Multiple input frequency locked loop
First Claim
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.
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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.
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Citations
29 Claims
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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:
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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. - View Dependent Claims (2, 3, 4, 5, 28, 29)
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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:
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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. - View Dependent Claims (7, 8, 23, 24, 25, 26, 27)
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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:
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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. - View Dependent Claims (10, 11, 12)
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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:
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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.
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14. A dual input locked loop circuit including:
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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. - View Dependent Claims (15, 16, 17, 18, 19, 20, 21, 22)
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Specification