Wave transmission lines and networks – Automatically controlled systems – With control of equalizer and/or delay network
Patent
1997-02-11
1998-08-04
Gensler, Paul
Wave transmission lines and networks
Automatically controlled systems
With control of equalizer and/or delay network
333 28R, 333167, H03H 703
Patent
active
057899932
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The field of the invention is that of transmission systems and more precisely concerns a device for equalizing the amplitude/frequency characteristic of a wide-band signal having an attenuation as a function of frequency characteristic that increases between its maximal and minimal frequencies. One preferred application of the invention is to correcting a signal that has passed through a transmission cable and the following description is given against this background.
Transmitting a microwave or intermediate frequency signal through a cable raises the problem known in itself of distortion of the signal at the receiving end. The signal transmitted, which is symmetrical about the center frequency (intermediate frequency or microwave frequency) at the transmitting end, has a lack of symmetry at the receiving end characterized by amplitudes of the received signal that are different below and above the center frequency. The distorted spectrum must then be returned to its original, symmetrical shape at the receiving end, using a frequency equalizer. The function of a frequency equalizer is therefore to modify the spectrum of a received signal to compensate for the attenuation characteristic of the cable, which is an increasing characteristic as a function of frequency.
FIG. 1 shows the spectrum of an intermediate frequency signal affected by "skew", i.e. a lack of symmetry in the amplitude/frequency characteristic. The center frequency f0 corresponds to the intermediate frequency and the minimal and maximal frequencies are f1 and f2, respectively. The bandwidth .DELTA.f, equal to f2-f1, is due to the modulation. The amplitudes of the frequencies less than and greater than f0 are not equal, which is characteristic of the skew.
FIG. 2 shows the attenuation as a function of frequency characteristic of a cable, for example that of a coaxial cable. The characteristic 20 has an attenuation of x.multidot..sqroot.f/f0 due to the properties of the transmission cable. The value of x is expressed in dB/m and corresponds to the amplitude attenuation of a signal at frequency f0. The frequencies f1 and f2 are not attenuated in the same manner, which causes the amplitude defect.
As previously indicated, the amplitude/frequency characteristic of a signal can be corrected using a frequency equalizer, for example an adaptive equalizer. All adaptive frequency equalizers include a correction device, for example a transversal filter, receiving an input signal and supplying a corrected output signal to a device for estimating the amplitude/frequency characteristic of the output signal. The estimator device controls the transversal filter to correct this characteristic.
FIG. 3 shows the structure of a frequency equalizer of this kind.
An intermediate frequency signal FI is applied to the input of a corrector device 1 the output of which is an intermediate frequency signal having a corrected amplitude/frequency characteristic. This signal is applied to two narrow-band filters 2, 3 the respective center frequencies f1 and f2 of which are in the spectrum of the signal FI, as shown in FIG. 1.
The amplitudes of the signals from the filters 2 and 3 allow for the deformation of the spectrum and it is therefore possible to control the correction device 1 so that the amplitude of the signals that it delivers are substantially identical on either side of the frequency f0. The device 1 is controlled by processor means 4 that generate correction coefficients applied to the filter 1. The filter 1, the narrow-band filters 2 and 3 and the processor means 4 form an intermediate frequency equalizer. This corrects for the amplitude/frequency response of the propagation medium, i.e. the amplitude/frequency characteristic of the processed signal, and the corrected spectrum is flat and symmetrical between f1 and f2, i.e. free of skew.
The corrected signal can then be applied to an amplifier having a gain which is determined by an automatic gain control device, this amplifier supplying a signal of constant amplitude. The ampli
REFERENCES:
patent: 2606970 (1952-08-01), Scott
patent: 4413240 (1983-11-01), Dyke
Comte Michel
Roche Nathalie
ALCATEL N.V.
Gensler Paul
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