Predistorter for linearization of electronic and optical signals

• US 5,252,930 A
• Filed: 07/21/1992
• Issued: 10/12/1993
• Est. Priority Date: 09/07/1989
• Status: Expired due to Term

• Current Assignee: Agere Systems Incorporated (Broadcom, Inc.)
• Original Assignee: Ortel Corporation (Emcore Corporation)
• Inventors: Blauvelt, Henry A.
• Primary Examiner(s): Mullins, James B.
• Application Number:
  US07/918,623
• Time in Patent Office: 448 Days
• Field of Search: 330/149, 328/163, 332/159-161, 455/50.1, 455/63
• US Class Current: 330/149
• CPC Class Codes: H03F 1/3252 using multiple parallel paths between input and output H03F1/3258, H03F1/3282, H03F1/3294 take precedence
• CPC Class Codes: H03F 1/3276 using the nonlinearity inherent to components, e.g. a diode
• CPC Class Codes: H04B 1/62 for providing a predistortion of the signal in the transmitter and corresponding correction in the receiver, e.g. for improving the signal/noise ratio
• CPC Class Codes: H04B 10/503 Laser transmitters
• CPC Class Codes: H04B 10/504 using direct modulation
• CPC Class Codes: H04B 10/58 Compensation for non-linear transmitter output

Abstract

An electronic circuit provides a linear output from an amplitude modulated transmission device such as an amplifier or a semiconductor laser which has inherent distortion. The distortion of the nonlinear device is compensated by applying a predistorted signal with distortion equal in magnitude and opposite in sign to the distortion introduced by the nonlinear device. The input signal is split into two paths with the primary part of the signal applied directly to the device, with a time delay to compensate for delays in the secondary path. The secondary path generates predistortion which is recombined with the primary signal in proper phase and amplitude for cancelling distortion in the output device. A distortion generator in the secondary path generates adjustable amplitude intermodulation signals. Filtering is used before the distortion generator to compensate for the dependence of the distortion of the nonlinear device on the frequencies of the fundamental signals. Filtering is used after the distortion generator to compensate for the dependence of the distortion of the nonlinear device on the frequency of the distortion. Phase of the distortion signal is adjusted to be in proper phase relation with the distortion of the device. Set points of the predistorter may be adjusted automatically. More than one secondary path may be used.

Claims

1. A predistortion circuit comprising:

means for splitting an input modulation signal into a primary electrical path and at least one secondary electrical path;

a distortion generator in the secondary electrical path for producing even order intermodulation products of the input frequencies and suppressing the fundamental frequencies;

means for combining signals on the primary and secondary signal paths into a single signal for modulating a nonlinear device with predictable distortion characteristics;

means in the secondary electrical path for adjusting the relative amplitude and phase of the signal from the distortion generator; and

filter means before the distortion generator for altering the input signal to the distortion generator as a function of frequency such that the dependence of the intermodulation distortion generated by the predistortion circuit on the frequencies of the input signal matches the frequency dependent distortion of the nonlinear device.

2. A predistortion circuit as recited in claim 1 comprising means in the secondary electrical path for adjusting the relative amplitude and phase of the distortion signal from the distortion generator as a function of frequency to match the distortion of the nonlinear device.

3. A predistortion circuit as recited in claim 1 wherein the distortion generator comprises a push-push amplifier circuit.

4. A predistortion circuit as recited in claim 1 further comprising:

means for detecting distortion remaining in the output of the nonlinear device; and

means for automatically changing the setting of the means for adjusting amplitude in response to the means for detecting, for minimizing distortion remaining in the output of the nonlinear device.

5. A predistortion circuit as claimed in claim 4 further comprising means for automatically changing the setting of the means for adjusting the phase of the inter-modulation products relative to the fundamental signals in response to the means for detecting, for minimizing distortion remaining in the output of the nonlinear device.

6. A predistortion circuit comprising:

means for splitting an input modulation signal into a primary electrical path and at least one secondary electrical path;

a distortion generator in the secondary electrical path for producing odd order intermodulation products of the input frequencies and suppressing even order intermodulation products;

means in the secondary electrical path for adjusting the amplitude and phase of the signal from the distortion generator;

post-filter means in the secondary electrical path after the distortion generator for altering the output signal from the distortion generator as a function of frequency;

pre-filter means before the distortion generator for altering the input signal to the distortion generator as a function of frequency; and

means for combining signals on the primary and secondary signal paths into a single signal for modulating a nonlinear device with predictable distortion characteristics.

7. A predistortion circuit as recited in claim 6 wherein at least one of the filter means is adjustable.

8. A predistortion circuit as recited in claim 6 wherein the distortion generator comprises a push-pull amplifier circuit.

9. A predistortion circuit as recited in claim 6 further comprising:

means for detecting distortion remaining in the output of the nonlinear device; and

means for automatically changing the setting of the means for adjusting amplitude in response to the means for detecting, for minimizing distortion remaining in the output of the nonlinear device.

10. A predistortion circuit as claimed in claim 9 further comprising means for automatically changing the setting of the means for adjusting the phase of the inter-modulation products relative to the fundamental signals in response to the means for detecting, for minimizing distortion remaining in the output of the nonlinear device.