# Patent

## Title
Low power, low voltage phase interpolator

## Abstract

A method for providing an interpolated output signal includes, in at least one aspect, receiving a plurality of phase signals applying each phase signal of the plurality of phase signals to switching elements of a first set of switching elements receiving a plurality of select signals, applying an asserted select signal to a first switching element of a second set of switching elements to provide a connection between a first switching element of the first set of switching elements and a first output terminal, and applying the asserted select signal to a second switching element of the second set of switching elements to provide a connection between a second switching element of the first set of switching elements and a second output terminal.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This disclosure claims the benefit of priority of U.S. application Ser. No. 11/006,189, filed on Dec. 6, 2004, issued on Jul. 24, 2012, as U.S. Pat. No. 8,228,110, and titled “LOW POWER, LOW VOLTAGE PHASE INTERPOLATOR,” the disclosure of which is hereby incorporated by reference in its entirety.
BACKGROUND
The following disclosure relates to electrical circuits and signal processing.
A phase interpolator is a circuit that can be used within an integrated circuit to adjust a phase of a signal (e.g., a clock signal). A phase interpolator can be used in a variety of applications. For example, a phase interpolator can be implemented within a receiver to adjust a phase of a sampling clock, or within a write precompensation circuit (e.g., of a hard disk drive system) to adjust a phase of a write clock.
FIG. 1 shows one example of a conventional phase interpolator 100. Phase interpolator 100 includes a differential pair having a pair of differential inputs PH1, PH1Bar (complement of PH1), PH2, PH2Bar (complement of PH2), and differential outputs OUT1, OUT1Bar (complement of OUT1). Phase interpolator 100 includes bias currents I1-I2, transistors M1-M4, and resistors R1-R2. In general, phase interpolator 100 provides a phase shift for a signal that is an interpolation between phase signals PH1 and PH2. Phase interpolator 100 provides the phase shift based on bias currents I1 and I2. For example, if bias current I1 is turned off, then phase interpolator 100 provides an output signal OUT1 having a phase that is substantially equal to that of phase signal PH2. And if bias currents I1 and I2 are substantially equal, then phase interpolator 100 provides an output signal OUT1 having a phase that is substantially in between those of phase signals PH1 and PH2.
As bias current I1 or I2 is changed, a common mode component of output signals OUT1 and OUT1Bar varies, and phase interpolator 100 therefore requires a certain amount of time to settle in order to provide an output signal having an accurate phase shift. In addition, phase interpolator 100 may require a large buffer to drive each of phase signals PH1, PH1Bar, PH2 and PH2Bar. Such buffers typically require a large power supply (e.g., greater than 1.5 Volts). A change in a bias condition (e.g., including a large power supply) can impact interpolator linearity.
SUMMARY
In general, in one aspect, this specification describes a phase interpolator that is operable to provide an interpolated output signal through an output node. The phase interpolator includes a first and second interpolator module, each having an output in communication with the output node. The first interpolator includes an input to receive a first plurality of input phase signals, and a selector to select one or more of the first plurality of input phase signals for interpolation at the output node of the phase interpolator. The second interpolator module includes an input to receive a second plurality of input phase signals, and a selector to select one or more of the second plurality of input phase signals for interpolation at the output node of the phase interpolator. Each of the selected ones of the first plurality of input signals and each of the selected ones of the second plurality of input signals are included in the interpolated output signal.
Particular implementations can include one or more of the following. The first interpolator module and the second interpolator module can each include one or more interpolator core cells. Each interpolator core cell can be operable to provide a single instance of a given input phase signal to the output node for interpolation. Each interpolator core cell can receive as inputs one or more select signals operable to select a given input phase signal to be provided to the output node. Each interpolator core cell can include only NMOS or PMOS transistors. The selector in the first interpolator module and the selector in the second interpolator module can be substantially in common. The phase interpolator can operate using a power supply that is less than 1.2 Volts. Each of the first plurality of input phase signals can have a different phase relative to other ones of the first plurality of input phase signals. Each of the second plurality of input phase signals can have a different phase relative to other ones of the second plurality of input phase signals. The first plurality of input phase signals, the second plurality of input phase signals, and the interpolated output signal can be differential signals.
The phase interpolator can further include a third interpolator module having an output in communication with each of the output of the first interpolator module and the output of the second interpolator module. The third interpolator module can include an input to receive a third plurality of input phase signals, and a selector to select one or more of the third plurality of input phase signals for interpolation at the output node of the phase interpolator.
In general, in another aspect, this specification describes a phase interpolator operable to provide an interpolated output signal through an output node. The phase interpolator includes a first and second interpolator module, each having an output in communication with the output node. The first interpolator includes a plurality of first switches to receive a corresponding first plurality of input phase signals, and a plurality of second switches in communication with corresponding ones of the first plurality of switches. Each of the plurality of second switches are operable to assert one or more of the first plurality of input phase signals for interpolation at the output node of the phase interpolator. The second interpolator module includes a plurality of third switches to receive a corresponding second plurality of input phase signals, and a plurality of fourth switches in communication with corresponding ones of the third switches. Each of the plurality of fourth switches are operable to assert one or more of the second plurality of input phase signals for interpolation at the output node of the phase interpolator. Each of the asserted ones of the first plurality of input signals and each of the asserted ones of the second plurality of input signals are included in the interpolated output signal.
Particular implementations can include one or more of the following. Each of the first, second, third and fourth transistors can comprise only NMOS or PMOS transistors. The interpolated output signal can be provided to a load. The load can be associated with one or more of a serial ATA (Advanced Technology Architecture) communication system, a read channel, a fiber channel, a wireless baseband communication system, an ethernet XAUI SERDES transceiver, a 1000 BaseTx network, a USB (Universal Serial Bus) 2.0 bus, or a PCI Express bus.
In general, in another aspect, this specification describes a disk drive system. The disk drive system includes a read channel configured to provide a data stream to be recorded onto a surface of a disk, and a write precompensation circuit operable to precompensate each data bit of the data stream. The write precompensation circuit includes a phase interpolator operable to provide a predetermined delay (through an output node) of a write clock to precompensate each data bit of the data stream. The phase interpolator includes a first and second interpolator module, each having an output in communication with the output node. The first interpolator includes an input to receive a first plurality of input phase signals, and a selector to select one or more of the first plurality of input phase signals for interpolation at the output node of the phase interpolator. The second interpolator module includes an input to receive a second plurality of input phase signals, and a selector to select one or more of the second plurality of input phase signals for interpolation at the output node of the phase interpolator. Each of the selected ones of the first plurality of input signals and each of the selected ones of the second plurality of input signals are included in the interpolated output signal. The disk drive system further includes a read/write head operable to write the precompensated data bits onto the surface of the disk.
Implementations can include one or more of the following advantages. A phase interpolator circuit is provided that consumes low power. In one implementation, the phase interpolator circuit operates based on a power supply that is substantially 1.2 Volts or lower.
In one implementation, a phase interpolator is provided that includes plural interpolator modules. Each interpolator module is designed so that only the output(s) of each interpolator module need be in communication with one another. To increase a resolution of the phase interpolator, additional interpolator modules can be added to the phase interpolator including coupling each additional interpolator module to the output(s) of the phase interpolator. Such a modular design reduces complexities in layout of a high resolution phase interpolator. In addition, a high resolution phase interpolator can be implemented within a smaller area of an integrated circuit chip relative to conventional phase interpolator designs.
The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.



DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic diagram of a conventional phase interpolator.
FIG. 2 is block diagram of a phase interpolator.
FIG. 3 is a timing diagram illustrating the input phase signals of the phase interpolator of FIG. 2.
FIG. 4 is a schematic diagram of an interpolator core cell within an interpolation module of FIG. 2.
FIG. 5 is a schematic block diagram of a hard disk drive system.



Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
FIG. 2 illustrates a block diagram of a phase interpolator 200. In one implementation, phase interpolator 200 includes circuitry for generating differential output signals OUT, OUTBar (complement of OUT) having a phase that is shifted relative to those of one or more differential input phase signals (e.g., phase signals PH1-PH8). Differential output signals OUT, OUTBar can be provided to a load 202. Load 202 can be associated with a variety of applications. For example, example applications include a serial ATA (Advanced Technology Architecture) communication system, a read channel, a fiber channel, a wireless baseband communication system, an ethernet XAUI SERDES transceiver, a 1000 BaseTx network, a USB (Universal Serial Bus) 2.0 bus, a PCI Express bus, and so on.
Phase interpolator 200 includes multiple interpolator modules, and, specifically, in the implementation shown, two interpolator modules 202, 204 are included. Though two interpolation modules 202, 204 are illustrated in FIG. 2 by way of example, phase interpolator 200 can contain additional interpolator modules. Each interpolator module contains one or more interpolator core cells (not shown) that are each operable to output a selected differential input phase signal for interpolation at outputs OUT, OUTBar, as discussed in greater detail below.
Interpolator module 202 receives as inputs differential phase signals (e.g., phase signals PH1, PH3, PH5 and PH7). Interpolator module 204 receives as inputs differential phase signals (e.g., phase signals PH2, PH4, PH6 and PH8). FIG. 3 shows a timing diagram, for one implementation, of phase signals PH1-PH8. Though (8) phase signals are shown (e.g., phase signals PH1-PH8), any number of phase signals can be interpolated through interpolator modules 202, 204. For example, in one implementation (4) phase signals are interpolated through interpolator modules 202, 204. As shown in FIG. 3, each phase signal PH1-PH8 has a delay time of ΔT*(i+1) [i=0, 1, . . . , 0] with respect to phase signal PH1. In the example of FIG. 3, in which a cycle of phase signal PH1 is T, the delay time ΔT is approximately equal to T/8 (e.g., 45°). Together, interpolator modules 202, 204 output differential output signals OUT, OUTBar that are an interpolation between any combination of phase signals PH1-PH8.
Each interpolator module 202, 204 is powered by a low voltage supply VDD. In one implementation, VDD is substantially equal to 1.2 Volts. Alternatively, VDD can be substantially lower than 1.2 Volts (e.g., 0.7-0.9 Volts). Unlike a conventional phase interpolator that may require a high voltage power supply, phase interpolator 200 does not require a large supply voltage in order to interpolate various phase signals.
Each interpolator module 202, 204 further receives as inputs one or more SELECT signals. Each SELECT signal is operable to select a given differential input phase signal (within a given interpolator core cell) for interpolation at outputs OUT, OUTBar, as discussed in greater detail below.
FIG. 4 illustrates one implementation of an interpolator core cell 400 within an interpolator module (e.g., interpolator module 202). Interpolator core cell 400 includes transistors M7-M22, voltage bias sources Vbias1, Vbias2, and a current source 402. In one implementation, current source 402 provides a bias current to one or more transistors within interpolator core cell 400. In one implementation, each of transistors M7-M22 are NMOS transistors or PMOS transistors.
The source of transistors M7-M14 is in communication with the drains of transistors M15-M22, respectively. The sources of transistors M15, M22 are respectively in communication with voltage bias sources Vbias1, Vbias2. The sources of transistors M18-M21 are in communication with a low-side power supply VSS (e.g., 0 Volts). The gates of transistors M7, M14 are in communication with select signal SEL1. The gates of transistors M8, M13 are in communication with select signal SEL2. The gates of transistors M9, M11 are in communication with select signal SEL3. The gates of transistors M10, M12 are in communication with select signal SEL4. The gates of transistors M15, M20 are in communication with input phase signal PH1. The gates of transistors M16, M21 are in communication with input phase signal PH3. The gates of transistors M17, M21 are in communication with input phase signal PH7. The gates of transistors M18, M22 are in communication with input phase signal PH5. Interpolator module 204 can include a similar interpolator core cell (not shown) having as inputs, in the example shown, input phase signals PH2, PH4, PH6 and PH8.
In operation, interpolator core cell 400 is operable to output a selected differential input phase signal for interpolation at outputs OUT, OUTBar based on select signals SEL1-SEL4. For example, to provide differential input phase signals PH1 (0°) and PH5 (180°) at the outputs OUT, OUTBar, respectively, interpolator core cell 400 can operate as follows. A control circuit or a switching circuit (not shown) asserts select signal SEL1, and deasserts each of select signals SEL2-SEL4. A conventional control circuit (or selector) can be used to assert or deassert each select signal SEL1-SEL4. In response to the voltage level settings of select signals SEL1-SEL4, transistors M7 and M14 are enabled (e.g., turned on) and transistors M8-M13 are each disabled (e.g., turned off). As phase signal PH1 goes high (and phase signal PH5 goes low), a high reference voltage appears at output OUT, and a low reference voltage appears at output OUTBar. As phase signal PH1 goes low (and phase signal PH5 goes high), a low reference voltage appears at output OUT, and a high reference voltage appears at output OUTBar.
As discussed above, interpolator modules 202, 204 can each include one or more interpolator core cells (e.g., interpolator core cell 400). In one implementation, each interpolator module 202, 204 of phase interpolator 200 includes (16) interpolator core cells that are each operable to output a given differential input phase signal for interpolation at outputs OUT, OUTBar. In this implementation, phase interpolator 200 can output a differential output signal OUT, OUTBar having a phase with a resolution (e.g., step size= 1/16th) between any two given input phase signals (e.g., input phase signals PH1-PH8). For example, to provide an output signal OUT having a phase that is ( 1/16th) between input phase signals PH1 and PH2, a control circuit (not shown) controls one of the (16) interpolator core cells of interpolator module 202 to provide a single instance of input phase signal PH1 at output OUT1. The control circuit also controls all (16) interpolator core cells of interpolator module 204 to provide (16) instances of input phase signal PH2 at output OUT1.
In general, each interpolator module 202, 204 can contain any number of interpolator core cells, and any number of phase input signals to provide various granularities of resolution for phase shifting a signal.
Phase interpolator 200 can be used with circuitry of a disk drive system 500, as shown in FIG. 5. Disk drive system 500 includes a read/write head 502, a write precompensation circuit 504, and a read channel 506.
In a write operation, a data stream to be recorded is provided by read channel 506 to write precompensation circuit 504. Write precompensation circuit 504 precompensates each data bit of the data stream and provides precompensated data to read/write head 502. In one implementation, phase interpolator 200 provide a predetermined delay (or phase shift) of a write clock to precompensate the data being sent to read/write head 502. Read/write head 502 locates an appropriate sector of a disk (not shown) and writes the precompensated data onto the disk.
A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, one or more of transistors M5-M22 can be PMOS transistors. Accordingly, other implementations are within the scope of the following claims.

## Claims

1. A method, comprising:
receiving a plurality of phase signals;
applying a first phase signal of the plurality of phase signals to a first pair of switching elements of a first set of switching elements to concurrently enable the first pair of switching elements;
applying a second phase signal of the plurality of phase signals to a second pair of switching elements of the first set of switching elements to concurrently enable the second pair of switching elements;
receiving a plurality of select signals, wherein a select signal of the plurality of select signals is asserted;
applying the asserted select signal to a first switching element of a second set of switching elements to provide a connection between a switching element of the first pair of switching elements and a first output terminal; and
applying the asserted select signal to a second switching element of the second set of switching elements to provide a connection between a switching element of the second pair of switching elements and a second output terminal.

2. The method of claim 1, wherein each phase signal of the plurality of phase signals has a different phase relative to other phase signals of the plurality of phase signals.

3. The method of claim 1, wherein the first phase signal of the plurality of phase signals applied to the first pair of switching elements of the first set of switching elements has a different phase relative to the second phase signal of the plurality of phase signals applied to the second pair of switching elements of the first set of switching elements.

4. The method of claim 1, further comprising:
providing an interpolated output signal based on output signals at the first output terminal and the second output terminal.

5. The method of claim 4, wherein the asserted select signal is a first asserted select signal, and the method further comprises:
applying a second asserted select signal to a third switching element of the second set of switching elements to provide a connection between another switching element of the second pair of switching elements and the first output terminal; and
applying the second asserted select signal to a fourth switching element of the second set of switching elements to provide a connection between another switching element of the first pair of switching elements and the second output terminal.

6. The method of claim 1, wherein each switching element of the first set of switching elements and the second set of switching elements comprises a transistor.

7. The method of claim 1, wherein the asserted select signal is a first asserted select signal, the method further comprising:
applying a third phase signal of the plurality of phase signals to a third pair of switching elements of the first set of switching elements to concurrently enable the third pair of switching elements; and
applying a second asserted select signal to a third switching element of the second set of switching elements to provide a connection between a switching element of the third pair of switching elements and the first output terminal.

8. The method of claim 7, further comprising:
applying a fourth phase signal of the plurality of phase signals to a fourth pair of switching elements of the first set of switching elements to concurrently enable the fourth pair of switching elements; and
applying the second asserted select signal to a fourth switching element of the second set of switching elements to provide a connection between a switching element of the fourth pair of switching elements and the second output terminal.

9. An apparatus, comprising:
a first output terminal and a second output terminal to provide an interpolated output signal; and
an interpolator circuit coupled to the first output terminal and the second output terminal, the interpolator circuit configured to:
receive a plurality of phase signals;
apply a first phase signal of the plurality of phase signals to a first pair of switching elements of a first set of switching elements to concurrently enable the first pair of switching elements;
apply a second phase signal of the plurality of phase signals to a second pair of switching elements of the first set of switching elements to concurrently enable the second pair of switching elements;
receive a plurality of select signals, wherein a select signal of the plurality of select signals is asserted;
apply the asserted select signal to a first switching element of a second set of switching elements to provide a connection between a switching element of the first pair of switching elements and the first output terminal; and
apply the asserted select signal to a second switching element of the second set of switching elements to provide a connection between a switching element of the second pair of switching elements and the second output terminal.

10. The apparatus of claim 9, wherein each phase signal of the plurality of phase signals has a different phase relative to other phase signals of the plurality of phase signals.

11. The apparatus of claim 9, wherein the first phase signal of the plurality of phase signals applied to the first pair of switching elements of the first set of switching elements has a different phase relative to the second phase signal of the plurality of phase signals applied to the second pair of switching elements of the first set of switching elements.

12. The apparatus of claim 9, wherein the asserted select signal is a first asserted select signal, and the interpolator circuit is further configured to:
apply a second asserted select signal to a third switching element of the second set of switching elements to provide a connection between another switching element of the second pair of switching elements and the first output terminal; and
apply the second asserted select signal to a fourth switching element of the second set of switching elements to provide a connection between another switching element of the first pair of switching elements and the second output terminal.

13. The apparatus of claim 9, wherein the interpolator circuit is further configured to:
operate on a supply voltage that is less than 1.2 volts.

14. A system, comprising:
a load;
a control circuit to provide a plurality of select signals, wherein a select signal of the plurality of select signals is asserted; and
an interpolator circuit to provide an interpolated output signal to the load, the interpolator circuit configured to:
receive a plurality of phase signals;
apply a first phase signal of the plurality of phase signals to a first pair of switching elements of a first set of switching elements to concurrently enable the first pair of switching elements;
apply a second phase signal of the plurality of phase signals to a second pair of switching elements of the first set of switching elements to concurrently enable the second lair of switching elements;
receive the plurality of select signals;
apply the asserted select signal to a first switching element of a second set of switching elements to provide a connection between a switching element of the first pair of switching elements and a first output terminal; and
apply the asserted select signal to a second switching element of the second set of switching elements to provide a connection between a switching element of the second pair of switching elements and a second output terminal.

15. The system of claim 14, wherein each phase signal of the plurality of phase signals has a different phase relative to other phase signals of the plurality of phase signals.

16. The system of claim 14, wherein the first phase signal of the plurality of phase signals applied to the first pair of switching elements of the first set of switching elements has a different phase relative to the second phase signal of the plurality of phase signals applied to the second pair of switching elements of the first set of switching elements.

17. The system of claim 14, wherein the interpolator circuit is further configured to:
provide the interpolated output signal based on output signals at the first output terminal and the second output terminal.

18. The system of claim 17, wherein the asserted select signal is a first asserted select signal, and the interpolator circuit is further configured to:
apply a second asserted select signal to a third switching element of the second set of switching elements to provide a connection between another switching element of the second pair of switching elements and the first output terminal; and
apply the second asserted select signal to a fourth switching element of the second set of switching elements to provide a connection between another switching element of the first pair of switching elements and the second output terminal.

19. The system of claim 14, wherein the load is associated with one or more of a serial ATA (Advanced Technology Architecture) communication system, a read channel, a fiber channel, a wireless baseband communication system, an ethernet XAUI SERDES transceiver, a 1000 BaseTx network, a USB (Universal Serial Bus) 2.0 bus, or a PCI Express bus.