising an amplifier having an input connected to one of said pair of modem input terminals, the other of said pair of modem input terminals being connected to one of said pair of modem output terminals to form a common input-output terminal, said amplifier having an inverted output and a non-inverted output, switching means connected to a source of carrier frequency to interrupt conduction through said switching means in accordance with the carrier frequency signal, means connecting said switching means from said amplifier inverted output to said common input-output terminals, first and second resistors connected from the inverted and non-inverted outputs of said amplifier, respectively, and means interconnecting said first and second resistors and the other of said pair of output terminals in a common summing node, said first and second resistors being proportioned so as to cause the input signal to said modem to be substantially cancelled at said output terminals.

4. An unbalanced modem in accordance with claim 3 wherein said amplifier comprises a first transistor having its base connected to the input of said amplifier, its collector connected to the said inverted output of said amplifier and its emitter connected to the non-inverted output of said amplifier, a third resistor is connected from the collector electrode of said first transistor to a source of biasing potential, and said switching means comprises a second transistor having base, collector, and emitter electrodes, means connecting the base electrode of said second transistor to said source of carrier frequency, and means connecting the collector-emitter path of said second transistor across said third resistor, said first and second resistors being proportioned with respect to said third resistor to provide said substantial cancellation of the modem input signal at said summing node.

5. An unbalanced modem in accordance with claim 3 wherein said means connecting said switching means from said amplifier inverted output to said common input-output terminals includes a serially connected capacitor which charges to an average potential equal to the average potential appearing at said inverted output of said amplifier, whereby the carrier frequency signal appearing at said output terminals is suppressed.

6. An unbalanced modem in accordance with claim 5 wherein said means connecting said switching means from said amplifier inverted output to said common input-output terminals additionally includes a source of d.c. bias, the magnitude of the d.c. bias being proportional to the magnitude of the transmitted carrier signal introduced at said output terminal.

7. An unbalanced modem having a pair of input and a pair of output terminals comprising an amplifier which includes first and second transistors, the base electrode of said first transistor being connected to one of said pair of modem input terminals, the collector electrode of sAid first transistor being connected to the base electrode of said second transistor, and the emitter electrode of said first transistor being connected to the collector electrode of said second transistor, said amplifier having an inverted output at the emitter electrode of said second transistor and a non-inverted output at the collector electrode of said second transistor, the other of said pair of modem input terminals being connected to one of said pair of modem output terminals to form a common input-output terminal, a first resistor connected between the non-inverted output terminal of said amplifier and said common input-output terminal, a second resistor connected from the inverted output terminal of said amplifier to a source of positive biasing potential to provide the biasing potentials for said first and second transistors, non-linear means comprising a third transistor having its base electrode connected to a source of carrier frequency signal and a capacitor serially connected with the collector-emitter path of said third transistor from said source of biasing potential to the inverted output of said amplifier, a third resistor connected to the inverted output of said amplifier, a fourth resistor connected to the non-inverted output of said amplifier, and means interconnecting said third resistor, said fourth resistor, and the other of said pair of output terminals in a common summing node, said first, second, third, and fourth resistors being proportioned such that the input signal appearing at said input terminals is substantially cancelled at said output terminals.