[time-nuts] question Alan deviation measured with Timelab and counters

John Miles john at miles.io
Wed Jan 14 01:26:11 EST 2015

> - Can I use Frequency difference mode from Timelab to monitor time intervals ?
> If no is there a way to use the time interval measurement from the counter with
> Timelab to plot ADEV ?

If you feed in frequency samples, it will convert them to phase-difference samples internally, so the program itself doesn't really care.  The use of frequency data has a few drawbacks such as less accurate ADEV plots due to the counter's dead time between readings, but it's the easiest way to get started and is perfectly usable for many purposes.

In general you should avoid letting the counter do any averaging.  Except in very specific circumstances, any apparent improvement in ADEV measurement floor will be illusory.   There are exceptions, but this isn't something you want to mess with until you're very comfortable with the rest of the measurement process.  Your counter's true ADEV measurement floor at t=1s should be assumed to be close to its single-shot resolution specification (e.g., 100 ps = about 1E-10).

> - In case the principle of plotting ADEV from Time Interval, what is the
> interpretation of the result ? The ADEV shows the relative stability between the
> two GPSDO... So, practically what does it bring ? And how to use this method if I
> want to characterize a device ?

An ADEV graph shows frequency stability statistics at different intervals, ranging from the rate at which the readings are returned from the counter (tau zero, at the left end of the plot) to a maximum interval that's related to how long you let the measurement run.  It's much too deep a subject to go into in an email; see http://www.ke5fx.com/stability.htm for more pointers.

Again, TimeLab always plots ADEV from time interval/phase data, even if you give it frequency readings.  ADEV is fundamentally a frequency stability metric, but it can be computed identically from either TI or frequency samples (assuming zero dead time).  
> - stupid question on Timelab. If I let Timelab in Auto to select the period
> between two samples (correctly detected), the time scale of the graph is wrong.
> For instance, a 3h plot stops at 2000s (0.5h)... Here again, I miss something but
> what ?

The TimeLab manual, for one thing. :)  Hit the books (specifically http://www.miles.io/TimePod_5330A_user_manual.pdf , page 31).

-- john, KE5FX
Miles Design LLC

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