Method and apparatus for synchronizing master and local time base systems

• US 4,005,266 A
• Filed: 07/14/1975
• Issued: 01/25/1977
• Est. Priority Date: 07/14/1975
• Status: Expired due to Term

• Current Assignee: Singer Company (SVP Holdings Ltd.)
• Original Assignee: Singer Company (SVP Holdings Ltd.)
• Inventors: Coralnick, Philmore, Lehr, Stanley, Blauth, Gary William
• Primary Examiner(s): Curtis, Marshall M.
• Application Number:
  US05/595,629
• Time in Patent Office: 561 Days
• Field of Search: 343/225, 178/69.5 R
• US Class Current: 375/356
• CPC Class Codes: G04G 7/00 Synchronisation radio-controlled time-pieces G04R
• CPC Class Codes: H03L 7/06 using a reference signal applied to a frequency- or phase-locked loop
• CPC Class Codes: H04J 3/0682 by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging

Abstract

A plurality of widely separated local time base systems are synchronized with a master time base system by means of digitally encoded radio signals. All of the time base systems comprise oscillator driven digital clocks which produce a cyclically recurring series of sequential time slots which are individually assigned to the master and local systems to provide time division multiple access to the master system from each local system. Coarse time synchronization is obtained by the transmission of a coarse synchronization signal from the master to each local system at the beginning of the series of slots in the master'"'"'s time base which resets all local time bases to the beginning of the series of slots. Fine synchronization is made during the individual time slot assigned to each local system by the transmission of fine synchronization request and fine synchronization reply signals which are utilized to produce a synchronization error signal in each local system. The error signal is employed to generate frequency and phase corrections for the local oscillator and time corrections for the local clock in accordance with a programmed control state sequence in which the oscillator and clock correction gains used for a particular correction are determined by the number of system cycles between fine corrections and the magnitude of the error. A digital filter technique is utilized which approximates the operation of a simple Kalman filter.

Claims

1. A time base synchronization system for synchronizing a local time base consisting of a cyclically recurring series of sequential time slots generated by a voltage controlled local oscillator and local digital main counter with a master time base consisting of a cyclically recurring series of sequential time slots generated by a master oscillator and master main digital counter comprising coarse sycnhronizing means for bringing said time bases into coarse synchronization, said coarse synchronizing means comprising

means coupled to the master main counter for generating a coarse synchronization signal at the beginning of the first time slot in said series of time slots in the master time base, and

means coupled to the local main counter and responsive to said coarse synchronization signal for adjusting the local main counter to begin generating the said first time slot in the series of slots in the local time base upon receipt of said coarse signal;

and

fine synchronizing means for bringing said time bases into fine synchronization, said fine synchronizing means comprising

means coupled to the local main counter for generating a fine synchronization request signal at the start of a predetermined one of the remaining time slots in the local time base,

means including a master secondary counter coupled to the master main counter for measuring the elapsed time between the receipt of said fine synchronization request signal by the master main counter and the beginning of said predetermined one time slot in the master time base and for generating a fine synchronization reply signal at the beginning of a time period before the end of said predetermined one slot in the master time base which is equal to said elaspsed time,

means including a local secondary counter coupled to the local main counter for measuring the elapsed time between the receipt of said fine synchronization reply signal by the local and the end of said predetermined one time slot in the local time base to produce a digital synchronization error signal,

means for computing from said digital synchronization error signal a frequency error, time error and phase error,

means for coupling said frequency and phase errors as corrections to said local oscillator and said time error as a correction to said local main counter,

synchronization state logic means obtaining as inputs said coarse synchronization signal and fine synchronization reply signal for generating a time varying gain output in dependence on the number of cycles of said time slots between error corrections, said gain outputs coupled respectively to said local oscillator and said local main counter to control the gain at which said correction signals are entered thereto,

error limit logic means coupled to said synchronization error measuring means for producing error limit signals when said error exceeds a predetermined range of values, and

sequencing logic means having as inputs said fine synchronization reply signal and the error limit signal output of said error limit logic means and having outputs coupled to to said synchronization state logic means and said means for computing and responsive to said fine synchronization and error limit signals for producing said sequencing signals to cause said synchronization system to follow a programmed control state sequence in which transfer between control states for large initial errors is made automatically for a predetermined number of sequentially occurring corrections and is dependent upon the magnitude of the synchronization error for subsequent corrections.

2. A time base synchronization system as claimed in claim 1 wherein

the local time base system is remotely located from the master time base system, and

said coarse and fine synchronization signals are transmitted between said local and master systems by means of wire.

3. A time base synchronization system as claimed in claim 1 wherein

the local time base system is remotely located from the master time base system, and

said coarse and fine synchronization signals are transmitted between said local and master systems by means of radio transmissions.

4. A time base synchronization system as claimed in claim 3 wherein said coarse synchronization, fine synchronization request and fine synchronization reply signals are digitally encoded radio signals.

5. A time base synchronization system as claimed in claim 1 wherein

said local oscillator adjusting means comprises an up-down digital counter having the output thereof coupled to the control voltage input of said local oscillator through a digital-to-analog converter,

said local oscillator correction signals comprise digital input signals to said up-down counter representing the magnitude and sign of the frequency correction, and

the local oscillator frequency correction gain is controlled by controlling the bit in said up-down counter where the digital input signals representing the magnitude of the frequency change are applied.

6. A time base synchronization system as claimed in claim 5 wherein phase adjustment of the local oscillator output is obtained by introducing a predetermined increment of frequency into the oscillator output for a predetermined time period and then removing the frequency increment at the end of said time period, to thereby adjust the phase of the oscillator output by an amount equal to the product of the frequency increment and said period of time.

7. A time base synchronization system as claimed in claim 1 wherein

the control state for the first cyclic correction has a unity main counter correction gain and a zero oscillator correction gain to allow for the correction of large initial phase errors,

the control states for the second through the fifth cyclic corrections have a unity main counter correction gain and a high oscillator correction gain to allow for the correction of large initial frequency errors, and

the steady state control states for the sixth and subsequent cyclic corrections have a low oscillator correction gain and a main counter correction gain which is equal to a low gain plus a variable gain which is a function of the number of time base cycles between corrections.

8. A time base synchronization system as claimed in claim 7 wherein the synchronization system remains in the control state for the sixth cyclic correction for so long as the measured synchronization error does not exceed a predetermined amount for a predetermined number of subsequent successive cyclic corrections and is returned to a control state having higher oscillator and counter correction gains if the error exceeds said predetermined amount for said predetermined number of subsequent cyclic corrections.

9. A time base synchronization system as claimed in claim 1 wherein the local main counter is corrected in time by causing it to count at double its normal rate upon receipt of said fine synchronization reply signal for control states having large main counter correction gains.

10. A time base synchronization system for synchronizing a local time base system with a master time base system, the master time base system adapted to transmit a coarse synchronization signal and a fine synchronization reply signal, in response to a request from the local time base system, said local time base system including means for receiving said coarse synchronization signal and fine synchronization signal, said synchronization system comprising:

a. a controllable oscillator;

b. means obtaining an input from said oscillator for counting the pulses therefrom, said means being reset to zero in response to receipt of said coarse synchronization signal and providing a fine synchronization request output signal at a preset count;

c. error measuring means for measuring the synchronization between the local and master time base systems having as inputs an output from said means for counting having a predetermined time relationship with said fine synchronization request output signal and said fine synchronization reply signal;

d. means having as an input the synchronization error output of said error measuring means for generating a frequency error signal, a phase error signal and a time error signal;

e. means for providing a control input to said oscillator; and

f. means for respectively coupling said frequency and phase error to said means providing control input and said time error to said means for counting.

11. Apparatus according to claim 10 wherein said oscillator is a voltage controlled oscillator and said means providing a control input to said oscillator comprise a digital to analog converter having its analog output coupled as an input to said oscillator and an up/down counter having its outputs coupled to the digital inputs of said digital to analog converter and wherein said means for coupling said frequency error signal comprise means for incrementing said counter up and down in dependence on the magnitude and sign of said frequency error.

12. Apparatus according to claim 11 and further including synchronization state control logic means having as inputs said coarse synchronization signal said fine synchronization reply signal said synchronization control logic means arranged to provide a frequency gain output and a time correction gain output said gain output being respectively coupled to said up/down counter and said means for counting to control the stage of each of said means at which said frequency error and time error are entered.

13. Apparatus according to claim 12 wherein said means for determining said frequency, phase and time errors comprise serial arithmetic computation means and further including sequencing logic means having as an input said fine synchronization reply signal and responsive to the receipt of each of said reply signals to carry out a sequence in which said frequency error, time error and phase error are sequentially determined in said arithmetic unit, said sequencing logic means further providing sequencing output signals to said means for selectively coupling said error signals.

14. Apparatus according to claim 13 wherein said synchronization state control logic is arranged to reduce said gain output as the number of fine synchronization replys increases.

15. A time base synchronization system as claimed in claim 14 wherein phase adjustment of the local oscillator output is made by changing the local oscillator output frequency by a predetermined increment of frequency for a predetermined period of time and then eliminating the incremental frequency change, to thereby adjust the phase of the oscillator output by an amount which is equal to the product of the incremental frequency change and the said time period.