Predistortion circuit for linearization of signals

Details Predistortion circuit for linearization of signals Predistortion circuit for linearization of signals

2000-01-19

2001-02-27

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Amplitude control

C327S355000, C327S362000, C330S149000, C375S285000, C375S296000, C359S199200

Reexamination Certificate

active

06194942

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a predistortion circuit for providing a linear output from a transmission device which has an output distorted from its input due to inherent nonlinearly, more particularly to a predistortion circuit that has a simple construction.
2. Description of the Related Art
It is well known to modulate the analog intensity of an optical source, such as a semiconductor laser, using an electric signal, in order to transmit analog signals, such as sound or video signals, on optical fibers. Distortion inherent in analog transmitters prevents an electrical modulation signal from being converted linearly to an optical signal, and instead distorts the signal. This is especially problematic with a multi-channel video transmission system, such as cable television. Therefore, a predistortion circuit is used to reduce the distortion inherent in nonlinear devices.
Referring to
FIG. 1
, optical power intensity of a nonlinear semiconductor laser in relation to current flowing therethrough is determined according to the following Equation 1:
P
out
=a
1
I+a
2
I
2
+a
3
I
3
+  (Equation 1)
where “P
out
” is the optical power intensity of the semiconductor laser, “I” is the current flowing through the semiconductor laser, and “a
1
,a
2
,a
3
, . . . ” are coefficients of nonlinear Taylor expansion.
Referring to
FIG. 2
, an input signal is split into a main path and a secondary path (P
1
, P
2
). “I
1
”, which is the current flowing through the main path (P
1
), and “I
2
”, which is the current flowing through the main path (P
2
), in relation to the signal voltage (V) can be determined according to the following Equations 2 and 3:
I
1
=b
1
V+b
2
V
2
+b
3
V
3
+  (Equation 2)
I
2
=c
1
V+c
2
V
2
+c
3
V
3
+  (Equation 3)
If the relationship of “P
out
” to the signal voltage (V) is in accordance with the following Equation 4:
P
out
=k
1
V+k
2
V
2
+k
3
V
3
+  (Equation 4)
By introducing I=I
1
=I
2
into Equation 1 and by comparing Equation 4, we can obtain the following:
k
1
=a
1
(
b
1
+c
1
)  (Equation 5.1)
k
2
=a
2
(
b
2
+c
2
)+
a
2
(
b
1
2
+c
1
2
+2
b
1
c
1
)  (Equation 5.2)
k
3
=a
3
(
b
3
+c
3
)+
a
2
(2
b
1
b
2
+2
c
1
c
2
+b
1
c
2
+b
2
c
1
)+
a
3
(
b
1
3
+c
1
3
+3
b
1
2
c
1
+3
b
1
c
2
1
)  (Equation 5.3)
If the coefficients (C
I
, I=1, 2, 3, . . . )of the nonlinear current (I
2
) flowing through the secondary path (P
2
) can be adjusted such that
k
2
=k
3
=. . . =k
n
=. . . =0  (Equation 6)
P
out
=k
1
V in Equation 4, which is the ideal aim.
Conventional predistortion circuits are disclosed in U.S. Pat. Nos. 4,992,754, 5,132,639, 5,252,930, and 5,424,680.
The following are some of the drawbacks of the predistortion circuits disclosed in the aforesaid U.S. Patents:
1. In U.S. Pat. Nos. 4,992,754 and 5,132,639, two predistortion generators are needed to provide second-order and third-order predistortion compensation, thereby resulting in greater complexity and higher costs.
2. In U.S. Pat. No. 5,252,930, only second-order predistortion compensation can be provided such that the distortion compensating effect is inferior.
SUMMARY OF THE INVENTION
Therefore, the object of the present invention is to provide a predistortion circuit of the aforesaid type which has a relatively simple construction.
According to the present invention, a predistortion circuit includes a first splitter, a first combiner, a distortion signal generator, a second splitter, a third splitter, an even-order signal processing unit, an odd-order signal processing unit, and a second combiner.
The first splitter splits an input signal into primary and secondary electrical paths.
The first combiner is connected to the first splitter via the primary electrical path for combining a main signal on the primary electrical path and a predistorted signal on the secondary electrical path into a single signal for modulating a nonlinear device with predictable distortion characteristics.
The distortion signal generator is disposed in the secondary electrical path and is connected to the first splitter for receiving a branch signal from the first splitter and for producing first and second intermodulation products from the branch signal.
The second splitter is disposed in the secondary electrical path and is connected to the distortion signal generator for receiving the first intermodulation products produced by the distortion signal generator and for outputting two sets of the first intermodulation products.
The third splitter is disposed in the secondary electrical path and is connected to the distortion signal generator for receiving the second intermodulation products produced by the distortion signal generator and for outputting two sets of the second intermodulation products.
The even-order signal processing unit is disposed in the secondary electrical path for receiving and processing one of the sets of the first intermodulation products and one of the sets of the second intermodulation products to generate even-order intermodulation products.
The odd-order signal processing unit is disposed in the secondary electrical path for receiving and processing the other one of the sets of the first intermodulation products and the other one of the sets of the second intermodulation products to generate odd-order intermodulation products.
The second combiner is disposed in the secondary electrical path and is connected to the even-order signal processing unit, the odd-order signal processing unit and the first combiner for receiving and combining the even-order intermodulation products generated by the even-order signal processing unit and the odd-order intermodulation products generated by the odd-order signal processing unit to obtain the predistortion signal that is provided to the first combiner.


REFERENCES:
patent: 5252930 (1993-10-01), Blauvelt
patent: 5304945 (1994-04-01), Myer
patent: 5363056 (1994-11-01), Blauvelt
patent: 5424680 (1995-06-01), Zazarathy et al.
patent: 5939920 (1999-04-01), Hiraizumi
patent: 6055278 (2000-04-01), Ho et al.
patent: 6081156 (2000-06-01), Choi et al.
patent: 6100757 (2000-08-01), Kotzamanis
patent: 6104241 (2000-08-01), Cova et al.

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