Abstract:
A phase-amplitude modulator for mobile communications employs a phase lock loop as an input to a nonlinear power amplifier providing phase information with amplitude information used to modulate the power amplifier output to synthesize phase-amplitude for the RF transmission signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of the Provisional Application Ser. No. 60/387,234 of the same title filed on Jun. 7, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to the field of modulators for Phase Amplitude Modulation (such as QAM or QPSK) transmission signal generation. More particularly, the invention provides a Phase-Amplitude modulator using a phase lock loop (PLL) to drive the power amplifier (PA) with phase and amplitude provided by either quadrature or polar input signals resulting in a low-cost, high power efficiency transmission solution for wireless communications. 
   2. Description of Related Art 
   As early as the 1950&#39;s, an “Envelop Elimination &amp; Restoration” scheme was practiced in transmitter technology, mainly to raise the RF power amplifier efficiency. This is described in detail by Leenaerts, et al ( Circuit Design for RF Transceivers , ISBN 0-7923-7551-3). The scheme, illustrated in  FIG. 1 , provides an alternative to traditional methods for adding the amplitude information at the power amplifier stage. 
   Traditional Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) transmitters consist of a quadrature I/Q modulator, depicted by  FIG. 2 , an up-converter and a linear RF power amplifier. A highly linear RF power amplifier is required, and a hi-order, low-loss band-pass filter is needed before the antenna to remove the side band signals as a result of the up-conversion. Both components are expensive, and both cause low power efficiency. 
   It is, therefore, desirable to provide modulation devices that could result in lower cost and higher power efficiency transmitters for QPSK and QAM signals for wireless applications. 
   SUMMARY OF THE INVENTION 
   The modulator of the present invention uses the output of a phase lock loop (PLL) to drive the power amplifier (PA). Because of the continuous phase of the PLL, the frequency spectrum of the PLL output contains very low spurious power. The amplitude information is added by modulation of the PA output, thereby synthesizing the phase-amplitude. In the structure of the invention, the phase of the input signal is employed to create the IF signal input to the phase detector (PD) of the PLL. The output of the PLL is connected to the PA. The input signal amplitude is then employed for modulating the gain of the PA to synthesize the RF output. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
       FIG. 1  is a schematic depiction of prior art Envelope Elimination and Restoration technology; 
       FIG. 2  is a schematic depiction of a traditional QPSK/QAM transmitter; 
       FIG. 3  is a schematic for a first embodiment of the present invention employing quadrature phase and amplitude information; 
       FIG. 4   a  is a schematic for a second embodiment employing polar phase and amplitude signal information with a direct digital synthesizer for IF generation; 
       FIG. 4   b  is a schematic for the second embodiment employing polar phase and amplitude signal information with a pulse width modulator for IF generation; 
       FIG. 4   c  is a schematic for the second embodiment employing polar phase and amplitude signal information with a phase lock loop for IF generation; 
       FIG. 4   d  is a schematic for the second embodiment demonstrating baseband filtering of the I/Q signals for polar phase and amplitude signal information for spectrum shaping; 
       FIG. 5  is a schematic for an alternative embodiment of the polar vector embodiment; 
       FIG. 6  is a schematic for a second alternative embodiment of a fully integrated polar vector modulator employing the present invention; 
       FIG. 7  is a quadrature plot of QPSK amplitudes for an embodiment of the invention; and, 
       FIG. 8  is a data plot for carrier and envelope data and the resulting signal output in a SPICE simulation using a multimode modulator according to the present invention as embodied in FIG.  3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings,  FIG. 3  illustrates a first embodiment of the invention. In this embodiment, the I/Q baseband vectors are used to create a precise phase-modulated intermediate carrier frequency (IF) by the means of a quadrature modulator employing analog to digital converters (A/D)  10  and  12  and phase shift architecture  14 . The resulting IF from the quadrature modulator then feeds the reference phase input of phase detector (PD)  16  of a PLL. The PD output signal is conditioned by low pass filter (LP)  18 . The oscillator frequency (OF) signal is mixed in the feedback loop and the voltage controlled oscillator (VCO)  20  output produces an up-converted frequency that is the sum of IF+OF, and preserves the original phase information of the I/Q signals. This signal then drives a PA  22 . A non-linear PA is used with a low-pass harmonic filter  23 , instead of a more expensive high order band pass filter to take advantage of the PLL&#39;s purer output spectrum. 
   The amplitude information of the input signal, essentially proportional to the magnitude of vector I+Q, is introduced at the PA by gain variation or output rail limiting in alternative embodiments. For the embodiment shown in  FIG. 3 , an amplitude generator  15  determines the amplitude from the outputs of the A/D converters. Since most PAs have varying gain and linearity characteristics, the closed-loop control is provided for the amplitude in the embodiment shown in FIG.  3 . This is done with an amplitude detector (ADET)  24  (implemented in various embodiments as an RF diode) and an analog transfer function (G(s))  26  (implemented in various embodiments as an operational amplifier). 
   For the I/Q scheme in  FIG. 3 , the PLL modulating signal contains amplitude information. For some modulation types, such as QPSK, the amplitude can momentarily become near zero. To avoid splash caused by the missing phase edges during phase transition, the I/Q carrier transition paths should avoid “zero” or “near zero” amplitude. For example, a transition from point A to point B shown in  FIG. 7  would produce a zero amplitude in IF momentarily. In the figure, r1 is the minimum carrier amplitude for Pi/4 QPSK, which is always not zero and r2 is the minimum carrier amplitude of an altered trace in a QPSK that does not cross the origin (known in the art as “Offset QPSK). The I/Q vector is modified to go around the origin so that at least there is a minimum magnitude r2. Optimum employment of the present invention uses Offset QPSK or alternatively QAM, GMSK or other non-zero crossing modulation techniques. 
     FIG. 4   a  shows a second embodiment of the present invention better suited for silicon implementation. In the embodiment shown, the I/Q vectors are not used at all, thus eliminating the requirement for the A/D converters for the I/Q analog signal. The Phase-amplitude information is used in polar form, i.e. the phase angle and magnitude. A direct-digital synthesizer (DDS)  28  receives the Phase signal and generates the IF signal provided to the PLL elements (PD  16 , LP  18  and VCO  20 ). 
     FIG. 4   b  shows the second embodiment wherein the DDS is replaced with a pulse width modulator (PWM)  28 ′ to provide digital synthesis of the phase to create the IF signal for the PLL. In another alternate embodiment shown in  FIG. 4   c , a second PLL  28 ″ is used in place of the DDS. The DDS or the alternatives of a PWM or second PLL and the PD, LP and VCO components of the PLL are contained on integrated circuit (IC)  30 . 
   The embodiment of  FIG. 5  enhances the cost reduction possible with the present invention by integrating the amplitude modulation function using a pulse width modulator (PWM)  32  onboard the IC which eliminates the necessity of converting digital amplitude information to analog prior to introduction to the circuit. 
     FIG. 6  shows an alternative embodiment wherein the components for the amplitude detection and transfer function generation for the PA control are implemented on the IC. The ADET  34  receives the signal from the PA and its output is routed through an A/D  36 . The digitized signal is mixed with the amplitude in a digital function generator (T(z))  38  which is then passed through digital to analog converter (D/A)  40  to provide the feedback signal for the PA. 
   For other embodiments, as an alternative to PA gain modulation, the amplitude modulation can also be accomplished by altering the power supply voltage on the PA transistor; changing the input matching parameters or using a RF attenuator. Also, the spectrum shaping can be achieved by deriving the phase and amplitude from the original I/Q signals to maintain the spectral requirement, using the formula Phase=Arctan(Q/I), Amplitude=SQRT(I 2 +Q 2 ) as shown in  FIG. 4   d . In addition, partial filtering on the phase can also be done in the PLL&#39;s low-pass filter. 
   SPICE simulation of an embodiment of the present invention as shown in  FIG. 3  demonstrates the efficacy of the circuit. In the simulation shown in  FIG. 8 , the carrier frequency in the model is 1900 MHz with a QPSK symbol rate of 1 MHz as shown in trace  42 . The fidelity of the modulation employing the present invention is shown in the comparison of the input signal of the base band I/Q amplitude provided in plot  44  shown with respect to the resultant amplitude envelope of the 1900 MHz carrier shown as signal  46 . 
   Having now described the invention in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.