Quadrature compensation for orthogonal signal channels

• US 5,400,363 A
• Filed: 05/07/1993
• Issued: 03/21/1995
• Est. Priority Date: 05/07/1993
• Status: Expired due to Term

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Loral Aerospace Corp. (MHR Holdings LLC (New York))
• Inventors: Waugh, Geoffrey S., Jacobson, Michael E., Wagner, Gary L.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Le, Amanda T.
• Application Number:
  US08/060,209
• Time in Patent Office: 683 Days
• Field of Search: 375/80, 375/81, 375/83, 375/97, 375/78, 375/88, 375/120, 329/307, 329/308, 329/309, 331/10, 331/12, 331/34
• US Class Current: 375/324
• CPC Class Codes: H04L 27/2273 associated with quadrature demodulation, e.g. Costas loop

Abstract

A system for processing inphase and quadrature data channels, such as a Costas loop QPSK demodulator, employs an additional feedback loop for adjustment of phase offset between carrier reference signals, this loop being in addition to the Costas error loop for control of frequency and phase of an oscillator which provides the regenerated carrier signal. The additional loop employs cross-channel products of demodulated inphase and quadrature data signals as does the Costas loop. The regenerated carrier is applied via an adjustable phase-offset unit to provide quadrature carrier reference signals to phase detectors of inphase and quadrature data channels. The phase offset unit includes a 90 degree hybrid circuit energized by the carrier signal at a main input port plus an adjustable fraction of the carrier power applied to an auxiliary input port. Adjustment of the relative magnitudes of the signals at the two input ports of the hybrid circuit in accordance with the sum of the cross-channel products provides a desired amount of phase offset between the two carrier reference signals in correspondence with the deviation from orthogonality of the components of the composite input signal. This removes crosstalk resulting from the deviation in orthogonality of the components of the composite input signal.

Claims

1. In a system for generation of a carrier for operation with inphase and quadrature data channels, the system comprising:

an inphase data channel and a quadrature data channel, each of said channels including a first multiplying means, a second multiplying means, and a limiter of signal amplitude interconnecting an output port of said first multiplying means with a first input port of said second multiplying means, said first multiplying means in each channel having an input port for receiving a composite input data signal having orthogonal components;

an oscillator outputting a reference carrier, and a variable phase offset means for coupling the carrier from said oscillator to each of said data channels, said first multiplying means in each of said channels having a second input port for receiving the carrier from said variable phase offset means, said variable phase offset means outputting inphase and quadrature components of said carrier differing in phase from each other by a nominal value of ninety degrees, the carrier inphase component being applied to said first multiplying means of said inphase channel and the carrier quadrature component being applied to said first multiplying means of said quadrature channel;

wherein, in said inphase channel, said second multiplying means multiplies together output signals of said limiter of said inphase channel and said first multiplying means of said quadrature channel to provide an inphase channel output signal and, in said quadrature channel, said second multiplying means multiplies together output signals of said limiter of said quadrature channel and said first multiplying means of said inphase channel to provide a quadrature channel output signal;

said system further comprises a first summing means for extracting a difference between the channel output signals to provide a control signal for said oscillator to establish a frequency of the carrier outputted by said oscillator; and

a second summing means for summing together the channel output signals to provide a command signal for said variable phase offset means to adjust the phase difference between the carrier components by shifting the phase difference away from ninety degrees, thereby to compensate for a deviation in orthogonality of the components of the input data signal.

2. A system according to claim 1 wherein said first multiplying means in each of said channels includes a multiplying element and a low-pass filter at an output port of the multiplying element, said first multiplying means functioning as a phase detector.

3. A system according to claim 2 wherein

in any one of said channels, there is connection between said limiter and said low-pass filter;

and

an output port of said first summing means includes a low-pass filter applying the control signal to said oscillator.

4. In a system for generation of a carrier for operation with inphase and quadrature data channels, the system comprising:

an inphase data channel and a quadrature data channel, each of said channels including a first multiplying means, a second multiplying means, and a limiter of signal amplitude interconnecting an output port of said first multiplying means with a first input port of said second multiplying means, said first multiplying means in each channel having an input port for receiving a composite input data signal having orthogonal components;

an oscillator outputting a reference carrier, and a variable phase offset means for coupling the carrier from said oscillator to each of said data channels, said first multiplying means in each of said channels having a second input port for receiving the carrier from said variable phase offset means, said variable phase offset means outputting inphase and quadrature components of said carrier differing in phase from each other by a nominal value of ninety degrees, the carrier inphase component being applied to said first multiplying means of said inphase channel and the carrier quadrature component being applied to said first multiplying means of said quadrature channel;

wherein, in said inphase channel, said second multiplying means multiplies together output signals of said limiter of said inphase channel and said first multiplying means of said quadrature channel to provide an inphase channel output signal and, in said quadrature channel, said second multiplying means multiplies together output signals of said limiter of said quadrature channel and said first multiplying means of said inphase channel to provide a quadrature channel output signal;

said system further comprises a first summing means for extracting a difference between the channel output signals to provide a control signal for said oscillator to establish a frequency of the carrier outputted by said oscillator;

a second summing means for summing together the channel output signals to provide a command signal for said variable phase offset means to adjust the phase difference between the carrier components, thereby to compensate for a deviation in orthogonality of the components of the input data signal; and

said variable phase offset means comprises a quadrature hybrid unit which includes a first port serving as a primary input port, a second port serving as an output port which provides one-half of power present at said first port with a phase lag of ninety degrees, a third port serving as an auxiliary input port, and a fourth port serving as a further output port providing one-half of power present at said first port with a phase lag of one hundred eighty degrees;

a power divider connecting a carrier input port of said variable phase offset means to said first and said third ports of said quadrature hybrid unit, said power divider applying a relatively large fraction of carrier power from said oscillator to said first port of said quadrature hybrid unit and a relatively small fraction of the carrier power to said third port of said quadrature hybrid unit; and

a variable attenuator interconnecting said power divider with said third port of said quadrature hybrid unit, said attenuator having circuitry for varying attenuation of a signal applied to said third port of said quadrature hybrid unit in accordance with the command signal of said second summing means, said second port of said quadrature hybrid unit outputting the inphase carrier component to said inphase channel, and said fourth port of said quadrature hybrid unit outputting the quadrature carrier component to said quadrature channel.

5. A system according to claim 4 wherein said variable phase hybrid means further comprises a delay line coupled to the fourth port of said quadrature hybrid unit to provide a fixed phase shift between said inphase and quadrature carrier components;

and

wherein an output port of said second summing means is provided with a low-pass filter for applying said command signal to said variable attenuator.