Low-pass filter for a PLL, phase-locked loop and semiconductor integrated circuit
First Claim
1. A low-pass filter comprising:
- first filtering means that receives an input signal to said low-pass filter and outputs a first voltage;
a circuit element included in said first filtering means for generating a first current in accordance with said first voltage;
current generating means for generating a second current at a given rate to said first current;
second filtering means that receives said second current and outputs a second voltage; and
adding means for adding said first voltage and said second voltage and outputting an output signal of said low-pass filter,wherein said given rate is a positive number smaller than 1.
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Abstract
The invention provides a low-pass filter suitably used as a loop filter for a PLL or a DLL that has a filtering characteristic equivalent to that of a conventional one and can be realized in a smaller circuit area. The low-pass filter includes first filtering means (31) for accepting, as an input, an input signal to the low-pass filter and outputting a first voltage; a circuit element (311) included in the first filtering means (31) for allowing a first current to flow in accordance with the first voltage; current generating means (32) for generating a second current at a given rate to the first current; second filtering means (33) for accepting, as an input, the second current and outputting a second voltage; and adding means (34) for adding the first voltage and the second voltage and outputting an output signal of the low-pass filter, in which the second current is set to be smaller than the first current.
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Citations
43 Claims
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1. A low-pass filter comprising:
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first filtering means that receives an input signal to said low-pass filter and outputs a first voltage; a circuit element included in said first filtering means for generating a first current in accordance with said first voltage; current generating means for generating a second current at a given rate to said first current; second filtering means that receives said second current and outputs a second voltage; and adding means for adding said first voltage and said second voltage and outputting an output signal of said low-pass filter, wherein said given rate is a positive number smaller than 1. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26)
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27. A feedback system for feeding back an output clock, generated in accordance with an input clock, to make said output clock attain a desired characteristic, comprising:
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a charge pump circuit for generating a charge current in accordance with a phase difference between said input clock and a clock resulting from the feedback; a loop filter for filtering said charge current; and output clock generating means for generating said output clock in accordance with an output signal from said loop filter, wherein said loop filter includes; first filtering means for filtering said charge current and outputting a first voltage; a circuit element included in said first filtering means for generating a first current in accordance with said first voltage; current generating means for generating a second current at a given rate to said first current; second filtering means for filtering said second current and outputting a second voltage; and adding means for adding said first voltage and said second voltage and outputting said output signal, and said given rate is a positive number smaller than 1. - View Dependent Claims (28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43)
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Specification