Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-05-25
2001-10-23
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S298000, C375S302000
Reexamination Certificate
active
06307894
ABSTRACT:
TECHNICAL FIELD
This invention relates to wireless communications and, more particularly, to power amplification using a direct-upconverting quadrature mixer topology.
BACKGROUND
A conventional power amplifier (PA) in a digital wireless transceiver receives and then amplifies a radio frequency (RF) signal. For many modulation schemes, including those that comply with the IS-95, PDC, PHS, DCT and certain CDMA standards, the RF signal has a non-constant envelope, which requires the power amplifier to operate with a high degree of linearity over a large dynamic range. As a result, the conventional power amplifier usually is biased for operation between the Class A and Class B modes. The best case (asymptotic) efficiency of a PA operating in this manner usually lies between 50% and 78% for constant envelope signals; the efficiency is even lower for non-constant envelope signals.
U.S. patent application Ser. No. 09/108,628, filed on Jul. 1, 1998, by Donald Brian Eidson and Robert Edmund Grange, and titled “Envelope Feedforward Technique with Power Control for Efficient Linear RF Power Amplification,” discloses a power amplification technique that allows operation in high efficiency modes (such as Class D or Class E), in some cases producing asymptotic efficiencies for constant envelope signals that approach 100%. In one implementation of this technique, a dual-gate field effect transistor (FET) acts in a manner similar to a conventional RF mixer, receiving at one dual-gate input a constant envelope RF signal containing phase component information, and receiving at the other dual-gate input an unmodulated signal containing the envelope component. In a digital system, the constant envelope RF signal and the unmodulated envelope signal are derived from the in-phase (I) and quadrature (Q) components provided by a baseband device.
SUMMARY
In one aspect, the invention features a power amplifier having a quadrature mixer and an oscillator circuit. The oscillator circuit generates two reference signals that are 90° out of phase with respect to each other. The quadrature mixer has two mixer elements that each receive one of the reference signals. Each mixer element receives a signal containing either the in-phase or the quadrature component of a baseband signal to be transmitted and uses the reference signal to upconvert and amplify the I or Q component of the baseband signal. The quadrature mixer also includes circuitry that combines the upconverted and amplified I and Q components to form an output signal.
In another aspect, the invention features a wireless transmitter that includes a baseband encoder, an oscillator circuit, and a quadrature mixer. The baseband encoder produces signals representing the in-phase and quadrature components of a baseband signal to be transmitted. The oscillator generates two reference signals that are 90° out of phase with respect to each other. The quadrature mixer uses each of the reference signals to upconvert and amplify one of the in-phase and quadrature components of the baseband signal.
In some embodiments, the reference signals have power levels much greater than the power levels of the I and Q components of the baseband signal. In these cases, the mixer elements include power elements, such as Gilbert cells or dual-gate FETs. The reference signal often drives a modulating port of the mixer, and the in-phase or quadrature component of the baseband signal often drives the reference port. In some cases, the reference port is biased to act as an ON/OFF switch, and the modulating port is biased to act as a current source. The ON/OFF switch is usually implemented as a power device.
Other embodiments include four mixer elements, each of which receives one of a group of signals representing the in-phase, negative in-phase, quadrature, and negative quadrature components of the baseband signal. In some of these embodiments, the oscillator circuit produces four reference signals, three of which are 90°, 180°, and 270° out of phase with respect to the other. A four-phase, ¼-wavelength branch tap is one tool for producing the four reference signals.
Other embodiments include a low-loss combiner coupled to the quadrature mixer elements. In some of these embodiments, the low-loss combiner includes a ceramic substrate.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
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Wofff et al, “Microwave Engineering and System Applications”, Copyright 1988 by John Wiley & Sons, Inc., pp. 387-388.
Eidson Donald Brian
Grange Robert Edmund
Conexant Systems Inc.
Pham Chi
Phu Phuong
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