Method and apparatus for generating an oversampling clock signal

• US 7,167,532 B1
• Filed: 04/26/2002
• Issued: 01/23/2007
• Est. Priority Date: 04/26/2002
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Isaac, Ronen, Suissa, Udi, Bronfer, Alexander
• Primary Examiner(s): Kim; Kevin
• Application Number:
  US10/133,609
• Time in Patent Office: 1,733 Days
• Field of Search: 375/355
• US Class Current: 375/355
• CPC Class Codes: H04L 7/042 Detectors therefor, e.g. correlators, state machines digital correlators in general G06F17/15

Abstract

A timing estimation mechanism operative to generate an oversampling clock signal for a large range of reference clock frequencies without requiring use of a PLL. The oversampling timing mechanism generates appropriate timing instances, typically for the purpose of sampling a received data signal in a digital communications system, without requiring a specific external clock source but rather by utilizing a clock source having any arbitrary frequency. The mechanism of the present invention is especially suited for use in applications where a specific external clock source (e.g., integer multiple of the data rate) is not available and wherein the implications of the use of a PLL cannot be tolerated. The oversampling clock estimation mechanism generates a clock signal which may be unevenly distributed over the symbol period, but whereby on average, the correct number of samples is produced over a specific time duration.

Claims

1. A method of determining oversampling timing from a reference clock source for an input symbol stream, said method comprising the steps of:

enabling reference clock cycles periodically an integer number of times during each symbol period, each reference clock cycle enabled resulting in a sampling instance;

enabling reference clock cycles zero or more additional times each symbol period to produce a minimum number of sampling instances per symbol period;

delaying sampling instances zero or more compensation reference clock cycles during each averaging period consisting of one or more symbol periods; and

wherein the timing of said sampling instances is controlled in accordance with the following

$A·<br><br>P=\left(\left(\frac{1}{f_{\mathrm{REF}}}·<br><br>\mathrm{chip\_step}\right)·<br><br>N+\mathrm{SBR}\right)·<br><br>A+K·<br><br>\frac{1}{f_{\mathrm{REF}}}$

wherein A is the averaging period representing the minimum number that can be represented by an integer number of periods of said reference clock, _fREF is the reference clock frequency, P is the symbol clock period, N is the number of chip steps in a symbol period, SBR is the symbol boundary remainder, and K is the number of compensation reference clock cycles to be added over the averaging period.

2. The method according to claim 1, adapted to be implemented in an Application Specific Integrated Circuit (ASIC).

3. The method according to claim 1, adapted to be implemented in a Field Programmable Gate Array (FPGA).

4. A method of determining oversampling timing from a reference clock source for an input symbol stream, said method comprising the steps of:

enabling reference clock cycles a predetermined number of times during each symbol period;

injecting a controlled amount of jitter into said oversampling timing by enabling zero or more additional reference clock cycles each symbol period; and

correcting any accumulated drift in said oversampling timing by enabling zero or more additional reference clock cycles after each averaging period of symbol times.

5. The method according to claim 4, wherein the timing of said sampling instances is controlled in accordance with the following

$A·$

P = ( ( 1 f REF ·

chip_step ) ·

N + SBR ) ·

A + K ·

1 f REF

wherein A is the averaging period representing the minimum number that can be represented by an integer number of periods of said reference clock, _fREF is the reference clock frequency, P is the symbol clock period, N is the number of chip steps in a symbol period, SBR is the symbol boundary remainder and K is the number of compensation reference clock cycles to be added over the averaging period.

6. The method according to claim 4, adapted to be implemented in an Application Specific Integrated Circuit (ASIC).

7. The method according to claim 4, adapted to be implemented in a Field 15 Programmable Gate Array (FPGA).

8. An apparatus for generating oversampling timing from a reference clock source, comprising:

a chip-step counter adapted to be clocked via said reference clock source and to generate a first terminal count every chip-step cycles of said reference clock;

an N step counter adapted to be clocked by said first terminal count signal and to generate a second terminal count every N chip steps, wherein N is a positive integer;

an averaging counter clock adapted to be clocked by said second terminal count signal and to generate a third terminal count signal every averaging period number of N chip steps;

a drift lookup table coupled to the output of said averaging counter and operative to indicate a number of compensation cycles to be added over each averaging period;

means for inserting zero or more reference clock cycles during each symbol period; and

means for inserting zero or more compensation reference clock cycles as indicated by said drift lookup table over each averaging period.

9. The method according to claim 8, wherein the timing of said sampling instances is controlled in accordance with the following

$A·$

P = ( ( 1 f REF ·

chip_step ) ·

N + SBR ) ·

A + K ·

1 f REF

wherein A is the averaging period representing the minimum number that can be represented by an integer number of periods of said reference clock, _fREF is the reference clock frequency,

P is the symbol clock period, N is the number of chip steps in a symbol period, SBR is the symbol boundary remainder and K is the number of compensation reference clock cycles to be added over the averaging period.

10. The apparatus according to claim 8, adapted to be implemented in an Application Specific Integrated Circuit (ASIC).

11. The apparatus according to claim 8, adapted to be implemented in a Field Programmable Gate Array (FPGA).

12. An apparatus for generating oversampling timing from a reference clock source for an input symbol stream, comprising:

means for enabling reference clock cycles a predetermined number of times during each symbol period;

means for injecting a controlled amount of jitter into said oversampling timing by enabling zero or more additional reference clock cycles each symbol period; and

means for correcting any accumulated drift in said oversampling timing by enabling zero or more additional reference clock cycles after each averaging period of symbol times.

13. The apparatus according to claim 12, adapted to be implemented in an Application Specific Integrated Circuit (ASIC).

14. The apparatus according to claim 12, adapted to be implemented in a Field Programmable Gate Array (FPGA).

15. A correlator for recovering the timing from an input symbol stream, comprising:

a plurality of symbol correlator cells, each symbol correlator cell comprising a plurality of data registers clocked by an oversampling clock and adapted to latch input samples spanning a symbol duration, wherein the number of input samples is dynamically set in accordance with an oversampling ratio signal;

means for latching an input data sample either on the rising or falling edge of a reference clock in accordance with an edge select signal;

means for generating an oversampling enable for enabling said plurality of data registers in each symbol correlator cell, said means comprising;

means for enabling reference clock cycles a predetermined number of times during each symbol period;

means for injecting a controlled amount of jitter into said oversampling timing by enabling zero or more additional reference clock cycles each symbol period;

means for correcting any accumulated drift in said oversampling timing by enabling zero or more additional reference clock cycles after each averaging period of symbol times; and

means for correlating said plurality of input samples with a reference value to generate a correlation result therefrom.

16. The correlator according to claim 15, wherein each symbol correlator cell is adapted to latch either seven or eight input samples.

17. The correlator according to claim 15, wherein the number of symbol correlator cells comprises 64.

18. The correlator according to claim 15, wherein a symbol period comprises 1 microsecond.

19. The correlator according to claim 15, wherein said reference clock ranges in frequency from 7 MHz to 100 MHz.

20. The method according to claim 15, wherein the timing of said sampling instances is controlled in accordance with the following

$A·$

P = ( ( 1 f REF ·

chip_step ) ·

N + SBR ) ·

A + K ·

1 f REF

wherein A is the averaging period representing the minimum number that can be represented by an integer number of periods of said reference clock, _fREF is the reference clock frequency, P is the symbol clock period, N is the number of chip-steps in a symbol period, SBR is the symbol boundary remainder and K is the number of compensation reference clock cycles to be added over the averaging period.

21. The correlator according to claim 15, adapted to be implemented in an Application Specific Integrated Circuit (ASIC).

22. The correlator according to claim 15, adapted to be implemented in a Field Programmable Gate Array (FPGA).