Predistortion circuit for an analog signal in a video communicat

Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control

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Details

327310, 327329, 327551, 330149, H03K 508

Patent

active

061408581

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention relates to a predistortion circuit comprising coupler/shunting device, comprising a second-harmonic generator, branch and adds this signal to the signal on the output of the primary branch.
The invention is applied to video communication networks which utilize analog television signals, more particularly, but not of necessity in transmitters transmitting a signal via an optical fiber. Certain components of such transmitters, for example, laser diodes, have an insufficient linearity for transmitting analog signals in the multiplex mode; therefore, a "predistortion" is introduced in the signal, that is to say, a signal having an amplitude which is as much as possible equal to that of the distortion caused by the component, but of opposite direction, by means of a distortion generator which comprises, for example, a second-harmonic generator mentioned above.


BACKGROUND OF THE INVENTION

A predistortion circuit is known from the document "Linearization of broadband optical transmission systems by adaptive predistortion", by M. Berstelmeier and W. Zschunke in FREQUENZ, Vol. 38 (1984), no. 9, pages 206-212. The predistortion technique used according to this document permits of obtaining interesting performance, but in a relatively small frequency range.


SUMMARY OF THE INVENTION

It is an object of the invention to permit the use of filters for increasing the range of frequencies used, without enhancing the manufacturing complexity too much.
For this purpose, the second-harmonic generator is formed by connections, first connections, the diodes of the first pair being interconnected by their cathodes and those of the other pair being interconnected by their anodes, cathodes and to the node of the two anodes, which form a symmetrical output.
The invention is thus based on the idea of using a symmetrical arrangement from the input of the harmonic generator onwards, for improving the performance thereof while providing a symmetrical output which makes the introduction of a symmetrical filter possible without additional cost. In addition, the spectral purity of the second-harmonic signal is better. A further advantage results from the following fact: the direction of the required correction is unknown beforehand, and thus the polarity necessary for the correction signal is unknown. An inverter circuit is thus habitually provided in the chain of the correction elements. Because of the symmetrical structure of the harmonic generators, such an inverter is no longer necessary because it is sufficient to cross the connections, if required.
Advantageously, the circuit comprises in a cascade combination with the second-harmonic generator, a symmetrical type of filter whose symmetrical input is connected to the second connections.
Thus, the frequency range in which the correction is effective is thus increased in a simple manner while the symmetrical drive of the filter on the output of the second-harmonic generator is possible without adding an additional symmetrizing circuit.
Preferably, the circuit comprises coupler/shunting device, comprising a third-harmonic generator which includes signals, conductive in the same direction, the first assembly being connected by its two ends to one of the third connections, the second assembly being connected by its two ends to the other third connection, respectively to the node of the two diodes of the first assembly and to the node of the two diodes of the second assembly, secondary branch and adds the output signal to the output signal of the primary branch.
The same principle as for the second-harmonic generator is applied with the same advantages to a third-harmonic generator. The spectral purity of the third-harmonic signal is better, the signal containing fewer even harmonics.
The circuit preferably comprises, in a cascade arrangement with the third-harmonic generator, a symmetrical type of filter whose symmetrical input is connected to the fourth connections.
Thus, the frequency range in which the correction is effective is furth

REFERENCES:
patent: 5321710 (1994-06-01), Cornish et al.
patent: 5589797 (1996-12-01), Gans et al.
patent: 5600472 (1997-02-01), Uesaka
patent: 5854811 (1998-12-01), Sasaki
patent: 5969575 (1999-10-01), Helms
Bertelsmeier et al, "Linearization of Broadband Optical Transmission Systems by Adaptive Predistortion", pp. 206-212, Sep. 1984.

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