Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
1999-11-12
2001-11-06
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C359S199200
Reexamination Certificate
active
06313701
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
The present invention relates to a method and to a signal predistorter circuit for eliminating the nonlinearities in the input/output characteristics of nonlinear stages, in particular for eliminating the third-order nonlinearities. More particularly, it relates to a method and to a predistorter circuit for external optical modulators of television signals.
2. Description of the Related Art
The distributing of television signals via CATV (CAble TeleVision) apparatus can be performed with optical fibres. Modulation of the optical signals can be achieved directly, by acting upon the optical source, usually a laser, or indirectly, via an optical modulator.
An optical modulator which makes it possible to perform the amplitude modulation of an optical signal, with a radiofrequency (RF) modulating signal (also known as the external signal) having very high frequencies (as for example other carriers of television channels which conventionally range from 40 to 860 MHz), consists for example of a device based on an interferometer of the Mach-Zehnder type constructed in lithium niobate (LiNbO
3
).
A characteristic which is required of an external modulator is linearity of modulation, very important for use in CATV apparatus with analogue type transmission, in particular when the modulating signal is not a single television channel, but consists of many television channels, for example from 40 up to 80 channels.
The electro-optical characteristic (optical output power versus radiofrequency input voltage) of modulators of this type (Mach-Zehnder interferometer) is typically nonlinear. To limit the distortion of the signals it is expedient to operate the modulator near as linear a stretch as possible of the characteristic.
For this purpose, to an electro-optical modulator of the Mach-Zehnder interferometer type there is applied a radiofrequency (RF) modulating signal to an electrode RF and, to the same electrode or to a second electrode, a d.c. supply voltage which determines the working point of the modulator.
A modulator of this type is for example marketed by the Applicant under the reference PIR PIM1510.
The modulating signal, applied to the RF input, consists, for example, of the set of modulated carriers of the television channels to be distributed to the user.
In the case of a Mach-Zehnder modulator the profile of the characteristic can be approximated by a sinusoid, and it is advantageous that the modulator operate near the point of inflection of the sinusoid, to correspond with an applied working point VQ voltage.
The modulation characteristic of the Mach-Zehnder modulator, referred to the working point, can be expressed through the relation:
P
u
=K
z
sin&bgr;
where:
P
u
is the optical output power
K
z
is a coefficient which depends on the characteristic of the Mach-Zehnder modulator.
&bgr;=&pgr;V/V
x
is the modulation index of the modulating signals, expressed in radiands
V is the variation in the applied voltage, with respect to the working voltage VQ
V
x
is a constant.
This characteristic with sinusoidal profile is distinguished by two values:
the value of the voltage, the said V&pgr;, which represents the voltage variation to be applied to the RF (radiofrequency) electrode so as to carry the optical power from the maximum value to the minimum value;
the value of the voltage VQ which should be applied to the supply electrode, so as to cause the working point to correspond to that of inflection of the characteristic with sinusoidal profile, i.e. with odd symmetry. In such a case the distortions of even order (comprising the second harmonic of the applied signals) vanish and the distortions of odd order take on a well defined value.
For example, in the case of a Mach-Zehnder modulator of the PIR PIM1510 type, produced by the Applicant, the aforesaid voltages can take the following values:
V&pgr;=4.3V and VQ=0.7V.
The value of the voltage VQ of the working point is not constant, but varies over time (for example through the build-up of static charge in the LiNbO
3
) and also with temperature.
Hence the value of the working voltage has to be continually adapted, using as information, for example, the presence and magnitude of the distortions of even order, or of the second-order intermodulation products, referred to overall as CSO (Composite Second Order).
In the case also of the operation of the modulator at the aforesaid working point, which minimizes the distortions of even order, the nonlinearity in the input/output characteristic causes residual distortions of odd order (mainly of third order) which get translated into intermodulation products or CTBs (Composite Triple Beats) and cross modulation (XMOD), which impair the quality of the signals reaching the user. Thus, the quality of television channels, with amplitude modulation of the carrier, is greatly influenced by the presence of the aforesaid intermodulation products, the overall level of which has to be maintained sufficiently low, for example over 65 dB below the level of the picture carrier of each channel, as to obtain good quality of the signals distributed to the users.
To limit these distortions to some extent it is expedient to chose a not too high depth of modulation for the modulating signals (where depth of modulation is to be understood as the maximum value, expressed as a percentage, of the modulation index &bgr;), for example around 3.5% or 4% per channel so as to operate as close as possible to the linear portion of the characteristic.
For the purpose of limiting the distortion introduced by the nonlinearity in the characteristic of the modulator, the maximum percentage usage of the modulator is usually about 40%. Thus, the maximum number of channels which can be applied, bearing in mind the aforesaid requirements relating to noise characteristics, is derived by considering that the various carriers, being uncorrelated, sum together statistically in phase quadrature; therefore, the sum of the channels is to be considered in terms of power. Hence, the maximum number of channels which can be applied to the modulator so as not to exceed the aforesaid percentage is around 100.
To reduce the residual distortions, and in particular the third-order distortions, the technique has been suggested of predistorting the modulating signals through the use of nonlinear elements, in such a way that the predistortion is compensated by the subsequent distortion on the part of the modulator. For this purpose a distorter circuit, with an input/output characteristic in respect of the radio frequency signals which is the inverse function of the input/output characteristic of the modulator, can be placed ahead of the modulator.
SUMMARY OF THE INVENTION
For the purposes of the present invention, a distorter circuit should be understood as an electrical circuit, in particular one comprising nonlinear circuits, which to correspond with an electrical signal delivered to its input, generates as output a signal which is a preset, nonlinear, function of the input signal.
This technique and circuit examples used for this purpose are for example explained in the article by M. Nazarathy et al. “Progress in Externally Modulated AM CATV Transmission Systems”, published in the Journal of Lightwave Technology, vol. 11, no. 1, 01/93, pages 82-104. In particular, this shows a circuit which uses diodes biased by a current source as nonlinear elements.
In Patent Application EP 0 620 661 and in the patent U.S. Pat. No. 5,172,068 distorter circuits are described which comprise diodes connected in parallel in such a way as to have opposite polarity with respect to the input signal. The diodes operate as nonlinear elements with a predetermined bias value. They are biased by voltage generators via a resistor or by current generators.
According to the present invention, the Applicant has found that the circuits of the above-mentioned references exhibit drawbacks.
In particular it has been noted that the amount of the distortions produced by them are in some cases in
Gobetti Gianluca
Mussino Franco
Notargiacomo Massimo
Ravasio Giuseppe
Zammarchi Claudio
Choe Henry
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pascal Robert
Pirelli Cavi E Sistemi S.p.A.
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