Mobile wireless phone and data communication have become increasingly popular. These applications, however, pose two special problems. First, the available bandwidth for transmitting information is limited due to the general shortage of available spectrum. As a result, both the amplitude and the phase of the carrier must be modulated to reduce the required bandwidth. Amplifying the amplitude modulated carrier without excessive distortion in the transmitter output stage imposes significant linearity constraints on the output stage amplifier.
Second, the power efficiency of the mobile transmitter is very important because the mobile end of the wireless communication link is usually battery powered. Typically, the transmitter output stage is the largest power consumer; hence, improvement in this stage is the most important. One of the most efficient power amplifiers are the class C and E RF amplifiers in which the output transistor conducts current only at the time when the collector-emitter voltage is at its lowest value. Unfortunately, class C and E amplifiers are very nonlinear and introduce substantial distortion of the amplitude modulation. Because of this distortion, class C and E amplifiers are used mainly in FM transmitters in which the amplitude or "envelope" of the RF carrier is constant, and hence, such distortion has no effect.
One method for avoiding this distortion with class C amplifiers and still allowing linear amplitude modulation is to generate two signals with constant amplitude using the class C amplifiers and then combining these signals. The amplitude modulation is achieved by modulating the relative phase of the two constant amplitude signals. Denote the two signals by V.sub.l and V.sub.1, respectively. EQU V.sub.1 =Vsin[.omega.t+mt(t)+a(t)] (1)
and EQU V.sub.2 =Vsin[.omega.t +mt(t)-a(t) ] (2)
Here, m(t) is the desired phase modulation of a carrier having angular frequency .omega., and a(t) and -a(t) are additional phase modulations of the two carriers. By adjusting a(t), the output of the sum signal can be varied from 0 to 2V. In particular, if an output voltage of V.sub.out is desired, a(t)=arccos(V.sub.out /2V). Methods for generating the phase angle, a(t), are described, for example, by D. C. Cox in "Linear Amplification with Nonlinear Components", IEEE Transactions on Communications, December 1974, pp. 1942-1945.
While this method, in principle, solves the problem of using class C amplifiers to provide the desired amplitude modulation, there are two problems in constructing a vectorial signal combiner for use in such a system. First, to minimize the power losses in the vectorial signal combiner itself, the combiner must consist of nominally reactive components only. This reduces the power dissipated in the combiner itself.
Second, the combiner must present a load to the two constant envelope amplifiers with unity power factor at all levels of the modulated output. A load with unity power factor presents an impedance that has no imaginary part, i.e., the voltage and current at the load are in phase. This constraint maximizes the overall power efficiency when the load is driven by class C or E type power amplifiers. Consider the case in which the constant envelope carriers are supplied by class C npn transistor stages. At all phase angles a(t), when the transistor conducts current, the sinusoidal voltage on the collector must be at its negative peak to minimize power dissipation in the transistors. This occurs only when the class C amplifier drives a load with unity power factor.
No combiner satisfying both conditions is taught in the prior art. The closest combiner to the above goals is a combiner described by H. Chireix in "High Power Outphasing Modulation", Proceedings of the Institute of Radio Engineers, November 1935, pp. 1370-1392. This combiner has two flaws. First it relies on fixed elements which are not tuned as a(t) changes. Using fixed elements results in a combiner which has a power factor that only approximates unity, and rapidly falls to zero for a(t)&gt;72 degrees. This restricts the range of output amplitudes that can be supplied by the circuit, and sacrifices efficiency even within that range. Second, the combiner uses uncoupled inductors to connect to the load. There is always a voltage drop across these inductors, even when the two amplifiers are in phase (a(t)=0). Therefore, a larger voltage swing is required at the amplifiers to support a given maximum voltage swing at the load. It is undesirable to generate large voltage swings in battery powered equipment.
Broadly, it is the object of the present invention to provide an improved vectorial signal combiner.
It is a further object of the present invention to provide a vectorial signal combiner that uses only reactive components.
It is a still further object of the present invention to provide a vectorial signal combiner that presents a load to the two constant envelope amplifiers with a unity power factor at all levels of modulated output.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.