Modulators – Phase shift keying modulator or quadrature amplitude modulator – Including logic element
Utility Patent
1999-03-24
2001-01-02
Pascal, Robert (Department: 2817)
Modulators
Phase shift keying modulator or quadrature amplitude modulator
Including logic element
C332S103000, C332S160000, C332S161000, C332S162000, C375S261000, C375S298000, C455S063300, C455S115200
Utility Patent
active
06169463
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a quadrature modulator.
The present invention further relates to a radio communication device comprising a quadrature modulator, and to a modulation method.
2. Description of Related Art
In the U.S. Pat. No. 5,012,208 a quadrature modulator is disclosed with adaptive suppression of carrier leakage in an in-phase and a quadrature branch of the quadrature modulator. By applying an appropriate dc-offset (direct current) to the in-phase and quadrature branches, such a carrier leakage can be substantially eliminated, when initially adjusting the quadrature modulation, and selecting matching components. It is described that many factors, such as temperature, frequency, load impedance, and carrier power variations may not be adequate to compensate for carrier leakage. In said U.S. Pat. No. 5,012,208 a complicated control loop is disclosed for adaptively adjusting the local oscillator's leak signal to the output of the rf-mixer, such a control loop having correlators and integrators in its feedback paths. At an output of an rf-power amplifier, which is coupled to the quadrature modulator, the control loop continuously measures and correlates the rf-power to be transmitted, and, in dependence thereto, continuously injects dc-offset compensation values at input side of the quadrature modulator.
In the U.S. Pat. No. 5,396,196 a similar type of carrier leakage compensation as in said U.S. Pat. No. 5,012,208 is disclosed. In addition thereto, the carrier leakage compensation control loop described therein needs a complicated Pseudo-Noise generator.
In the Japanese Abstract No. 07202961, in a transmitter with a quadrature modulator, a carrier leakage compensation circuit is disclosed. A dc-level detector measures dc-levels at a base band side of the quadrature modulator. On the basis of such measured base band dc-offset signals, an adjusting circuit adjusts dc-levels of the base band quadrature signals.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a quadrature modulator with a simple but robust carrier leakage compensation means.
It is another object of the invention to provide a quadrature modulator in which carrier leakage compensation is performed at predetermined instants.
It is still another object of the invention to provide a quadrature modulator in which possible dc-offsets or other disturbing effects generated by compensation means itself are eliminated.
It is yet another object of the invention to easily control application of carrier leakage compensation.
It is yet another object of the invention to eliminate effects of cross talk.
In accordance with the invention, a quadrature modulator is provided comprising:
an in-phase quadrature modulation branch comprising a first series arrangement of a first digital-to-analog converter, a first anti-aliasing filter, a first summing means, and a first mixer;
a quadrature modulation branch comprising a second series arrangement of a second digital-to-analog converter, a second anti-aliasing filter, a second summing means, and a second mixer;
a local oscillator means for providing a first local oscillator signal and a second local oscillator signal to said first and second mixers, respectively;
a third summing means coupled to respective outputs of said first and second mixers, said third summing means providing a quadrature modulated signal;
carrier leakage measurement means for measuring a first carrier leakage signal of said first local oscillator signal in said in-phase modulation branch, and a second carrier leakage signal of said second local oscillator signal in said quadrature branch;
a controllable signal generating means for generating a first monotonously increasing signal and a second monotonously increasing signal; and
holding means for holding values of said first and second monotonously increasing signals, said holding means being coupled to said first and second summing means so as to form feedback paths;
said carrier leakage measurement means adopting a first state in which said first and second carrier leakage signals are measured, and a second state in which said controllable signal generating means is controlled to stop generating said first and second monotonously increasing signals, said second state being adopted from said first state during measurement of said first and second carrier leakage signals.
The invention is based upon the insight that usually parameters influencing carrier leakage are slowly varying with time so that there is no need for continuous compensation. Based upon this insight, it was realized that a simple but robust carrier leakage compensation could be implemented being operative at predetermined points in time, such at switching on power, or, when used in a communications apparatus, possibly also at channel switching. When used in a communication device such as a broad band CDMA communications device, in which, in principle, no channel switching is needed, there is only a needed to apply carrier leakage compensation at power on of the communications device. In such a case, after power on, power to components used for carrier leakage compensation could even be switched off, thus achieving power savings in such a portable communications device.
Preferably, said carrier leakage detection means comprises a first series arrangement of a first synchronous detector and a first comparator, and a second series arrangement of a second synchronous detector and a second comparator, respective output signals of said first and second comparators controlling said controllable signal generating means to stop generating said first and second monotonously increasing signals. Herewith, carrier leakage is accurately measured, while at the same time a well-defined criterion is available for stopping carrier leakage compensation.
Preferably, said controllable signal generating means comprises a first counter, and a first state machine coupled between said carrier leakage measurement means, and a second counter, and a second state machine coupled between said carrier leakage measurement means, said first and second state machines feeding clock pulses to said first and second counters when said carrier leakage means is in said first state, and stopping to feed clock pulses to said counter when said carrier leakage means is in said second state. Herewith, a very simple means is obtained for digitally generating a controllable amount of dc-offset compensation, until the carrier leakage measurement means stop the dc-offset compensation.
For simply injecting an analog dc-compensation signal representing the digitally generated compensation, digital-to-analog converters are provided. Filters are provided for filtering double frequency carrier components that are introduced by synchronous detectors included in the measurement means. Before these synchronous detectors, amplifiers are provided to suppress dc-offsets introduced by the synchronous detectors, the synchronous detectors otherwise introducing additional carrier leakage.
REFERENCES:
patent: 5012208 (1991-04-01), Makinen et al.
patent: 5396196 (1995-03-01), Blodgett
patent: 5793817 (1998-08-01), Wilson
patent: 07202961A (1995-08-01), None
Mohindra Rishi
Stroet Petrus M.
Glenn Kimberly
Pascal Robert
Philips Electronic North America Corp.
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