[time-nuts] Time interval measurement vs dual mixer method

[cdelect at juno.com]

Luciano, 

I don't know of a commercial version.

As you have seen as the two signals move away from close phase
coincidence the system noise level will increase.

This is because the common offset oscillator noise will only cancel when
the phases are closely matching.

This is mainly important for the lower Tau (like 1 to 10 seconds).

I will normally adjust the phase of my reference or DUT so that myTIC is
reading 0.00000XX (close to phase match) and very slowly rising. (with my
setup most DUT will age downward).

Then I start logging the data.

A typical run with a 1 week aged quartz will show the TIC count slowly
increase (and may wrap) and then as the aging continues the count will
reverse and eventually go "below" zero and wrap.

A phase plot will show a nice parabolic looking curve going up and then
as the aging crosses it will curve down.

If there are wraps the plotter program can take them out.

Any long plot of Quartz WILL wrap!

Now if you plot the AD you will see a normal type plot for a good Quartz.

If the TIC count increased well past coincidence only the Higher Taus
will be accurate.

For the lower Tau you need to cut off the data past where it climbed too
high.

I typically will setup as described and run a short log of say 5 minutes.

I might only keep the first 100 Seconds and plot the AD against that.

This will give you an accurate plot for the lower Tau.

These two plots can then be combined to give you the whole range.

The attached plot of an FE405B illustrates this.

The Red plot is against a very good FTS 1200 (2X10-13th at 1 thru just
past 10 Sec)

The blue continuation is against a very good HP 5065A (1.5X10-13th at 100
Sec)

So if the lower Tau are important only use logged data that are in near
phase match!

Hope this helps!

Cheers,

Corby
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