Amplifiers – With amplifier condition indicating or testing means
Reissue Patent
2000-04-19
2001-10-16
Shingleton, Michael B (Department: 2817)
Amplifiers
With amplifier condition indicating or testing means
C330S136000, C330S149000
Reissue Patent
active
RE037407
ABSTRACT:
FIELD OF THE INVENTION
The 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 INVENTION
As 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
MHz
Both 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 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.
SUMMARY OF THE INVENTION
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.
In accordance with a first embodiment of the polar envelope-responsive correction scheme of the present invention, an RF/microwave input signal to be amplified by the power amplifier of interest is initially subdivided or split into two signal components, a first of which is conveyed through a main (M) signal path through the amplifier, and a second of which is conveyed through a reference (R) signal path separate from the main path through the amplifier. Before passing through the RF amplifier of interest, the main path signal is processed by an amplitude and phase adjustment circuit. A portion of the RF power amplifier output is fed back through a directional coupler and fixed gainset attenuator to be compared with the signal on the reference path. The comparison is made by the fast phase amplitude controller (PAC).
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.
The phase-amplitude controller compares the composite amplitude and phase of the delayed input signal spectrum delivered via the reference path to its R input with the corresponding amplitude and phase components of the output signal spectrum of amplifier delivered to its M input, and generates respective amplitude and phase adjustment control voltages V
A
and V
&PHgr;
. These amplitude and phase adjustment control voltages, which are coupled through respective loop filters to control the amplitude and phase adjustment circuits in such a manner as to maintain constant gain and phase through the main signal path between the RF input at the input signal splitter to the directional coupler output port. The amplitude adjustment control voltage V
A
is proportional to the difference between the amplitudes of the envelopes of the signals, and the phase control signal V
&PHgr;
is proportional to the phase difference between the envelopes of the signals applied to the M and R input ports of the fast phase-amplitude controller.
The PAC responds to changes in main path gain and phase caused by changes in RF input power, DC power supply voltages, time, temperature and other variables. The feedback signal from the directional coupler controls the amplitude/gain and phase adjustment circuit, so that constant gain and transmission phase through the RF amplifier are maintained. The control system bandwidth is normally set several times above the highest frequency component contained in the envelope of the RF signal passing through the amplifier.
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.
The amplitude/gain and phase adjustment circuit has a phase range greater than the maximum phase change through the main signal path under all conditions of RF input power, temperature, supply voltage, signal frequency or other environmental variables impacting the transmission phase of the main signal path. Also, its amplitude range is greater than the maximum gain change through the main signal path under any combination of such conditions. Moreover, the high frequency response of the feedback loops are controlled to ensure that envelope frequency components beyond the loop bandwidth do not significantly degrade the raw performance of the power amplifier.
To avoid control loop stability problems, the amplitude and phase transfer functions of the amplitude/gain and phase adjustment circuit are smooth, monotonic functions of the control voltages V
A
and V
&PHgr;
generated by the fast phase-amplitude controller. The bandwidth of the PAC and amplitude and phase control elements are significantly greater than the overall loop bandwidth, so that the loop bandwidth is set by the responses of the loop filters.
Pursuant to a first embodiment, the amplitude/gain and phase adjustment circuit comprises a fast variable attenuator and a fast phase shifter coupled in cascade. The variable attenuator is phase-linear and has an extremely fast, flat and low insertion loss, with a dynamic range of more that 12 dB and a control bandwidth greater than 20 MHz. Such an attenuator is preferably implemented as a MESFET-based attenuator circuit, having a quadrature hybrid circuit that is terminated in identical variable resistors coupled in parallel with respective (GaAs) MESFETs. The MESFETs are operated as variable resistors, having their gate electrodes coupled to receive the above-referenced control voltage V
A
generated by the phase-amplitude controller.
When the control input voltage V
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 t
Baskin Brian L.
Brubaker David Lee
Eisenberg John A.
Mucenieks Lance Todd
Robertson, III Charles Stuart
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Shingleton Michael B
Spectrian Corporation
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