Stable composite clock
AssigneeLockheed Martin Corporation
InventorStephen R. McReynolds
Life of this patent
Estimating the term…
Abstract
Apparatus and method for producing a composite clock signal with optimized stability characteristics from individual clocks which have different stabilities or variances. The composite clock signal includes weighted individual clock signals. In a first (PPN) case, an optimum composite clock is a scale-factor-weighted linear combination of clock signals. In another (non-PPN) case, the optimum stable composite clock is the output of a linear filter of the input clocks signals. Both the phases and frequencies of the individual clocks over time are estimated.
Claims3
A composite clock, comprising:
- a plurality of individual clocks, each producing a clock signal, and the ith clock having known Allan variance curve give by a standard curve times a constant λi;
- reading means coupled to said plurality of individual clocks, for simultaneously reading said clock signals;
- and
- means coupled to said reading means and to a source of said variance information, for generating a composite clock signal according to
- x
- ->
- c
-
- (
- t
- )
- =
- ∑
- i
- =
- 1
- n
-
- K
- i
- *
-
- x
- ->
- i
-
- (
- t
- )
- (
- 57
- )
- where
- {right arrow over (x)}c=Composite clock state (estimate of phase and/or frequency produced by the composite clock mechanism);
- * represents multiplication;
- {right arrow over (x)}i=Individual clock states read out by the hardware
- Ki=the gain chosen to optimize the stability of the composite clock, such that
- K
- i
- =
- (
- 1
- /
- λ
- i
- )
- /
- (
- ∑
- j
- =
- 1
- n
-
-
- 1
- /
- λ
- j
- )
- .
- (
- 58
- )
A composite clock comprising:
- a plurality of individual clocks, each producing a clock signal, and the ith clock having known Allan variance curve given by a standard curve times a constant vector
- λ
- →
- i
- =
- [
- λ
- x
- i
- λ
- y
- i
- ]
- ;
- λ
- x
- i
- =
- scale factor for short term (phase random walk) stability errors,
- λiy=scale factor for long term (frequency random walk) stability errors,
- reading means coupled to said plurality of individual clocks, for simultaneously reading said clock signals;
- and
- means coupled to said reading means and to a source of said variance information, for generating a composite clock signal, the vector
- [
- x
- c
- y
- c
- ]
- according to
- [
- x
- .
- c
- y
- .
- c
- ]
- =
- [
- y
- c
- +
- u
- v
- ]
- (
- 60
- )
- where
- [
- x
- c
- y
- c
- ]
- is the phase and frequency;
- xc is phase;
- and
- yc is the frequency,
- u is the estimate of the composite clock phase noise,
- v is the estimate of the composite clock frequency noise, and u and v are given by
- [
- u
- v
- ]
- =
- ∑
- j
- =
- 1
- n
-
-
- K
- →
- j
- *
- [
- x
- .
- j
- -
- y
- j
- y
- .
- j
- ]
- (
- 61
- )
- where
- [
- x
- i
- y
- i
- ]
- are phase and frequency estimates of the ith clock,
- the optimized gains are given by
- K
- →
- i
- =
- [
- K
- x
- i
- K
- y
- i
- ]
- (
- 62
- )
- K
- x
- i
- =
- (
- 1
- /
- λ
- x
- j
- )
- /
- (
- ∑
- j
- =
- 1
- n
-
-
- 1
- /
- λ
- x
- j
- )
- (
- 63
- )
- K
- y
- i
- =
- (
- 1
- /
- λ
- y
- j
- )
- /
- (
- ∑
- j
- =
- 1
- n
-
-
- 1
- /
- λ
- y
- j
- )
- .
- (
- 64
- )
Dependencies are inferred from the claim text.