Multiple input frequency locked loop

• US 5,751,777 A
• Filed: 05/03/1996
• Issued: 05/12/1998
• Est. Priority Date: 05/03/1996
• Status: Expired due to Term

• Current Assignee: Microsemi Frequency And Time Corporation
• Original Assignee: Symmetricom Incorporated (Microchip Technology Incorporated)
• Inventors: Zampetti, George
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Roundtree, Joseph
• Application Number:
  US08/642,814
• Time in Patent Office: 739 Days
• Field of Search: 375/376, 375/200, 375/327, 375/356, 375/362, 331/1, 331/16, 331/25, 331/69, 331/70, 331/158, 455/20, 455/12.1, 455/76, 455/86, 455/260
• US Class Current: 375/376
• CPC Class Codes: G04G 7/00 Synchronisation radio-controlled time-pieces G04R
• CPC Class Codes: H03L 7/08 Details of the phase-locked loop
• CPC Class Codes: H03L 7/087 using at least two phase detectors or a frequency and phase detector in the loop
• CPC Class Codes: H03L 7/143 by switching the reference signal of the phase-locked loop
• CPC Class Codes: H04J 3/0688 Change of the master or reference, e.g. take-over or failure of the master

Abstract

A dual locked loop is disclosed comparing preferably a GPS signal with an E1 signal and the E1 signal with the output of the loop. The GPS signal is low pass filtered to provide a low pass filtered GPS versus E1 signal that is used as a calibration for a closed loop having a second low pass filter for filtering the comparisons of the E1 and the output signal. By appropriately selecting the filter parameters, the output stability can track the stability of the local oscillator driving the NCO for short term stability, the medium term stability of the E1 signal and the long term stability of the GPS signal.

Claims

1. A multiple source loop responsive to at least two input signals and a oscillator signal, each of the three signals having a frequency associated therewith, each signal having differing frequency stability characteristics such that one signal nominally has the best long term stability, a second signal nominally has the best intermediate term stability and a local signal having nominally the best short term stability, the dual source loop including:

first difference detection means for detecting a first difference between the two input signals, the first difference comprising one of a group of frequency and phase differences,

second difference detection means for detecting a second difference between one of the input signals and the oscillator signal, the second difference comprising one of a group of frequency and phase differences;

an oscillating means to provide the oscillator signal having short term, intermediate term and long term stability characteristics;

filtering means responsive to both of the differences to control the oscillating means such that the short term stability of the oscilliator signal tracks the short term stability of the first oscillator signal, the intermediate term stability of the first signal and the long term stability of the second signal.

2. The loop of claim 1, wherein the filtering means comprises:

a first low pass filter responsive to the first difference detector and having a first frequency response;

a second filter responsive to the second difference detector and having a second frequency response; and

the oscillator being responsive to the output of both of the filters.

3. The loop of claim 2, wherein the output of the first filter is coupled to the input of the second filter.

4. The loop of claim 3, wherein the output of the first filter is coupled to the output of the second filter.

5. The loop of claim 2, wherein each filter has at least one parameter that alters the frequency response of the filter;

means for determining the relative stability of the signals with respect to each other; and

means for altering the filter coefficients to maximize the overall stability of the output signal.

28. The loop of claim 3, wherein the output of the first filter is subtracted from the output of the first detection means.

29. The loop of claim 1, wherein the difference detectors measure the phase difference with zero dead time.

6. A method of providing a clock signal having a stability characteristic curve based upon the three separate input signals, each of the three signals having its own stability characteristic curve, each of the stability characteristic curves of the input signals being alterable in response to environmental conditions, the method comprising:

comparing at least one for the frequency and the phase of each input signals with at least one of the frequency and the phase of another of the input signals;

filtering the comparisons with alterable filters;

outputting an oscillatory signal based upon the filtering; and

adjusting the filtering such that the frequency stability for the output is maximized.

7. The method of claim 6, wherein the stability curves of the adjusting of the filters tracks causes the stability curve for the output for the oscillator to substantially track the most stable portions of the stability curve for each of the three input signals.

8. The method of claim 6, wherein the outputting of the oscillatory signal is done through one and only one synthesizer.

23. The method of claim 6, wherein one of the input signals is responsive to the output oscillatory signal, whereby a closed loop is formed.

24. The method of claim 6, wherein a fourth input signal is provided, the method further including further comparing at least the frequency or the phase of the fourth input signal with at least the phase or the frequency signals of two of the other input signals, the filtering of the comparisons involving filtering said further comparisons.

25. The method of claim 6, wherein each of the input signals has a nominal stability curve with short term, intermediate term and long term stability, the curves for the three input signals having cross-over points, one of the nominal curves having the best short term stability, a second having the best nominal intermediate stability and a third having the best nominal long term stability, the method further comprising altering the filtering such that as the stability curves of the various signals alters, the overall stability of the output is maximized.

26. The method of claim 25, wherein the method further includes filtering in bandwidths, the altering of the filters altering the bandwidths of the filters.

27. The method of claim 25, wherein the filtering acts as a low pass filter to the signal with the best short term stability, a bandpass filter to the signal with the best intermediate term stability and a highpass filter to the signal with the best long term stability.

9. A loop apparatus for providing an oscillatory signal having a frequency stability based upon at least three input signals having different stability characteristics, whereby combining the stability characteristics of the three signals provides an optimal stability characteristic, the loop apparatus comprising:

comparison means for determining one of a phase and a frequency difference for two different pairs of the input signals;

filtering means responsive to the comparison means to generate control information;

no more than one synthesizer, said synthesizer having a frequency stability characteristic and responsive to the control information to provide an output signal such that the control information causes the frequency stability characteristic to substantially track the optimal stability characteristic.

10. The loop apparatus of claim 9, wherein each of the signals includes noise and biases the loop further including

estimation means for estimating corrections of at least some of the differences.

11. The loop apparatus of claim 10, wherein the filtering means performs filtering on the differences with altering filtering parameters.

12. The loop apparatus of claim 11, wherein the filter parameters are altered to maximize the frequency stability of the output signal.

13. A method for generating an output signal having a frequency stability based upon a locally generated signal, a signal received over a landline connection and a signal received over a radio link, the method comprising:

repeatedly measuring one of a frequency and phase difference information between the locally generated signal and the landline signal to provide first differences;

repeatedly measuring one of a frequency and a phase difference between the radio link signal and one of the locally generated signal and landline signal to provide second differences;

filtering the first differences with a first filter to provide a first filtered differences;

combining the first filtered differences with the second differences to form a first combined differences;

filtering the first combined differences to provide second filtered set;

combining the first combined set with the second filtered differences to form second combined differences; and

controlling an oscillator with the second combined differences to provide an output clock signal having a frequency dependent upon at least the landline signal and the radio signal.

14. A dual input locked loop circuit including:

a first difference detector detecting either a phase or a frequency difference between two signals;

a second difference detector detecting either a phase or a frequency difference between one of the first two signals and a third signal;

an open loop responsive to the output of the first detector providing a filtered output of the first difference detector; and

a closed loop responsive to the output of the open loop and the second difference detector to generate the third signal.

15. A dual locked loop as set forth in claim 14, the dual locked loop including a third difference detector detecting either a phase or a frequency difference between a fourth signal and the third signal;

and

the closed loop also being responsive to the third difference detector.

16. The dual locked loop as set forth in claim 15, wherein each signal has a nominal short, medium and long term stability, the third signal is generated by a local oscillator having nominally the best of the short term stability, and nominally inferior medium and long term stabilities;

the fourth and said one of the first two signals having nominally approximately the same, short, medium and long term stability and the medium term stability of the fourth signal and said one of the first two signals being better than the stability of the other of the first signal and the third signal, and the other of the first two signals having the best long term stability.

17. The dual locked loop of claim 16, wherein the control loops include filters for filtering the measured differences according to predetermined filter algorithms based upon parameters.

18. The dual locked loop of claim 17, wherein the parameters are altered as the stability of the various signals changes.

19. The dual locked loop of claim 15, wherein the closed loop comprises a weighted average responsive to the measured differences between the third signal and said one of the first two signals and the measured differences between the fourth signal and the third signal.

20. The dual locked loop of claim 19, wherein the weighted average is obtained by implementing through a processor a first closed loop on said one of the first two signals with the third signal to provide a first software loop output and a second closed loop on the fourth signal and the third signal to provide a second software loop output and taking a weighted average of the two software loop outputs.

21. The dual locked loop of claim 19, wherein the weighted average is based upon the relative stability of said one of the first two signals and the fourth signal.

22. The dual locked loop of claim 21, wherein the weighting further takes into account the relative stability of the other of the first two signals.