Broadband predistortion linearizer

Details Broadband predistortion linearizer Broadband predistortion linearizer

2002-12-04

2004-09-07

Nguyen, Khanh Van (Department: 2817)

Amplifiers

Hum or noise or distortion bucking introduced into signal...

C330S286000

Reexamination Certificate

active

06788139

ABSTRACT:

The invention relates to a predistorsion linearizer for an amplifier having a broad frequency band, in particular in the microwave range.
BACKGROUND OF THE INVENTION
When the power requirements of electronic signals are large, use is made of an amplifier which is caused to operate close to its saturation power. By way of example, mention can be made of the transmission requirements of telecommunications satellites for which traveling wave tube amplifiers are used.
Operating the amplifier close to saturation gives it a response that is not linear. More precisely, when the input power is well below saturation power, then output power is substantially proportional to input power. However, as input power comes closer to saturation power, gain decreases, and takes on the value 1 at saturation. This non-linearity also affects the phase of the output signal: phase remains constant when the amplifier is operating away from its saturation zone, but for certain amplifiers (and in particular traveling wave tube amplifiers) phase decreases as power approaches the saturation value.
To remedy that drawback, it is common practice to use predistorsion linearizer devices, particularly with tube amplifiers for which both gain and phase decrease as input power approaches the saturation value. Such a device is placed upstream from the amplifier or tube to be linearized. It provides an output signal whose amplitude and phase vary as a function of input power in non-linear manner so that both the amplitude and the phase of the signal output by the amplifier vary in linear manner as a function of the signal input to the predistorsion device.
More precisely, in the linearizer device, predistorsion is applied both to gain and to phase so as to keep gain and phase constant for input power values well below saturation power and also provide gain and phase which increase with increasing input power approaching saturation power, thus compensating the decrease in gain and phase due to the amplifier.
This principle is shown in
FIG. 1
where there can be seen, for example, a tube power amplifier
10
for use on board a spacecraft, such as a satellite, in order to power one or two transmit antennas (not shown) in a telecommunications system. The signal to be amplified is applied to the input
12
of the amplifier
10
via a linearizer device
14
of the predistorsion type.
The graph of
FIG. 1
a
plots output power P
s
up the ordinate axis as a function of input power P
e
along the abscissa axis. In this graph, curve
16
corresponds to the linearizer device
14
. It can be seen that at low input powers the curve
16
presents a linear portion
16
1
and that for higher levels of input power it presents a non-linear portion
16
2
of slope that is steeper than that of the linear portion
16
1
.
The graph of
FIG. 1
b
shows the way in which the amplifier
10
operates, in this case a traveling wave tube. This graph is analogous to that of
FIG. 1
a
. Curve
18
has a first portion
18
1
that is linear and a second portion
18
2
that is non-linear for higher values of output power P
S
when this power comes close to the saturation power P
M
. In its portion
18
2
, the slope is smaller than the slope of the linear portion
18
1
and it comes close to zero.
In the graph of
FIG. 1
c
, there can be seen the relationship between the signal P
e
at the input
20
of the device
14
and the signal P′
s
at the output
22
of the amplifier
10
. It can be seen that the curve showing variation in P′
s
as a function of P
e
presents variation
24
that is linear all the way to saturation P
M
.
The graphs of
FIGS. 1
a
to
1
c
also show, in dashed lines, the variations in the phase &phgr; of the output signals as a function of the powers P
e
or P′
e
of the input signals. In
FIG. 1
a
it can be seen that the phase &phgr;
1
of the signal on the output of the device
14
remains constant (flat portion
28
) for lower values of input power P
e
and increases in non-linear manner (portion
30
) for input powers that approach saturation power.
In
FIG. 1
b
, it can be seen that the phase &phgr;
2
of the signal on the output
22
of the amplifier
10
is constant for smaller values of the power P′
e
on the input
12
and decreases in non-linear manner (portion
34
) when this input power comes close to saturation power.
In
FIG. 1
c
, it can be seen that the phase of the signal on the output
22
remains constant (straight line
36
).
Various types of predistorsion linearizers have already been studied. A first family of predistorsion linearizers is described, for example, in document U.S. Pat. No. 4,992,754 in the name of Blauvelt et al., assigned to Ortel Corp. (USA). According to the teaching of that document, the payload signal is applied to a delay line while the correction signal is generated in a parallel branch, with amplitude adjustment of the signal so as to make it equal to the non-linearity of the amplified signal, and phase adjustment of the signal so as to obtain phase opposition relative to the payload signal propagating along the delay line. The payload signal and the correction signal are added by means of a microwave combiner or coupler, and subsequently applied to the inlet of the power microwave amplifier. The non-linearities observed at the outlet from the power amplifier stage are thus substantially diminished if not eliminated.
Mention can also be made of another known type of predistorsion linearizer, e.g. as disclosed in document U.S. Pat. No. 4,068,186 to Sato et al., in the name of KDD (Japan). That linearizer is designed to work at very high frequencies to mitigate the non-linearities of a traveling wave tube (TWT) type amplifier or of a klystron. As the non-linearity generator, Sato et al. teach the use of a low power TWT. That type of amplifier introduces a small amount of delay (due to the finite propagation speed of electrons in a vacuum) in the amplified signal relative to the non-amplified signal. In order to synchronize the signals from the non-linear path and the linear path, it is therefore necessary to introduce a delay line in the correcting branch. The device of Sato et al. is relatively complicated because there are two corrector members upstream and downstream from a power splitter and a power combiner that are connected together by the two paths referred to as the linear path and the non-linear path. The linear path also comprises a microwave amplifier whose operating point is selected to be close to saturation in order to generate non-linearities which depend on the power of the input signal. In order to be able to adjust the output level from the predistorsion linearizer device without acting on the gain of the amplifier, a variable attenuator is situated at the outlet of the amplifier. The non-linear path has another member for correcting the phase/frequency characteristics. The signals from the two paths are applied to the two inputs of a second combiner coupler where they are added together (also with a phase shift &thgr; between the two signals). This combiner coupler can be a 3 dB hybrid coupler, for example, thus giving rise to a further phase shift of &thgr;=&pgr;/2.
That prior art device therefore comprises two members for correcting the non-linear amplitude/frequency characteristic together with a member for correcting the phase/frequency characteristic. Embodiments of such corrector members are described in the Sato et al. document, and the text of that description is expressly incorporated by reference in the present application for its description of prior art embodiments.
The non-linear characteristics of those members add to the non-linear characteristics of the microwave amplifier. The transfer functions of all those members vary in disparate manner as a function of the frequency and the amplitude of the signals, or indeed as a function of the temperature of the components. Precautions are taken so that the non-linear signal as generated in that way is added to the payload signal in phase opposition so as to cancel the no

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