Phase locked loop using a schmitt trigger block
First Claim
1. A phase locked loop (PLL) circuit using a Schmitt trigger to achieve a very small steady state phase error, said PLL circuit comprising:
- a phase comparator block for detecting a phase difference between an input reference signal and a feedback signal to produce a phase difference signal indicative of the phase difference;
a low-pass filter block for producing a control voltage;
a charge pump block, placed between said phase comparator block and said low-pass filter block, for charging or discharging said low-pass filer block according to the phase difference signal;
a Schmitt trigger block, connected to said low-pass filter block, consisting of a plurality of Schmitt trigger circuits, each Schmitt trigger circuit having the amount of hysteresis which depends on a damping factor ζ
of the PLL circuit as well as temperature and voltage coefficients of the control voltage, a hysteresis curve having the midpoint which is set by a current voltage characteristics of said charge pump block, said Schmitt trigger block producing Schmitt output signals which vary in response to the control voltage;
a control logic circuit placed between said Schmitt trigger block and said charge pump block, said control logic circuit producing control output signals which vary in response to the Schmitt output signals, the control output signals commanding said charge pump block to turn ON or OFF;
a voltage controlled oscillator (VCO), connected to said low-pass filter block, for generating a VCO output signal whose frequency is determined by the control voltage; and
a frequency divider, placed between said voltage controlled oscillator and said phase comparator block, for frequency dividing the VCO output signal by a division ratio N, said frequency divider producing a divided signal as the feedback signal which is supplied to said phase comparator block.
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Accused Products
Abstract
A phase locked loop (PLL) circuit is described which uses a Schmitt trigger block (28) to achieve a very small steady state phase error at an input of a phase comparator block (21) over the entire PLL lock voltage range. The amount of hysteresis which each Schmitt trigger circuit (281, 282) in the Schmitt trigger block (28) has depends on the damping factor ζ of the PLL circuit as well as the temperature and voltage coefficients of a VCO'"'"'s input voltage. The midpoint of the positive and the negative thresholds of the hysteresis curve of each Schmitt trigger circuit (281, 282) is set by the current voltage characteristics of charge pump circuits in a charge pump block (22). Responsive to the PLL'"'"'s lock voltage (VCNT), the Schmitt trigger block (28) commands a control logic circuit (29) to turn ON or turn OFF as the case may be PMOS pump UP transistors to that of NMOS pump DOWN transistors. It is this ratio which determines the PLL'"'"'s steady state phase error. In one embodiment, a frequency divider (25) is used between the VCO (24) and the phase comparator block (21). In another embodiment, this divider (25) is removed and the output of the VCO is fed directly back to the phase comparator block.
93 Citations
14 Claims
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1. A phase locked loop (PLL) circuit using a Schmitt trigger to achieve a very small steady state phase error, said PLL circuit comprising:
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a phase comparator block for detecting a phase difference between an input reference signal and a feedback signal to produce a phase difference signal indicative of the phase difference; a low-pass filter block for producing a control voltage; a charge pump block, placed between said phase comparator block and said low-pass filter block, for charging or discharging said low-pass filer block according to the phase difference signal; a Schmitt trigger block, connected to said low-pass filter block, consisting of a plurality of Schmitt trigger circuits, each Schmitt trigger circuit having the amount of hysteresis which depends on a damping factor ζ
of the PLL circuit as well as temperature and voltage coefficients of the control voltage, a hysteresis curve having the midpoint which is set by a current voltage characteristics of said charge pump block, said Schmitt trigger block producing Schmitt output signals which vary in response to the control voltage;a control logic circuit placed between said Schmitt trigger block and said charge pump block, said control logic circuit producing control output signals which vary in response to the Schmitt output signals, the control output signals commanding said charge pump block to turn ON or OFF; a voltage controlled oscillator (VCO), connected to said low-pass filter block, for generating a VCO output signal whose frequency is determined by the control voltage; and a frequency divider, placed between said voltage controlled oscillator and said phase comparator block, for frequency dividing the VCO output signal by a division ratio N, said frequency divider producing a divided signal as the feedback signal which is supplied to said phase comparator block. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A phase locked loop (PLL) circuit using a Schmitt trigger to achieve a very small steady state phase error, said PLL circuit comprising:
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a phase comparator block for detecting a phase difference between an input reference signal and a feedback signal to produce a phase difference signal indicative of the phase difference; a low-pass filter block for producing a control voltage; a charge pump block, placed between said phase comparator block and said low-pass filter block, for charging or discharging said low-pass filer block according to the phase difference signal; a Schmitt trigger block, connected to said low-pass filter block, consisting of a plurality of Schmitt trigger circuits, each Schmitt trigger circuit having the amount of hysteresis which depends on a damping factor ζ
of the PLL circuit as well as temperature and voltage coefficients of the control voltage, a hysteresis curve having the midpoint which is set by a current voltage characteristics of said charge pump block, said Schmitt trigger block producing Schmitt output signals which vary in response to the control voltage;a control logic circuit placed between said Schmitt trigger block and said charge pump block, said control logic circuit producing control output signals which vary in response to the Schmitt output signals, the control output signals commanding said charge pump block to turn ON or OFF; a voltage controlled oscillator (VCO), connected to said low-pass filter block, for generating a VCO output signal whose frequency is determined by the control voltage; and supplying means, placed between said voltage controlled oscillator and said phase comparator block, for directly supplying, as the feedback signal, the VCO output signal to said phase comparator block. - View Dependent Claims (9, 10, 11, 12, 13, 14)
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Specification