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POWER LINE CARRIER/LORAN-C INTERFERENCE MITIGATION TECHNIQUE

IP.com Disclosure Number: IPCOM000005435D
Original Publication Date: 1982-Jan-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 1 page(s) / 71K

Publishing Venue

Motorola

Related People

Robert V. Janc: AUTHOR

Abstract

Electric utllltles utlllze a technique called direct transfer trip to provide protective relaying of their hlgh voltage transmlsslon system. As part of this technique, a guard tone or CW power line carrier in the LF frequency band is injected on the transmission line at a level of approximately 1 watt. A problem arlses in operating a LORAN-C navigation receiver in the vicinity of such transmission lines since the LORAN-C receiver also operates in the LF frequency band. In particular, the phase locked loops utilized by the LORAN-C receiver to track the navigation signals lose lock and the LORAN-C receiver provides er- roneous navigation information. This problem can be eliminated by adding a blanking circuit of ap- propriate timing at the output of the transmitter utilized to inject the guard tone into the transmission line to create short gaps in the guard tone (see Figure 1).

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MOmmLA Technical Developments Volume 2 January 1982

8

POWER LINE CARRIER/LORAN-C INTERFERENCE MITIGATION TECHNIQUE By Robert V. Jane

   Electric utllltles utlllze a technique called direct transfer trip to provide protective relaying of their hlgh voltage transmlsslon system. As part of this technique, a guard tone or CW power line carrier in the LF frequency band is injected on the transmission line at a level of approximately 1 watt. A problem arlses in operating a LORAN-C navigation receiver in the vicinity of such transmission lines since the LORAN-C receiver also operates in the LF frequency band. In particular, the phase locked loops utilized by the LORAN-C receiver to track the navigation signals lose lock and the LORAN-C receiver provides er- roneous navigation information. This problem can be eliminated by adding a blanking circuit of ap- propriate timing at the output of the transmitter utilized to inject the guard tone into the transmission line to create short gaps in the guard tone (see Figure 1).

   The phase locked loops in a LORAN-C receiver will lose lock in the presence of a strong (i.e., interference-to-signal ratio greater than 25 dB) CW interferer. Because all modern LORAN-C receivers are hard-limited and use digital phase locked loops, the time to lose lock is independent of the strength of the'interferer once it controls the output of the limiter. Rather, the time to lose lock is a function of the step response of the phase locked loop and the coherence of the input samples to the phase locked loop As a result, CW interferers that are coherent with the input samples will cause loss of lock in a ti...