Carrier-recovery loop with stored initialization in a radio receiver

• US 6,072,842 A
• Filed: 11/12/1997
• Issued: 06/06/2000
• Est. Priority Date: 11/21/1996
• Status: Expired due to Term

• Current Assignee: DSP Group Incorporated (Synaptics Incorporated)
• Original Assignee: DSP Group Incorporated (Synaptics Incorporated)
• Inventors: Janesch, Stephen T., Bell, Ed, Schnizlein, Paul G.
• Primary Examiner(s): Bocure, Tesfaldet
• Application Number:
  US08/968,029
• Time in Patent Office: 937 Days
• Field of Search: 375/326, 375/327, 375/371, 375/372, 375/373, 375/376, 329/306-309
• US Class Current: 375/326
• CPC Class Codes: H03L 7/093 using special filtering or amplification characteristics in the loop H03L7/087 - H03L7/091 take precedence
• CPC Class Codes: H03L 7/0991 the oscillator being a digital oscillator, e.g. composed of a fixed oscillator followed by a variable frequency divider H03L7/0995 takes precedence; fixed oscillators with means for selecting among various phases H03L7/0814
• CPC Class Codes: H03L 7/146 by using digital means for generating the oscillator control signal H03L7/141, H03L7/143 take precedence
• CPC Class Codes: H04L 1/24 Testing correct operation
• CPC Class Codes: H04L 2027/0028 at passband only
• CPC Class Codes: H04L 2027/0046 Open loops
• CPC Class Codes: H04L 2027/0067 Phase error detectors
• CPC Class Codes: H04L 2027/0069 Loop filters
• CPC Class Codes: H04L 27/22 Demodulator circuits; Receiver circuits
• CPC Class Codes: H04L 27/2273 associated with quadrature demodulation, e.g. Costas loop
• CPC Class Codes: H04L 7/0334 Processing of samples having at least three levels, e.g. soft decisions

Abstract

A carrier-recovery loop for a receiver in a communication system with features that facilitate initialization of the loop. The carrier-recovery loop is a PLL that uses a feedback signal to keep a recovery oscillator phase-locked to the carrier of a received signal. In the present invention, an initializing value of the feedback signal is stored in a memory and provided to a digitally controlled recovery oscillator (DCO). This initializing value brings the recovered signal to an initial frequency that approximates the carrier frequency. When the receivers starts to acquire a phase-lock with the carrier, the carrier-recovery loop is in a condition close to the desired phase lock. Preparing the DCO in this manner imparts a significant improvement to the carrier-recovery loop. The response time for the loop to acquire a phase lock depends in part on its initial frequency offset from the carrier. In general, reducing this initial offset reduces the loop'"'"'s acquisition time. By thus anticipating the frequency of the carrier, this carrier-recovery loop can have an improved acquisition time to reach phase lock. The initializing value of the feedback signal can be generated by recording a sample of the feedback signal when the carrier-recovery loop is phase-locked to a received signal or to an on-board crystal oscillator. The invention also includes a mechanism to correct drifts in the crystal oscillator'"'"'s frequency.

Claims

1. A method for initializing a carrier-recovery loop in a communications receiver, wherein said communications receiver receives a received signal with a received frequency, wherein said carrier-recovery loop comprises a digitally controlled oscillator (DCO), the method comprising:

storing an initializing value of a digital feedback signal in a memory;

providing said initializing value from said memory to said DCO,

said DCO generating a preliminary recovered signal with a preliminary recovered frequency in response to said providing said initializing value to said DCO;

providing said digital feedback signal to said DCO;

said carrier-recovery loop acquiring a phase lock with said received signal in response to said providing said digital feedback signal to said DCO, wherein in said phase lock with said received signal said DCO generates a recovered signal with a recovered frequency;

wherein said recovered frequency approximates said received frequency, and wherein said preliminary recovered signal is useable in said communication receiver to reduce an acquisition time for said acquiring said phase lock with said received signal.

2. The method of claim 1, wherein said carrier recovery loop further comprises a phase detector and a loop filter, and wherein said acquiring said phase lock with said received signal comprises:

providing said received signal, said preliminary recovered signal, and said recovered signal to said phase detector;

said phase detector generating a digital phase-error signal and providing said digital phase-error signal to said loop filter, wherein said digital phase-error signal indicates a phase difference between said recovered signal and said received signal;

said loop filter generating said digital feedback signal in response to said phase-error signal;

said DCO adjusting a frequency of said DCO in response to said digital feedback signal.

3. The method of claim 1, wherein said storing said initializing value of said digital feedback signal in said memory comprises storing a sample of said digital feedback signal as said initializing value in said memory.

4. The method of claim 3, wherein said carrier-recovery loop is used in a time-division duplexing communications unit, wherein said communications unit alternates between receiving data and transmitting data, and wherein said providing said initializing value from said memory to said DCO and said acquiring said phase lock with said received signal occur prior to said receiving data.

5. The method of claim 3, wherein said carrier-recovery loop is used in a time-division multiple-access communications unit, wherein said communications unit alternates between receiving data and transmitting data, and wherein said providing said initializing value from said memory to said DCO and said acquiring said phase lock with said received signal occur prior to said receiving data.

6. The method of claim 3 wherein said storing said initializing value of said digital feedback signal in said memory comprises storing said sample of said digital feedback signal as said initializing value in said memory after said carrier-recovery loop has acquired a prior phase lock with said received signal.

7. The method of claim 1, wherein said carrier-recovery loop further comprises a stable reference oscillator that generates a stable reference signal at a stable reference frequency, and wherein said storing said initializing value in said memory comprises:

providing said stable reference signal to said phase detector;

said carrier-recovery loop achieving a phase lock with said stable reference signal; and

said memory storing a sample of said digital feedback signal resulting from said phase lock with said stable reference signal as said initializing value;

wherein said initializing value represents said stable reference frequency of said stable reference oscillator.

8. The method of claim 7, wherein:

said received signal is generated by a remote transmitter at a remote transmitter frequency; and

said stable reference oscillator is adjustable to compensate for slow frequency drifts;

the method further comprising;

storing a first initializing value when said carrier-recovery loop has acquired said phase lock with said received signal;

storing a second initializing value when said carrier-recovery loop has acquired said phase lock with said stable reference signal;

calculating a difference using said first initializing value and said second initializing value, wherein said difference represents a frequency offset between said stable reference frequency and said remote transmitter frequency; and

adjusting said stable reference oscillator to nullify said frequency offset between said stable reference frequency and said remote transmitter frequency.

9. The method of claim 7, wherein:

said receiver is comprised in a local time-division duplexing (TDD) transceiver;

said received signal is generated by a remote transmitter in a remote TDD transceiver at a remote transmitter frequency; and

said stable reference oscillator is adjustable to compensate for slow frequency drifts;

the method further comprising;

storing a first initializing value when said carrier-recovery loop has acquired said phase lock with said received signal;

storing a second initializing value when said carrier-recovery loop has acquired said phase lock with said stable reference signal;

calculating a local difference using said first initializing value and said second initializing value, wherein said local difference represents a local frequency offset between said stable reference frequency and said remote transmitter frequency;

receiving a remote difference from said remote TDD transceiver, wherein said remote difference represents a remote frequency offset between a stable reference frequency in said remote transceiver and a local transmitter frequency of said local TDD transceiver; and

adjusting said stable reference oscillator to match said local frequency offset with said remote frequency offset.

10. The method of claim 7, wherein:

said receiver is comprised in a local time-division duplexing (TDD) transceiver;

said received signal is generated by a remote transmitter in a remote TDD transceiver at a remote transmitter frequency; and

said stable reference oscillator is adjustable to compensate for slow frequency drifts;

the method further comprising;

receiving a first value from a remote TDD transceiver, wherein said first value represents a local transmitter frequency of said local TDD transceiver with respect to a stable reference frequency in said remote transceiver;

storing a second value when said carrier-recovery loop has acquired said phase lock with said received signal, wherein said second value represents said remote transmitter frequency of said remote TDD transceiver with respect to said stable reference frequency of said stable reference oscillator; and

adjusting said stable reference oscillator to match said first value with said second value.

11. The method of claim 8, or of claim 9, or of claim 10, wherein said adjusting said stable reference oscillator occurs after time periods over which said stable reference oscillator has significant frequency drifts.

12. The method of claim 8, or of claim 9, or of claim 10, wherein said stable reference oscillator comprises a piezoelectric crystal coupled to a varactor diode, and wherein said adjusting said stable reference oscillator comprises changing a bias on said varactor diode.

13. A carrier-recovery loop in a communications receiver that receives a received signal with a received frequency and generates a recovered signal with a recovered frequency, said carrier-recovery loop comprising:

a signal input that receives said received signal;

a phase detector that receives said received signal from said signal input and receives said recovered signal, wherein said phase detector generates a digital phase-error signal that indicates a phase difference between said received signal and said recovered signal;

a loop filter that receives said digital phase-error signal from said phase detector and generates a digital feedback signal in response to said digital phase-error signal;

a memory that stores an initializing value of said digital feedback signal;

a digitally controlled oscillator (DCO) coupled to said memory and coupled to said loop filter;

wherein;

during an initialization of said carrier-recovery loop, said DCO receives said initializing value from said memory, said DCO generates a preliminary recovered signal in response to said initializing value, and said DCO provides said preliminary recovered signal to said phase detector;

after said initialization said DCO receives said digital feedback signal from said loop filter, said DCO generates said recovered signal in response to said digital feedback signal, and said DCO provides said recovered signal to said phase detector; and

said initialization of said carrier-recovery loop reduces an acquisition time for said carrier-recovery loop to acquire a phase lock with said received signal, wherein in said phase-lock said recovered frequency matches said received frequency.

14. The carrier-recovery loop of claim 13 further comprising:

a multiplexer, wherein said multiplexer receives said digital feedback signal from said loop filter, wherein said multiplexer receives said initializing value from said memory, and wherein said DCO is coupled to said multiplexer;

wherein;

said multiplexer is operable to provide said initializing value to said DCO during said initialization of said carrier-recovery loop; and

said multiplexer is operable to provide said digital feedback signal to said DCO after said initialization of said carrier-recovery loop.

15. The carrier-recovery loop of claim 13, wherein said memory is coupled to said loop filter, and wherein said memory is operable to store a sample of said digital feedback signal as said initializing value.

16. The carrier-recovery loop of claim 15, wherein said memory stores said sample of said digital feedback signal as said initializing value after said carrier-recovery loop has acquired said phase lock.

17. The carrier-recovery loop of claim 15, wherein said carrier-recovery loop is used in a time-division duplexing communications unit, wherein said communications unit alternates between receiving data and transmitting data, and wherein said initialization of said carrier-recovery loop occurs prior to said receiving data.

18. The carrier-recovery loop of claim 15, wherein said carrier-recovery loop is used in a time-division multiple access communications unit, wherein said communications unit alternates between receiving data and transmitting data, and wherein said initialization of said carrier-recovery loop occurs prior to said receiving data.

19. The carrier-recovery loop of claim 15, further comprising:

a stable reference oscillator that generates a stable reference signal at a stable reference frequency; and

a front-end multiplexer with a first input and a second input, wherein;

said first input is coupled to said signal input;

said second input is coupled to said stable reference oscillator;

said phase detector is coupled to said front-end multiplexer;

said front-end multiplexer is operable to provide said received signal to said phase detector; and

said front-end multiplexer is operable to provide said stable reference signal to said phase detector;

wherein said memory is operable to store said initializing value when said carrier-recovery loop is locked to said stable reference signal so that said initializing value represents said stable reference frequency of said stable reference oscillator.

20. The carrier-recovery loop of claim 19, wherein:

said stable reference oscillator is adjustable to compensate for slow frequency drifts;

said received signal is generated by a remote transmitter at a remote transmitter frequency;

said memory is operable to store a first initializing value when said carrier-recovery loop has acquired said phase lock with said received signal; and

said memory is operable to store a second initializing value when said carrier-recovery loop has acquired said phase lock with said stable reference signal;

the carrier-recovery loop further comprising a trimming unit for said stable reference, wherein;

said trimming unit receives said first initializing value and said second initializing value from said memory;

said trimming unit calculates a difference using said first initializing value and said second initializing value, wherein said difference represents a frequency offset between said stable reference frequency and said remote transmitter frequency; and

said trimming unit adjusts said stable reference oscillator to nullify said frequency offset between said stable reference frequency and said remote transmitter frequency.

21. The carrier-recovery loop of claim 19, wherein:

said receiver is comprised in a local time-division duplexing (TDD) transceiver;

said received signal is generated by a remote transmitter in a remote TDD transceiver at a remote transmitter frequency;

said stable reference oscillator is adjustable to compensate for slow frequency drifts;

said memory is operable to store a first initializing value when said carrier-recovery loop has acquired said phase lock with said received signal; and

said memory is operable to store a second initializing value when said carrier-recovery loop has acquired said phase lock with said stable reference signal;

the carrier-recovery loop further comprising a trimming unit for said stable reference, wherein;

said trimming unit receives said first initializing value and said second initializing value from said memory;

said trimming unit calculates a local difference using said first initializing value and said second initializing value, wherein said difference represents a local frequency offset between said stable reference frequency and said remote transmitter frequency;

said trimming unit receives a remote difference from said remote TDD transceiver, wherein said remote difference represents a remote frequency offset between a stable reference frequency in said remote transceiver and a local transmitter frequency of said local TDD transceiver; and

said trimming unit adjusts said stable reference oscillator to match said local frequency offset with said remote frequency offset.

22. The carrier-recovery loop of claim 19, wherein:

said receiver is comprised in a local time-division duplexing (TDD) transceiver;

said received signal is generated by a remote transmitter in a remote TDD transceiver at a remote transmitter frequency;

said stable reference oscillator is adjustable to compensate for slow frequency drifts; and

said memory is operable to store a remote transmitter value when said carrier-recovery loop has acquired said phase lock with said received signal;

the carrier-recovery loop further comprising a trimming unit for said stable reference, wherein;

said trimming unit receives said remote transmitter value from said memory;

said local TDD transceiver receives a local transmitter value from said remote TDD transceiver and said local TDD transceiver provides said local transmitter value to said trimming unit; and

said trimming unit adjusts said stable reference oscillator to match said local transmitter value with said remote transmitter value.

23. The carrier-recovery loop of claim 20, or of claim 21, or of claim 22, wherein said trimming unit adjusts said stable reference oscillator after time periods over which said stable reference oscillator has significant frequency drifts.

24. The carrier-recovery loop of claim 20, or of claim 21, or of claim 22, wherein said stable reference oscillator comprises a piezoelectric crystal coupled to a varactor diode, and wherein said trimming unit controls a bias on said varactor diode to adjust said stable reference oscillator.