Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2000-09-25
2004-06-29
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S297000, C455S114300
Reexamination Certificate
active
06757338
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a linearizer for predistorting a signal in order to compensate for non-linearities; and, in particular, to a predistortion linearizer capable of providing a predistorted signal through the use of even order intermodulation components.
DESCRIPTION OF THE PRIOR ART
Communications systems handling a single carrier usually employ high efficiency and low linearity amplifiers. Multi-carrier systems, however, require highly linear amplifiers at the expense of efficiency. An ideal amplifier would exhibit both perfect linearity and high efficiency.
Intermodulation distortion products are produced by non-linearities in amplifiers handling radio frequency(RF) signals such as the multiple carrier signals found in cellular telephone systems or in various other types of personal communications systems(PCS). Distortions can be caused by amplitude compressions or phase shifts that may occur in relatively large amplitude signals. The resulting spurious signals, when added to the spectrum of the information bearing signals, may degrade the quality of associated communications.
While the phenomena are fairly complex, the transfer function of an amplifier exhibiting non-linearities can be approximated by a Taylor series, that is, a polynominal with terms of the form a
n
x
n
. The first order effect (a
l
x) is the desired linear function. Distortion is caused by the second order term (square term), the third order term(cubic term), and so forth. For many high frequency systems, the even order terms produce harmonics that are out of the working bandwidth of a system and therefore can be readily discarded. Typically, only odd order (third, fifth, seventh, etc.) intermodulation products reside within the working spectrum of the system and thus are of a concern but with decreasing deleterious effects in that order.
Intermodulation distortion is highly undesirable because it causes interference, crosstalk, and other deleterious effects on the performance of a system employing a high power amplifier. One of the conventional approaches employed to counteract the distortion introduced by an amplifier involves predistortion of an input signal to the amplifier, wherein the input signal to the amplifier is predistorted in such a way that the intentionally added distortion effectively cancels out the distortion generated by the amplifier to thereby provide an undistorted output signal.
Referring to
FIG. 1
, there is illustrated a conventional predistortion circuit
50
for predistorting a RF signal, which includes two power dividers
10
and
12
, an error amplifier
14
, vector modulators
16
and
20
, a signal canceler
18
, a delay line
22
and a combiner
24
.
The power dividers
10
and
12
sequentially divide an input RE signal having, e.g., two carriers into three RF signals RF
1
, RF
2
and RF
3
. The error amplifier
14
amplifies the RF
1
to generate a distorted RF signal having therein amplified carriers and intermodulation distortion components. The vector modulator
16
adjusts the amplitude and polarity (i.e., phase) of the RF
2
in such a way that the adjusted RF signal has carriers whose amplitudes are substantially identical to those of the amplified carriers of the distorted RF signal, but with a 180 degree phase shift. At the signal canceler
18
, the adjusted and the distorted RF signals are combined to extract the intermodulation distortion components. The vector modulator
20
varies the amplitudes and phase of the predistortion components extracted by the canceler
18
. The delay line
22
serves to delay the RF signal RF
3
to compensate for the time delay required in producing the predistortion components. The combiner
24
combines the delayed RF
3
signal with the predistortion components to provide a main amplifier (not shown) with a predistorted RF signal having therein intentionally added distortion components to offset the distortion to be generated by the amplifier itself.
In the predistortion circuit described above, however, the predistortion components are not produced in an independently controlled manner. Therefore, The predistortion should be focused on the cancellation of the most deleterious intermodulation product, i.e., the third order component. Accordingly, if non-linear characteristics of an error amplifier for producing a predistorted signal and a main amplifier for amplifying the signal are different, which would be the case in general, higher order, e.g., fifth and seventh order, intermodulation products may not be effectively removed, deteriorating the overall performance of the system.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a linearization circuit capable of providing independently controlled predistorting components.
In accordance with the present invention, there is provided a predistortion linearizer for predistorting a radio frequency(RF) signal having one or, more fundamental frequencies, comprising:
a divider for dividing the RF signal into a first and a second RF signals;
a generator, responsive to the second RF signal, for generating one or more even order harmonics of the fundamental frequencies; and
a producer for producing one or more odd order harmonics from the fundamental frequencies and the even order harmonics to provide a predistorted RF signal including the fundamental frequencies and the odd order harmonics.
REFERENCES:
patent: 4992754 (1991-02-01), Blauvelt et al.
patent: 6313701 (2001-11-01), Mussino et al.
patent: 6538509 (2003-03-01), Ren
patent: 6693974 (2004-02-01), Jin et al.
Kevin Morris, Peter Kenington, “Power Amplifier Linearization using Predistortion Techniques” IEE, Savoy Place, London, 1997.*
Jaehyok Yi, Youngoo Yang, Myungkyu Park, Wonwoo Kang and Bumman Kim, “Analog Predistortion Linearizer For High Power RF Amplifier” IEEE MTT-S Int. Microwave Sympo. Dig., Jun. 5, 2000, pp. 1511-1514.
Kim Bum-man
Woo Young Yun
Yang Youn-goo
Corrielus Jean B.
Ghulamali Qutbuddin
Katten Muchin Zavis & Rosenman
Postech Foundation
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