Broadband predistortion linearizer with automatic temperature compensation for microwave amplifiers

A predistortion linearizer for microwave power amplifiers has a linear channel and a non-linear channel. Both channels are made up of the same units. Each channel includes a variable phase-shifter, a variable attenuator and an amplifier. Each channel can further include a fixed attenuator. The two channels are coupled at their ends by couplers each introducing a phase-shift of 90.degree. so that the total phase-shift is 180.degree.. Because the components of the two channels are the same, any variations with the frequency or the amplitude of the signal or with the operating temperature cancel out. The linearizer has applications in telecommunications and in particular in multicarrier systems.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The invention concerns telecommunication by means of electromagnetic waves 
and in particular microwave amplifier devices used in transmit and/or 
receive equipment in such telecommunication systems. To be more precise, 
the invention concerns a predistortion linearizer for microwave amplifiers 
intended to alleviate the effects of non-linearity due to different 
amplification conditions in the various operating regimes of the 
equipment. 
2. Description of the prior art 
The person skilled in the art knows that to operate efficiently an 
amplifier, especially a power amplifier, must operate near saturation, and 
hence the need for a linearizer. Close to saturation the linearity of the 
amplifier is strongly degraded compared to the linearity of the same 
amplifier used with signals of lower amplitude than those required for 
operation under saturated conditions. 
A non-linearity correction device known as a linearizer can be used to 
increase the linear dynamic range of an amplifier without compromising the 
electrical efficiency achieved near saturation. There are three types of 
linearizers: the feedback type, the feed forward type and the 
predistortion type. 
The feedback type linearizer is suitable for amplifiers operating at 
relatively low frequencies in which variations in the signal 
characteristics (and therefore in the linearity of the amplifier) are 
slower than the loop response time, i.e. the time between detection of a 
non-linearity and its correction. On the other hand, this configuration is 
very difficult to use for amplifying microwave signals, as it is too slow 
(compared to the signal frequency) for results to be satisfactory. 
Thus for microwave signals it is preferable to use either a feed forward 
linearizer or a predistortion linearizer. The principle of operation is 
based on the extraction by a coupler of part of a signal before 
amplification which is then processed by various active and passive 
electronic components to produce a non-linear correction signal having the 
same non-linearity characteristics as the wanted signal to be corrected, 
but with the opposite phase. In a feed forward linearizer circuit adding 
this correction signal to the amplified wanted signal achieves the 
required linearity. On the other hand, a predistortion linearizer supplies 
a correction signal with the signal to be amplified at the input of the 
microwave power amplifier the non-linearity of which is to be corrected. 
Various types of predistortion linearizer have already been designed. A 
first family of predistortion linearizers is described in U.S. Pat. No. 
4,992,754, BLAUVELT et al., assigned to ORTEL Corp. (USA), for example. 
According to this document, the wanted signal is applied to a delay line 
and the correction signal is produced in a parallel branch of the circuit 
with the amplitude adjusted so that it is equal to that of the 
non-linearity of the amplified signal, the phase of the signal being 
varied so that it is in phase opposition to the wanted signal in the delay 
line. The wanted signal and the correction signal are added by means of a 
microwave coupler (power combiner) and are then fed to the input of the 
microwave power amplifier. The non-linearities at the output of the power 
amplifier stage are significantly reduced or even eliminated by this 
means. 
Another type of predistortion linearizer is known from U.S. Pat. No. 
4,068,186 SATO et al., assigned to KDD (Japan). This linearizer, shown 
schematically in FIG. 1, is designed to operate at high frequencies and to 
alleviate the non-linearity of a klystron or TWT (travelling wave tube) 
type amplifier. SATO et al. teach the use of a low-power TWT as a 
non-linearity generator. This type of amplifier introduces a slight 
time-delay of the amplified signal relative to the unamplified signal, due 
to the finite propagation speed of electrons in a vacuum. It is therefore 
necessary to include a delay line 3 in the corrector branch of the circuit 
to synchronize the non-linear and linear channel signals. 
The signal to be amplified is applied to the input 1 of the SATO et al. 
linearizer, which is connected to a first amplitude-frequency 
characteristic corrector 10 the output of which is fed to a coupler (power 
divider) 2 which splits the signal into two parts fed to the inputs of 
respective transmission lines, a first of which includes a delay line 3 
and the second of which includes a microwave amplifier 4 generating 
non-linear distortion, for example a low-power TWT. The fourth branch of 
the coupler 2 is terminated by a matched load 8. The coupler 2 
conventionally introduces a phase shift .theta. between the two output 
signals. The coupler can be a 3 dB hybrid coupler, for example, in which 
case the phase shift is .theta.=.pi./2=90.degree.. 
The main channel (or linear channel) includes a microwave amplifier 4 the 
operating point of which is chosen near saturation to generate 
non-linearities which depend on the power of the input signal. A variable 
attenuator 13 is provided at the output of the amplifier 4 so that the 
output level of the predistortion linearizer device can be varied without 
altering the gain of the amplifier 4. 
In the SATO et al. disclosure the other (nonlinear) channel includes a 
phase-frequency characteristic corrector 9 in addition to the delay line 
3. The signals from the two channels are applied to two inputs of a 
coupler (power combiner) 5 which adds them together (still with a phase 
shift .theta. between the two signals). This coupler can be a 3 dB hybrid 
coupler, for example, which introduces a further phase shift 
.theta.=.pi./2. The fourth branch of the coupler 5 is terminated by a 
matched load 7. The signal obtained by adding the two signals at the two 
inputs is then fed to a second amplitude-frequency characteristic 
corrector 12. The wanted signal, complete with the predistortion, is then 
fed to the output 6 of the device from which it is fed to the input of a 
microwave power amplifier. 
This prior art device therefore includes two nonlinear amplitude-frequency 
characteristic correctors 12 and 10 plus a phase-frequency characteristic 
corrector 9. Embodiments of such corrector units are described in the SATO 
et al. document, the disclosure of which is hereby incorporated by way of 
reference, constituting description of the prior art. 
The non-linear characteristics of these units are added to those of the 
microwave amplifier 4. The transfer functions of all these units vary in a 
disparate manner with the signal frequency and amplitude and with the 
temperature of the components. The non-linear signal produced in this way 
is added to the wanted signal with the opposite phase to cancel the 
non-linearity of the microwave power amplifier (not shown), but 
cancellation is obtained only in relatively narrow frequency band, over a 
relatively narrow range of input power and at a given temperature. 
A transfer function with gain and phase increasing with the input signal 
level can be obtained by varying the characteristics of the corrector 
units 9, 10, 12. This is usually the required response for linearizing a 
power TWT. 
A transfer function with the gain increasing with the input signal level 
but the phase decreasing with the latter can be obtained by a different 
adjustment of the corrector units 9, 10, 12. This is the response required 
to linearize a solid state power amplifier. 
Another broadband microwave linearizer is described in the article by A. M. 
KHILLA "Novel broadband linearizers and their application in power 
amplifiers for satellite transponders and ground stations", published in 
Proceedings Second European Conference on Satellite Communications, Liege, 
Belgium, 22-24 Oct. 1991, pp 229-234, published by ESA (European Space 
Agency), publication No SP-332, the content of which is hereby 
incorporated by way of reference, constituting a description of the prior 
art. 
This document teaches the use of a predistortion circuit for linearizing a 
broadband amplifier of a Ku band satellite transponder. As in the SATO et 
al. document, the circuit has two branches connected at their ends by two 
3 dB hybrid couplers, each introducing a phase-shift of 90.degree.. It 
further teaches the use of equal electric lengths in the two branches, a 
configuration that is relatively rare in other predistortion circuits 
described in the literature. 
The operating principle of predistortion linearizers as described in the 
prior art documents cited hereinabove can be used in microwave 
applications. Nevertheless, it has a number of major drawbacks which make 
it difficult to use. 
A first drawback is inherent in the fact that the initial adjustments of 
the amplitude-frequency characteristic corrector circuits (10, 12) and the 
phase-frequency characteristic corrector circuit (9) are often difficult 
and time-consuming, and constitute a task which is even more complicated 
in the usual case in which the electrical lengths of the two channels are 
very different. 
A second drawback results from the fact that the two channels comprise 
components of different kinds, with different responses to temperature 
variations; this causes variations in the characteristics of the corrector 
units and therefore in the overall transfer function of the device, these 
variations being highly temperature-sensitive. 
A further drawback of the prior art devices is that the optimum bandwidth 
of the non-linearity corrector device is limited by the differences 
between or by different variations in the electrical lengths of the two 
channels, depending either on the frequency or the amplitude of the 
signals to be amplified or on the operating conditions and in particular 
the temperature. 
An object of the invention is to alleviate the drawbacks of the prior art. 
SUMMARY OF THE INVENTION 
To this end, the invention proposes a predistortion linearizer for 
microwave amplifiers including: 
an input divider having one input and two outputs with a relative 
phase-shift of 90.degree. between said two outputs; 
a linear first channel and a non-linear second channel, said linear and 
non-linear channels having substantially the same electrical length and 
comprising identical circuits; and 
an output combiner having two inputs and one output, with a relative 
phase-shift or 90.degree. between said two inputs. 
The divider is preferably a 3 dB hybrid coupler. The combiner is preferably 
a 3 dB hybrid coupler. The divider is preferably a branch-line coupler. 
The combiner is preferably a ring coupler. 
The invention provides various technologies for fabricating the circuits of 
the linearizer, which can be implemented at least in part in the MMIC 
technology. The linearizer is preferably implemented at least in part in 
the MIC technology. The invention also consists in a microwave amplifier 
including a linearizer in any of the variants defined hereinabove. 
The invention will be better understood and its various features and 
advantages will emerge from the following detailed description of one 
embodiment of the invention and from the appended drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The figures are given by way of non-limiting example to illustrate the main 
features of the invention and its variants. Like reference numbers refer 
to like elements of both figures. Equivalent means can be substituted for 
the means shown in the figures without this departing from the scope of 
the invention. 
FIG. 2 shows one embodiment of a linearizer of the invention. The diagram 
is similar in many ways to the FIG. 1 diagram showing the prior art. As in 
the preceding figure, the signal to be amplified is fed to the input 1 of 
the linearizer which is connected to the input of a coupler (power 
divider) 22 one branch of which is terminated by a matched load 8. The 
signal is thus divided into two parts applied to respective inputs of two 
transmission lines. The coupler (power divider) 22 conventionally 
introduces a phase-shift .theta.between the two signals fed from its 
outputs to the two transmission lines. The coupler 22 can be a 3 dB hybrid 
coupler, for example, which introduces a phase-shift .theta.=.pi./2 
(=90.degree.). In a preferred embodiment of the invention the coupler 22 
can be a microstrip branch-line coupler, for example. 
The first transmission line (the non-linear channel) includes an amplifier 
14 generating non-linear distortion of similar amplitude to that produced 
by the power amplifier to be linearized, a variable attenuator 13 and a 
variable phase-shifter 29. 
The second transmission line (the linear channel) includes a variable 
phase-shifter 19, a variable attenuator 13 and a microwave amplifier 4. 
The signals on the two channels are then added by a coupler (power 
combiner) 15 one branch of which is terminated by a matched load 7. The 
predistortion correction signal is supplied from the output 6 of the 
coupler (power combiner) 15 to a microwave power amplifier (not shown). 
The coupler (power combiner) 15 conventionally introduces a phase-shift 
.theta.between the two signals supplied to its inputs from the two 
transmission lines, before they are added together. The coupler 15 can be 
a 3 dB hybrid coupler, for example, which introduces a phase-shift 
.theta.=.pi./2 (=90.degree.). In a preferred embodiment of the invention 
the coupler 15 can be a ring coupler, also capable of introducing the 
required phase-shift. 
In addition to the essential components of the invention referred to above, 
other components can be introduced into each channel, provided that the 
electrical lengths of the components introduced into each channel are 
identical and provided that any variations in their characteristics 
(transfer functions) as a function of temperature or operating conditions 
(input signal power, frequency) in particular are the same. 
In a preferred embodiment of the invention each channel further includes a 
fixed attenuator (not shown in FIG. 2). These fixed attenuators can have 
different attenuation values but their electrical lengths are the same. In 
this way, the electrical lengths of all the circuits of each channel being 
identical, the linearizer device of the invention operates correctly over 
a very wide band of frequencies, since the transfer function of each 
channel varies with frequency in the same manner. 
The design described above with reference to FIG. 2 therefore features an 
inherent phase-shift of 180.degree. between the two channels, this total 
phase-shift resulting entirely from the phase-shifts introduced by the 
coupler (power divider) 22 and the coupler (power combiner) 15. As the 
circuits on the two channels are identical and the settings of the 
identical circuits are approximately the same in both channels, any 
variations with temperature of the transfer functions of the two channels 
are added with opposite phase and cancel out. The device of the invention 
is therefore automatically temperature compensated. 
Because the electrical lengths of the two channels are identical, the 
transfer function of the device of the invention is independent of 
frequency. The device operates correctly over a very wide band of 
frequencies. 
The person skilled in the art knows that the relative phase-shift 
.DELTA..phi. of the signals on the linear channel and the non-linear 
channel to obtain a global transfer function of the linearizer 
corresponding to a TWT microwave power amplifier is always in the order of 
.DELTA..phi.=-170.degree., with an amplitude difference .DELTA.V on the 
two channels in the order of 3 dB. For a solid state microwave amplifier, 
on the other hand, the relative phase-shift of the signals on the two 
channels is in the order of .DELTA..phi.=+170.degree.. In both cases the 
inherent phase-shift of 180.degree. between the two channels "presets" the 
linearizer of the invention, which saves time in the fine adjustment of 
the variable phase-shifters of the linearizer and ensures that the two 
variable phase-shifters are set to similar values, and therefore to nearby 
operating points, the latter obtained by component bias values which are 
virtually the same. This maintains identical electrical lengths in both 
channels. 
In a preferred embodiment of the invention fixed attenuators are added to 
each channel. In the previously mentioned application of linearizing a 
solid state or TWT microwave power amplifier the fixed attenuator in the 
non-linear channel must be set to an attenuation value 3 dB greater than 
the attenuation value of the fixed attenuator in the linear channel. In 
this way, if small amplitude adjustments are required in one of the two 
channels, this can be done without significant modification to the 
operating point and therefore without significant modification to the 
electrical lengths of the two channels. 
In this embodiment the two channels are inherently set very close to the 
optimum point defined by .DELTA..phi.=.+-.170.degree., .DELTA.V=3 dB. 
Final adjustment of the phase difference or the amplitude difference 
between the signals in the linear and non-linear channels is then effected 
by means of attenuator or phase-shifter circuits in each of the two 
channels. The fact that the inherent amplitudes and phases of the signals 
propagating in the two channels are very close to the optimum setting 
means that the biasing conditions of the fine adjustments circuits 
(attenuators and phase-shifters) will be similar. Their transfer functions 
will remain virtually identical because any adjustment required will 
necessarily be a small adjustment from this inherent setting of 
.DELTA..phi.=180.degree., .DELTA.V=3 dB. 
Tests have been carried out to verify the effectiveness and the correct 
operation of the device of the invention. In these tests the linearizer 
described was associated with a TWT for which at saturation the required 
gain expansion was 4 dB and the required phase expansion was 40.degree.. 
It was found that the linearizer was inherently near the required setting 
and that fine adjustment could be completed very quickly. It was also 
found that the linearizer of the invention operated perfectly over a wide 
band of frequencies without any change of setting. A first breadboard 
implementation had a bandwidth in the C band of 20%. 
The variation in the gain expansion curve of the linearizer was found to be 
negligible and the variation in the phase expansion curve was found to be 
very small (less than 5.degree. at saturation) for a range of temperature 
from -10.degree. C. to +60.degree. C. 
The circuits of the two channels of the linearizer device of the invention 
can be implemented in techniques familiar to the person skilled in the 
art, in particular in the MMIC or MIC technology. The circuits can be 
implemented collectively or individually, and then interconnected by 
microstrip circuits, or by hybrid techniques. The device of the invention 
lends itself naturally to implementation in the MMIC technology in that 
the circuits of each channel must be identical and can be easily 
reproduced from a library of standard MMIC components. 
Finally, the linearizer device of the invention is intended to form part of 
a microwave power amplifier the linearity of which it improves over a wide 
band of frequencies and under varying operating conditions. The invention 
also provides a microwave amplifier including a linearizer as described 
hereinabove. 
To summarize the advantages of the invention, the proposed predistortion 
linearizer device comprises linear and non-linear channels made up of the 
same components for which the bias conditions and the operating points are 
identical or virtually identical. As a result the global transfer function 
of the device is not affected by variations in the temperature of the 
device, or at worst is only very slightly affected. The device of the 
invention can be regarded as automatically temperature compensated. 
Also, because the electrical lengths of the paths are identical or 
virtually identical, the device operates in a very wide band of 
frequencies. 
Further, the novel use of a branch-line coupler at the input and a ring 
coupler at the output associated with the fact that the two channels are 
constructed from the same devices means that the device of the invention 
has an inherent phase (and, in one specific embodiment, an inherent 
amplitude) corresponding to a TWT or solid state amplifier application. 
The device of the invention is therefore "preset". 
Of course, the invention is not limited to the examples discussed and 
described above, but can be applied to any implementation using one or 
more means equivalent to the means described by way of example to 
implement the same functions to obtain the same results.