Apparatus and method to hold PLL output frequency when input clock is lost
First Claim
1. A clock conditioning circuit comprising:
- a phase detector circuit configured to provide an analog tuning signal indicative of a phase relationship between a reference clock to be conditioned and a generated clock;
a controlled oscillator configured to produce the generated clock, with the generated clock having an output frequency adjustable in response to an analog tuning signal applied to a control signal input of the controlled oscillator;
mode control circuitry switchable between at least two modes, including a tracking mode and a holdover mode, with the mode control circuitry operating in the tracking mode to couple the analog tuning signal to the control signal input of the controlled oscillator and with the mode control circuitry operating in the holdover mode to couple an analog holdover signal to the control signal input of the controlled oscillator; and
converter circuitry configured to produce a digital representation of the analog tuning signal when the mode control circuitry is in the tracking mode and to provide the analog holdover signal to the control signal input when the mode control circuitry is in the holdover mode, with the analog holdover signal being based upon one of the digital representations produced when the mode control circuitry was in the tracking mode.
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Abstract
A clock conditioning circuit including a phase detector circuit configured to provide an analog tuning signal indicative of a phase relationship between a reference clock to be conditioned and a generated clock. The controlled oscillator is configured to produce the generated clock, with the generated clock having an output frequency adjustable in response to an analog tuning signal applied to a control signal input of the controlled oscillator. Converter circuitry is provided to produce a digital representation of the analog tuning signal when the mode control circuitry is in a tracking mode. In the event the reference clock is lost, the mode control circuitry switches to a holdover mode so as to provide an analog holdover signal to the control signal input based upon the digital representations produced just prior to the loss of the reference clock.
23 Citations
24 Claims
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1. A clock conditioning circuit comprising:
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a phase detector circuit configured to provide an analog tuning signal indicative of a phase relationship between a reference clock to be conditioned and a generated clock; a controlled oscillator configured to produce the generated clock, with the generated clock having an output frequency adjustable in response to an analog tuning signal applied to a control signal input of the controlled oscillator; mode control circuitry switchable between at least two modes, including a tracking mode and a holdover mode, with the mode control circuitry operating in the tracking mode to couple the analog tuning signal to the control signal input of the controlled oscillator and with the mode control circuitry operating in the holdover mode to couple an analog holdover signal to the control signal input of the controlled oscillator; and converter circuitry configured to produce a digital representation of the analog tuning signal when the mode control circuitry is in the tracking mode and to provide the analog holdover signal to the control signal input when the mode control circuitry is in the holdover mode, with the analog holdover signal being based upon one of the digital representations produced when the mode control circuitry was in the tracking mode. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method of conditioning a clock signal comprising:
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producing an analog tuning signal indicative of a phase difference between a reference clock to be conditioned and a generated clock; providing a controlled oscillator to produce the generated clock, with the generated clock being generated in response to an analog signal applied to a control input of the controlled oscillator; operating in a tracking mode of operation where the analog tuning signal is coupled to the control input of the controlled oscillator; generating a digital representation of the analog tuning signal during the tracking mode of operation; and switching to a holdover mode of operation where the analog tuning signal is replaced with an analog holdover signal, with the analog holdover signal being generated based upon the digital representation of the analog tuning signal generated during the previous tracking mode of operation. - View Dependent Claims (13, 14, 15, 16, 17)
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18. A clock conditioning circuit for use with a controlled oscillator configured to produce a generated clock, with the generated clock having an output frequency adjustable in response to an analog tuning signal applied to a control signal input of the controlled oscillator, said clock conditioning circuit comprising:
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a phase detector circuit configured to provide, when a controlled oscillator is present, the analog tuning signal indicative of a phase relationship between a reference clock to be conditioned and the generated clock; mode control circuitry switchable between at least two modes, including a tracking mode and a holdover mode, so that when a controlled oscillator is present and the mode control circuitry is operating in the tracking mode, the analog tuning signal is coupled to the control signal input of the controlled oscillator and so that when a controlled oscillator is present and the mode control circuitry is operating in the holdover mode, an analog holdover signal is coupled to the control signal input of the controlled oscillator; and converter circuitry configured to produce a digital representation of the analog tuning signal when the mode control circuitry is in the tracking mode and to provide the analog holdover signal to the control signal input when the mode control circuitry is in the holdover mode, with the analog holdover signal being based upon one of the digital representations produced when the mode control circuitry was in the tracking mode. - View Dependent Claims (19, 20, 21, 22, 23, 24)
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Specification