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(12) United States Patent ao) Patent No.: us 6,680,991 Bi
Gutierrez (45) Date of Patent: Jan. 20,2004
(58) Field of Search 375/324, 326,
375/327, 344, 354, 357, 371, 373, 374, 375, 376; 331/1 A, 1 R, 18, 25, 34, 37, 40; 327/212, 159, 156; 370/395.62, 503, 516; 455/182.2, 192.2, 255
(56) References Cited
U.S. PATENT DOCUMENTS
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* cited by examiner
Primary Examiner—Chieh M. Fan
(74) Attorney, Agent, or Firm—Fenwick & West LLP
A lock detector is described for detecting a difference between the frequencies of a first and a second input signal. The lock detector includes first and second beat generators configured to generate corresponding beat signals based on the first and second input signals. The second beat signal is phase shifted relative to the first beat signal. A chatter elimination module combines the two chattery beat signals to produce a third corresponding beat signal that is substantially free of chatter. Using this clean beat signal and either of the input signals, a lock detection module produces a lock detection signal, which indicates whether the difference between the frequencies of the first and second input signals is within a prescribed tolerance.
21 Claims, 4 Drawing Sheets