Polar envelope correction mechanism for enhancing linearity of RF/microwave power amplifier

Abstract

Linearity of an RF / microwave power amplifier is enhanced by an amplitude and phase distortion correction mechanism based upon signal envelope feedback, that operates directly on the RF signal passing through the power amplifier. A phase-amplitude controller responds to changes in gain and phase through the RF / microwave power amplifier signal path caused by changes in RF input power, DC power supply voltages, time, temperature and other variables, and controls the operation of a gain and phase adjustment circuit, so as to maintain constant gain and transmission phase through the RF / microwave power amplifier.

Description

FIELD OF THE INVENTIONThe present invention relates in general to communication systems, and is particularly directed to a polar envelope responsive correction mechanism for reducing amplitude and phase distortion in a microwave and RF power amplifier that is designed to amplify signals whose bandwidth is in the KHz to low MHz spectral range.BACKGROUND OF THE INVENTIONAs the wireless communications market continues to expand, the accompanying need for increased capacity is forcing a move from analog modulation techniques, such as frequency modulation (FM), to digital modulation formats, such as time division multiple access (TDMA) and code division multiple access (CDMA), which have bandwidths listed in Table 1.TABLE 1 US DAMPs (TDMA) 30 kHz GSM 277 kHz CDMA 1.23 MHzBoth TDMA and CDMA modulation require somewhat greater linearity than can be routinely obtained in an uncorrected, high efficiency class AB power amplifier. Unfortunately, conventional correction mechanisms, such as feed...

AI Technical Summary

Benefits of technology

The present invention takes advantage of the relatively modest bandwidth of digital modulation formats such as those shown in Table 1, by providing an amplitude and phase distortion correction strategy that is based upon signal envelope feedback. In particular, the present invention provides a simple, yet efficient and effective way of improving the linearity of a non-linear microwave / RF power amplifier employed for digital modulation formats having signal bandwidths in the KHz to low MHz range, through the use of a polar envelope-responsive correction (PEC) mechanism, which operates directly on the RF signal passing through the power amplifier, requires no baseband information, and reduces the AM-to-PM and AM-to-AM distortion which causes spectral regrowth of digital modulation formats such as TDMA, CDMA and GSM. The correction mechanism of the invention enables the power amplifier to comply with linearity and spectral regrowth requirements imposed by both government regulatory agencies (such as the Federal Communications Commission (FCC)) and industry standards.

A delay line is usually installed in the reference path to reduce phase differences between the main and reference paths which arise from the larger delay of the main path. This ensures that the main and reference path signals arrive at the phase amplitude controller at the same time.

In order to maintain stability, the inverse of the time delay (1 / t) through the feedback path is greater than the bandwidth occupied by the envelope of the signal being processed. The amplitude control loop and the phase control loop interact minimally with one another and have adequate bandwidth to process all of the frequency components contained in the spectrum of the input signal envelope. The response of both of the gain and phase control loops is such that each loop has adequate gain and phase margin to prevent instability and unacceptable transient behavior.

When the control input voltage V.sub.A to each MESFET is biased at a first DC voltage value (e.g. -5V), current flow through the MESFETs' source-drain paths is cut off, so that the MESFETs are rendered non-conductive or placed in the high impedance state. As a result, all of the RF energy delivered to the input of the fast variable attenuator and the quadrature hybrid attenuator will be absorbed by the MESFET's parallel 50 ohm resistors, thereby providing maximum attenuation. As the control voltage V.sub.A applied to the MESFET gate terminals is increased (e.g., from -5V toward zero volts), the effective resistance of the MESFETs and their parallel resistors decreases, since the parallel resistors become shunted by the monotonically decreasing resistance of the MESFETs, whereby more and more RF energy is reflected to the fast variable attenuator's RF output port.

For this purpose, the gain / phase adjuster in the main signal path is implemented as a slow gain / phase adjuster, connected in cascade with a fast gain / phase adjuster. The control voltages V.sub.A and V.sub..PHI. generated by the phase / amplitude controller for the two gain / phase adjusters are coupled through respective slow and fast pair of loop filters. Such separate loops not only reduce the slew rate requirements imposed on loop components but also reduce the amount of the range of the amplitude and phase adjuster that must be traversed at fast rates. As a result, the relatively slow and fast loops can be centered in an optimal portion of the control component's (variable attenuator, phase shifter, vector modulator) operating curve. This greatly eases the task of obtaining stable, wide bandwidth control loops. Namely, dividing each loop into a relatively fast loop that tracks the modulation envelope of the signal incident on the RF amplifier, and a relatively slow loop that tracks only environmentally induced changes facilitates the design of stable wideband polar envelope correction loops.

Problems solved by technology

Unfortunately, conventional correction mechanisms, such as feed forward or predistortion schemes, are complex, inefficient, and prohibitively expensive to be practical solutions for correcting distortion in the majority of single carrier linear power amplifiers.

Since baseband I and Q data is usually not available at the power amplifier, baseband correction techniques which make use of baseband signals, such as baseband cartesian feedback, are also not applicable.

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