Reducing non-linearity in a digital-to-time converter (DTC) due to unequal successive input codes specifying respective delays
A digital-to-time converter (DTC) of a fractional-n PLL contains a delay generator and a current-drawing block. The current drawn by the delay generator from the power supply contains: (A) a fixed component having a first current magnitude regardless of magnitude of a second code specifying the desired delay; and (B) a variable component having a magnitude determined only by magnitude of the second code. The current-drawing block draws correction-current from the power supply, whose magnitude is determined only by a complement of the second code.
1 . An open-loop modulator (OLM) circuit for generating a fractional output clock having a frequency which is a desired fraction of that of a reference clock, wherein said desired fraction comprises an integer component and a fractional component, said OLM comprising:
a delta-sigma modulator (DSM) coupled to receive said desired fraction and to generate a sequence of first codes and a corresponding sequence of second codes, wherein said sequence of first codes comprises a set of a lower value and a set of a higher value together representing said desired fraction on average;
a first divider coupled to receive said reference clock and each first code of said sequence of first codes, and to generate a divided signal having a period that is a multiple of the period of said reference clock, wherein the multiple is said each first code;
a digital-to-time converter (DTC) coupled to generate said fractional output clock from said divided signal, wherein said DTC comprises:
a delay generator, powered by a power supply, coupled to receive said divided signal and to delay an edge of interest of said divided signal according to corresponding second code of said sequence of second codes to generate said fractional output clock, wherein the current drawn by said delay generator from said power supply comprises:
a fixed component having a first current magnitude regardless of magnitude of said corresponding second code; and
a variable component having a magnitude determined only by magnitude of corresponding second code of said sequence of second codes,
a current-drawing block coupled to draw a correction-current from said power supply, wherein a magnitude of said correction-current is determined only by a complement of the corresponding second code of said sequence of second codes,
wherein said current-drawing block operates to ensure that a sum of said correction-current and said variable component drawn by said delay generator is constant regardless of magnitude of said corresponding second code,
wherein total current drawn from said power supply by said delay generator and said current-drawing block equals an aggregate of said sum and said fixed component.
2 . The OLM circuit of claim 1 , wherein said current-drawing block is a current digital-to-analog converter (DAC) which receives said complement as input and draws said correction-current to convert said complement to current.
3 . The OLM circuit of claim 2 , wherein said DTC comprises:
an input buffer coupled to receive said divided signal and to generate a buffered signal, wherein said buffered signal is provided to said delay generator; and
an output buffer coupled to receive said fractional output clock and to generate a buffered fractional output clock,
wherein said delay generator comprises:
an asymmetric inverter drawing said variable component; and
a symmetric inverter drawing at least a part of said fixed component.
4 . The OLM circuit of claim 2 , wherein said DSM is a first-order delta-sigma modulator.
5 . The OLM circuit of claim 4 , wherein said complement of said second code comprises a plurality of bits, wherein magnitude of said correction-current is proportional to a magnitude of a subset of bits of said plurality of bits,
wherein said subset of bits comprises a pre-determined number of most-significant bits (MSBs) of said plurality of bits.
6 . The OLM circuit of claim 5 , wherein said current DAC comprises:
a programmable current source operable to source a reference current from a bias voltage-source, wherein a first terminal of said programmable current source is coupled to said bias voltage-source;
a first transistor, wherein a first current terminal of said first transistor is coupled to a control terminal of said first transistor, wherein a second current terminal of said first transistor is coupled to a constant reference potential, wherein a second terminal of said programmable current source is coupled to said first current terminal of said first transistor;
a plurality of switches;
a plurality of transistors having control terminals coupled together and to said control terminal of said first transistor, wherein a first current terminal of each transistor of said a plurality of transistors is coupled to said power supply via a respective switch of said plurality of switches, wherein a second current terminal of each transistor of said a plurality of transistors is coupled to said constant reference potential,
wherein each switch operates, when closed to connect to, and when open to disconnect from, said power supply a respective transistor of said plurality of transistors;
wherein each switch is operable to be closed or open by a corresponding bit of said subset of bits,
wherein said plurality of transistors is operable to sink said correction-current.
7 . The OLM circuit of claim 6 , further comprising a look-up table containing corresponding values of said reference current for each range of a respective operating frequency range of said fractional output clock,
wherein a magnitude of said reference current is configured based on said look-up table.
8 . A digital-to-time converter (DTC) for generating a fractional output clock from a divided signal based on a sequence of delay-codes, said DTC comprising:
a delay generator, powered by a power supply, coupled to receive said divided signal and to delay an edge of interest of said divided signal according to each delay-code of said sequence of delay-codes to generate said fractional output clock, wherein the current drawn by said delay generator from said power supply comprises:
a fixed component having a first current magnitude regardless of magnitude of said corresponding delay-code; and
a variable component having a magnitude determined only by magnitude of corresponding delay-code of said sequence of delay-codes,
a current-drawing block coupled to draw a correction-current from said power supply, wherein a magnitude of said correction-current is determined only by a complement of the corresponding delay-code of said sequence of delay-codes,
wherein said current-drawing block operates to ensure that a sum of said correction-current and said variable component drawn by said delay generator is constant regardless of magnitude of said corresponding delay-code,
wherein total current drawn from said power supply by said delay generator and said current-drawing block equals an aggregate of said sum and said fixed component.
9 . The DTC of claim 8 , wherein said current-drawing block is a current digital-to-analog converter (DAC) which receives said complement as input and draws said correction-current to convert said complement to current.
10 . The DTC of claim 9 , wherein said DTC comprises:
an input buffer coupled to receive said divided signal and to generate a buffered signal, wherein said buffered signal is provided to said delay generator; and
an output buffer coupled to receive said fractional output clock and to generate a buffered fractional output clock,
wherein said delay generator comprises:
an asymmetric inverter drawing said variable component; and
a symmetric inverter drawing at least a part of said fixed component.
11 . The DTC of claim 10 , wherein said complement of said delay-code comprises a plurality of bits, wherein magnitude of said correction-current is proportional to a magnitude of a subset of bits of said plurality of bits,
wherein said subset of bits comprises a pre-determined number of most-significant bits (MSBs) of said plurality of bits.
12 . The DTC of claim 11 , wherein said current DAC comprises:
a programmable current source operable to source a reference current from a bias voltage-source, wherein a first terminal of said programmable current source is coupled to said bias voltage-source;
a first transistor, wherein a first current terminal of said first transistor is coupled to a control terminal of said first transistor, wherein a second current terminal of said first transistor is coupled to a constant reference potential, wherein a second terminal of said programmable current source is coupled to said first current terminal of said first transistor;
a plurality of switches;
a plurality of transistors having control terminals coupled together and to said control terminal of said first transistor, wherein a first current terminal of each transistor of said a plurality of transistors is coupled to said power supply via a respective switch of said plurality of switches, wherein a second current terminal of each transistor of said a plurality of transistors is coupled to said constant reference potential,
wherein each switch operates, when closed to connect to, and when open to disconnect from, said power supply a respective transistor of said plurality of transistors;
wherein each switch is operable to be closed or open by a corresponding bit of said subset of bits,
wherein said plurality of transistors is operable to sink said correction-current.
13 . The DTC of claim 12 , wherein said DTC is designed to operate for a plurality of operating frequency ranges of said fractional output clock,
wherein said DTC comprises a look-up table containing corresponding values of said reference current for each operating frequency range of said plurality of operating frequency ranges,
wherein a magnitude of said reference current is configured based on said look-up table.
14 . A system comprising:
a line card coupled to receive a data packet, said line card to re-time said data packet with reference to a selected clock, and to transmit a re-timed packet;
a first timing card to generate a first clock; and
a power supply,
wherein said line card comprises:
a phase-locked loop (PLL) coupled to receive said first clock, said PLL to generate a reference clock locked to said first clock; and
an open-loop-modulator (OLM) coupled to receive said reference clock and to generate a fractional output clock having a frequency which is a desired fraction of that of said reference clock, wherein said OLM comprises:
a delta-sigma modulator (DSM) coupled to receive said desired fraction and to generate a sequence of first codes and a corresponding sequence of second codes, wherein said sequence of first codes comprises a set of a lower value and a set of a higher value together representing said desired fraction on average;
a first divider coupled to receive said reference clock and each first code of said sequence of first codes, and to generate a divided signal having a period that is a multiple of the period of said reference clock, wherein the multiple is said each first code;
a digital-to-time converter (DTC) coupled to generate said fractional output clock from said divided signal, wherein said DTC comprises:
a delay generator, powered by a power supply, coupled to receive said divided signal and to delay an edge of interest of said divided signal according to corresponding second code of said sequence of second codes to generate said fractional output clock, wherein the current drawn by said delay generator from said power supply comprises:
a fixed component having a first current magnitude regardless of magnitude of said corresponding second code; and
a variable component having a magnitude determined only by magnitude of corresponding second code of said sequence of second codes,
a current-drawing block coupled to draw a correction-current from said power supply, wherein a magnitude of said correction-current is determined only by a complement of the corresponding second code of said sequence of second codes,
wherein said current-drawing block operates to ensure that a sum of said correction-current and said variable component drawn by said delay generator is constant regardless of magnitude of said corresponding second code,
wherein total current drawn from said power supply by said delay generator and said current-drawing block equals an aggregate of said sum and said fixed component.
15 . The system of claim 14 , wherein said current-drawing block is a current digital-to-analog converter (DAC) which receives said complement as input and draws said correction-current to convert said complement to current.
16 . The system of claim 15 , wherein said DTC comprises:
an input buffer coupled to receive said divided signal and to generate a buffered signal, wherein said buffered signal is provided to said delay generator; and
an output buffer coupled to receive said fractional output clock and to generate a buffered output clock,
wherein said delay generator comprises:
an asymmetric inverter drawing said variable component; and
a symmetric inverter drawing at least a part of said fixed component.
17 . The system of claim 15 , wherein said DSM is a first-order delta-sigma modulator.
18 . The system of claim 17 , wherein said complement of said second code comprises a plurality of bits, wherein magnitude of said correction-current is proportional to a magnitude of a subset of bits of said plurality of bits,
wherein said subset of bits comprises a pre-determined number of most-significant bits (MSBs) of said plurality of bits.
19 . The system of claim 18 , wherein said current DAC comprises:
a programmable current source operable to source a reference current from a bias voltage-source, wherein a first terminal of said programmable current source is coupled to said bias voltage-source;
a first transistor, wherein a first current terminal of said first transistor is coupled to a control terminal of said first transistor, wherein a second current terminal of said first transistor is coupled to a constant reference potential, wherein a second terminal of said programmable current source is coupled to said first current terminal of said first transistor;
a plurality of switches;
a plurality of transistors having control terminals coupled together and to said control terminal of said first transistor, wherein a first current terminal of each transistor of said a plurality of transistors is coupled to said power supply via a respective switch of said plurality of switches, wherein a second current terminal of each transistor of said a plurality of transistors is coupled to said constant reference potential,
wherein each switch operates, when closed to connect to, and when open to disconnect from, said power supply a respective transistor of said plurality of transistors;
wherein each switch is operable to be closed or open by a corresponding bit of said subset of bits,
wherein said plurality of transistors is operable to sink said correction-current.
20 . The system of claim 19 , further comprising a look-up table containing corresponding values of said reference current for each range of a respective operating frequency range of said fractional output clock,
wherein a magnitude of said reference current is configured based on said look-up table.