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| + | ====== Allan Variance, Challenges and Opportunities ====== | ||
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| + | {{:precision_timing:symmetricom_-_allan_variance_-_challenges_and_opportunities.pdf|}} | ||
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| + | The attached white paper from Symmetricom may be of interest. | ||
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| + | Among other things it explains why a common technique is largely a waste of time for tau greater than the inverse PLL bandwidth. | ||
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| + | It also indicates that using a bandpass filter centered on the beat frequency really should be used in DMTD systems. | ||
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| + | However the associated filter phase shift tempco will require that the filter components have low tempcos and will require elaborate temperature control if analog filters are used. | ||
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| + | The best compromise would be to use a high end sound card and use digital filters. | ||
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| + | One can then interpolate to accurately time stamp the zero crossings of the bandpass filter signals. | ||
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| + | Although this method doesn't make optimum use of the signal samples it will implement a more ideal DMTD albeit with a noise level of around 1E-13/Tau. | ||
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| + | However, one is still left with the problem of calibrating the sampling clock. | ||
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| + | Bruce | ||