[time-nuts] Measuring frequency

[Dr Bruce Griffiths]

Hal Murray wrote:
> If I understand things correctly, when a box like the 5334 takes a sequence 
> of frequency measurements, each measurement has a start time and a stop time. 
>  For each measurement, you get out the number of ticks (including fraction) 
> on the input signal between those times converted into reasonable units.
>
> What if a sequence of measurements combined the stop from one measurement 
> with the start of the next measurement?  Then any error in one measurement 
> has the same magnitude but opposite sign in the next measurement.  How much 
> does that help in the downstream data processing?
>
> That would be easy to implement that with a FPGA.
>
> This seems like an obvious idea.  What's it called and/or what is a clump of 
> non-linked measurements called?
>
>
>
>   
Hal

Its already been implemented in various "counters" (HP5371, HP5372 etc.).
That's how zero dead time sequential measurements are taken with a 
reciprocal counter.
The time stamp at every Nth input signal zero crossing with positive 
slope (or negative if selected) is recorded.
The corresponding frequencies are then calculated in software
The advantage is that only a single interpolator is required for each 
measurement channel.
For frequency measurements any time offset in the interpolator and other 
circuitry cancels on subtraction.

Zero deadtime between measurements is highly desirable in that there is 
no need to correct for the effect of finite deadtime when calculating 
the Allan deviation, MVAR and other statistical measures of frequency 
stability. Assumptions about the statistical characteristics of the 
oscillator instability are necessary to make the deadtime corrections.

Bruce