Digital signal clock recovery

• US 5,943,378 A
• Filed: 08/01/1996
• Issued: 08/24/1999
• Est. Priority Date: 08/01/1996
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Keba, James Michael, Powell, II, Clinton C.
• Primary Examiner(s): Bocure, Tesfaldet
• Application Number:
  US08/691,081
• Time in Patent Office: 1,118 Days
• Field of Search: 375/355, 375/371, 375/373, 375/360, 327/141
• US Class Current: 375/373
• CPC Class Codes: H04L 7/0083 taking measures against momentary loss of synchronisation, e.g. inhibiting the synchronisation, using idle words or using redundant clocks
• CPC Class Codes: H04L 7/0334 Processing of samples having at least three levels, e.g. soft decisions

Abstract

A clock recovery circuit for recovering a symbol clock (226) includes a level decoder (210) for determining one of a plurality of received states of a demodulated signal (105) during each symbol period of the symbol clock (226). Each of the plurality of received states corresponds to one of at least two modulation levels. The level decoder (210) generates for each of the plurality of received states a sign signal (212) having transitions at central threshold transition times and a magnitude signal (211). An edge selector (220) determines selected central threshold transition times. A synchronizable clock (225) is synchronized by the selected central threshold transition times, resulting in a significant reduction of symbol clock (226) jitter.

Claims

1. A clock recovery element for recovering a symbol clock having a symbol period from a demodulated signal that includes multilevel digital symbols, wherein each of the multilevel digital symbols has been generated having one of at least two modulation levels, said clock recovery element comprising:

a level decoder for determining one of a plurality of received states of the demodulated signal during each symbol period, wherein each of the plurality of received states corresponds to one of the at least two modulation levels, and wherein said level decoder generates for each of the plurality of received states a sign signal having transitions at central threshold transition times and a magnitude signal that indicates the magnitude of the demodulated signal;

an edge selector, coupled to said level decoder, for determining selected central threshold transition times based on the value of the magnitude signal before and after each transition; and

a synchronizable clock, coupled to said edge selector, wherein said synchronizable clock is synchronized by the selected central threshold transition times.

2. The clock recovery element according to claim 1, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of equal magnitudes.

3. The clock recovery element according to claim 1, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of equal magnitudes which do not include one or more small magnitudes.

4. The clock recovery element according to claim 1, wherein the selected central threshold transition times of the sign signal are crossing transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of approximately equal magnitudes.

5. The clock recovery element according to claim 1, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the sign signal has transitioned in a predetermined direction.

6. A radio receiver for recovering a symbol clock having a symbol period from a modulated signal that includes multilevel digital symbols, wherein each of the multilevel digital symbols has been generated having one of at least two modulation levels, said radio receiver comprising:

a receiver element for generating a demodulated signal from the modulated signal;

a clock recovery circuit comprising

a level decoder for determining one of a plurality of received states of the demodulated signal during each symbol period, wherein each of the plurality of received states corresponds to one of the at least two modulation levels, and wherein said level decoder generates for each of the plurality of received states a sign signal having transitions at central threshold transition times and a magnitude signal that indicates the magnitude of the demodulated signal;

an edge selector, coupled to said level decoder, for determining selected central threshold transition times based on the value of the magnitude signal before and after each transition; and

a synchronizable clock, coupled to said edge selector, wherein said synchronizable clock is synchronized by the selected central threshold transition times; and

a latching device, coupled to said synchronizable clock and said demodulated signal, for latching the received states during each symbol period.

7. The radio receiver according to claim 6, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of equal magnitudes.

8. The radio receiver according to claim 6, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of equal magnitudes which do not include one or more small magnitudes.

9. The radio receiver according to claim 6, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of approximately equal magnitudes.

10. The radio receiver according to claim 6, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the sign signal has transitioned in a predetermined direction.

11. A method used in a receiver for recovering a symbol clock having a symbol period from a demodulated signal that includes multilevel digital symbols, wherein each of the multilevel digital symbols has been generated having one of at least two modulation levels, said method comprising the steps of:

determining one of a plurality of received states of the demodulated signal during each symbol period, wherein each of the plurality of received states corresponds to one of the at least two modulation levels, and wherein for each of the plurality of received states a sign signal is generated having transitions at central threshold transition times and a magnitude signal is generated that indicates the magnitude of the demodulated signal;

determining selected central threshold transition times from the central threshold transition times based on the value of the magnitude signal before and after each transition; and

synchronizing a symbol clock by the selected central threshold transition times.

12. The method according to claim 11, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of equal magnitudes.

13. The method according to claim 11, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of equal magnitudes which do not include one or more small magnitudes.

14. The method according to claim 11, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the plurality of received states substantially one half symbol period before and after each of the selected transition times are of approximately equal magnitudes.

15. The method according to claim 11, wherein the selected central threshold transition times of the sign signal are central threshold transition times for which the sign signal has transitioned in a predetermined direction.