RF fiber optic transmission utilizing dither

• US 5,598,288 A
• Filed: 07/31/1995
• Issued: 01/28/1997
• Est. Priority Date: 07/31/1995
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Northrop Grumman Corporation
• Inventors: Collar, Stuart J.
• Primary Examiner(s): Chin, Wellington
• Assistant Examiner(s): Negash, Kinfe-Michael
• Application Number:
  US08/509,634
• Time in Patent Office: 547 Days
• Field of Search: 359/111, 359/154, 359/161, 359/173, 359/180, 359/181, 359/188-189, 375/243, 341/131
• US Class Current: 398/193
• CPC Class Codes: H04B 10/2507 for the reduction or elimination of distortion or dispersion

Abstract

A low cost RF link is implemented by summing a wideband amplitude noise dither signal with a band limited RF signal. The summed signal is then amplitude limited. The output of the limiter can be applied directly to the digital fiber optic transmitter. At the remote end of the link, the original signal can be recovered by removing the noise dither signal. This can easily be accomplished with a narrow band filter centered on the original signal. This system uses the same optical fiber transmitters and receivers as the traditional system, but eliminates the need for the ADCs, DACs and parallel/serial format conversion hardware. Adding amplitude dither to the signal mitigates the intermodulation distortion that would normally be associated with the non-linear operation of amplitude limiting. In this way, multiple narrow band signals can be transmitted down a single fiber using digital fiber optic transmitters and receivers without distortion due to intermodulation. The technique of the invention has application in commercial systems requiring low cost transmission of analog narrow band voice or data signals over a digital fiber optic link. The inherent EMI/RFI immunity of optical fiber would make this especially attractive for industrial applications.

Claims

1. A fiber optic RF link comprising:

means for introducing an RF signal, including multiple narrow band signal components, to a summing junction;

a noise source for generating a dither signal;

means connected to the output of the noise source for amplifying the dither signal prior to its introduction to the summing junction;

means located at the summing junction for adding the RF signal and the dither signal to create a dithered RF signal to be transmitted;

amplitude limiting means connected at its input to the output of the summing junction for linearly quantizing the dithered RF signal to two levels;

fiber optic transmitter means connected at its input to an output of the limiting means for converting the dithered RF signal from an electrical to optical form;

a fiber optic cable connected at its input to an output of the fiber optic transmitter for propagating the dithered RF signal;

fiber optic receiver means, connected at its input to the output of the cable, for reconverting the dithered RF signal from optical to electrical form; and

a narrow bandpass filter means connected at its input to an output of the fiber optic receiver and having a narrow bandwidth centered at the frequency of the RF signal for removing the dither signal and recovering the original RF signal.