US 5,850,305 AGranted

Adaptive predistortion control for optical external modulation

AssigneeScientific-Atlanta, Inc.

InventorRezin E. Pidgeon

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Abstract

A communication system, such as a cable television system, includes an optical transmitter (FIG. 2) that includes adaptive predistortion control circuitry (104). The adaptive predistortion control circuitry (104) measures the distortion produced by television band signals processed in the communication system. In the optical transmitter (FIG. 2), the distortion signal produced by the television signals is a relatively high-level signal from which necessary predistortion control signals may be more easily processed. Additionally, the adaptive predistortion control circuitry (104) detects both the amplitude and the polarity of distortion within the optical output (FIG. 2) of the optical transmitter, which enables the control circuit (104) to determine the correct polarity and level adjustments that should be made to the predistorter (102) included in the optical transmitter.

Claims4

Dependent claims

An external modulation fiber optic CATV transmitter, comprising:

an optical source for providing an optical signal;
a predistorter for generating a distortion cancellation signal in response to a radio-frequency input;
an electro-optic modulator for receiving said optical signal and said distortion cancellation signal and modulating said optical signal in response to said distortion cancellation signal;
and
an adaptive predistortion control circuit for providing a predistortion cancellation control signal and a modulation bias control signal, said circuit comprising
a receiver for receiving said modulated optical signal output by said modulator;
a filter for filtering intermodulation distortion from said modulated optical signal to obtain a first distortion component;
a distortion generator for receiving a radio-frequency input signal from said predistorter and generating a second distortion component in response to said radio-frequency input signal;
a mixer for synchronously demodulating a product of said first and second distortion components to obtain a correction signal;
a lowpass filter;
and
an integrator, said lowpass filter and said integrator generating said predistortion cancellation control signal and said modulation bias control signal in response to residual intermodulation distortion in the output of said electro-optic modulator;
wherein said predistorter modifies said distortion cancellation signal in response to said predistortion cancellation control signal, and wherein a bias of said electro-optic modulator is adjusted in response to said modulation bias control signal, thereby reducing distortion of said modified optical signal.

10 more elements

Dependencies are inferred from the claim text.

Facts

Application: 08/769,692Filed 18 Dec 1996
Granted: 15 Dec 1998
Term ends: Not estimatedThe source records don’t support an estimate.

Classification8 codes

H04B 10/5051
Using external modulation: using a series, i.e. cascade, combination of modulators
H04B 10/50572
Using a feedback signal generated by analysing the optical output: to control the modulating signal amplitude including amplitude distortion
H04B 10/50575
Using a feedback signal generated by analysing the optical output: to control the modulator DC bias
H04B 10/50593
Using a feed-forward signal generated by analysing the optical or electrical input: to control the modulating signal amplitude including amplitude…