Analog phase locked loop holdover
First Claim
1. A phase locked loop, comprising:
- a phase detector coupled to a reference clock signal and a feedback signal for generating positive and negative phase detection signals corresponding to the phase difference between the reference clock signal and the feedback signal;
an integrator coupled to the positive and negative phase detection signals for generating an output voltage in response to pulses from the phase detector;
a voltage controlled oscillator coupled to the output voltage of the integrator for generating a local oscillator signal with an oscillation frequency proportional to the output voltage of the integrator;
a feedback circuit coupled to the local oscillator signal for generating the feedback signal; and
an analog holdover circuit for generating an input to said integrator when said phase detector stops receiving the reference clock signal.
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Abstract
A phase locked loop (PLL) circuit is provided having: (1) a phase detector coupled to a reference clock signal and a feedback signal for generating positive and negative phase detection signals corresponding to the phase difference between the reference clock signal and the feedback signal; (2) an integrator coupled to the positive and negative phase detection signals for generating an output voltage proportional to the pulse width of either the positive or negative phase detection signals, the integrator including an operational amplifier having positive and negative inputs; (3) a voltage controlled oscillator coupled to the output voltage of the integrator for generating a local oscillator signal with an oscillation frequency proportional to the output voltage of the integrator; (4) a feedback circuit coupled to the local oscillator signal for generating the feedback signal; and (5) an analog holdover circuit for generating an input to the integrator when the phase detector stops receiving the reference clock signal.
17 Citations
36 Claims
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1. A phase locked loop, comprising:
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a phase detector coupled to a reference clock signal and a feedback signal for generating positive and negative phase detection signals corresponding to the phase difference between the reference clock signal and the feedback signal;
an integrator coupled to the positive and negative phase detection signals for generating an output voltage in response to pulses from the phase detector;
a voltage controlled oscillator coupled to the output voltage of the integrator for generating a local oscillator signal with an oscillation frequency proportional to the output voltage of the integrator;
a feedback circuit coupled to the local oscillator signal for generating the feedback signal; and
an analog holdover circuit for generating an input to said integrator when said phase detector stops receiving the reference clock signal. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. An improved phase locked loop having a phase detector for comparing a reference signal with a feedback signal and for generating phase pulses having a pulse width corresponding to the phase difference between the reference signal and the feedback signal, an integrator coupled to the phase pulses for generating a voltage level corresponding to the pulse width of the phase pulses, and a voltage controlled oscillator coupled to the voltage level for generating the feedback signal, the improvement comprising:
an analog holdover circuit for generating an input to the integrator when the phase detector stops receiving a reference clock signal. - View Dependent Claims (12, 13, 14, 15, 16, 17)
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18. A phase locked loop, comprising:
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means for detecting the phase difference between a reference clock signal and a feedback signal;
means for generating a phase voltage corresponding to the phase difference between the reference clock signal and the feedback signal;
means for generating the feedback signal, wherein the feedback signal is characterized by a frequency that varies in proportion to the phase voltage; and
means for generating an input to the integrator when the phase detector stops receiving a reference clock signal. - View Dependent Claims (19, 20, 21, 22, 23, 24, 25)
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26. An improved phase locked loop having a phase detector, a voltage controlled oscillator, and an integrator coupled between the phase detector and the voltage controlled oscillator, wherein the integrator includes a positive input and a negative input, the improvement comprising:
an analog holdover circuit coupled to at least one of the positive or the negative inputs of the integrator, said analog holdover circuit being operative to generate an input to the integrator when the phase detector stops receiving a reference clock signal. - View Dependent Claims (27, 28, 29, 30, 31, 32, 33)
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34. A method of controlling a phase locked loop having a phase detector, a voltage controlled oscillator, and an integrator coupled between the phase detector and the voltage controlled oscillator, comprising the steps of:
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storing the voltage output level from the integrator when there is a reference signal input present at the phase detector;
detecting a reference signal failure; and
generating a driving signal for the integrator when said reference signal failure is detected, said driving signal being operative to drive the integrator to output a voltage level that is equivalent to the stored voltage level. - View Dependent Claims (35, 36)
generating a DC voltage level based on the stored voltage level;
comparing the generated DC voltage level to the stored voltage level; and
coupling the output of the comparison to the integrator.
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36. The method of claim 35 wherein said coupling step comprises the steps of:
coupling a digital gain attenuator chopper between the comparison output and the integrator to reduce the loop gain.
Specification