I recently made some measurements between 3 oscillators. It wasn't a
true 'Three-Cornered Hat' measurement because the measurements were made
sequentially. When I do the three-cornered hat calculation for the
hopefully 'better' oscillator, I end up trying to take the square root
of a negative number. Is that a red flag that the data is invalid? If
I ignore the minus sign, the results seem reasonable and I can
successfully calculate from the result for each oscillator back to the
measured results.
Ed
On 5/1/2013 12:08 PM, Tom Van Baak wrote:
Folks,
I asked about 3 cornered comparisons some time ago, and now have plenty
of data to start exploring, and way more than the 3 oscillators I had
when I asked six months ago.
Apart from Stable32, which I do not have access to, are there any free
tools which will allow me to perform 3 cornered comparisons between my
oscillators?
Thanks,
david
I use Excel on the { tau, ADEV } pairs when I want to get fancy, or just a
calculator for something quick. You can sometimes simply eyeball it on a
composite log-log adev plot.
I'll give the formula is below, but to understand, first consider this
backwards example:
Suppose for some tau the ADEV of three oscillators is 6e-12, 8e-12, and 10e-12,
respectively. But -- you don't know that yet -- because all you have is
pairwise measurements. The assumption is that noise is rms additive. Let's do
the numbers:
When you measure A vs. B you should get 1.00e-11, since that is sqrt( 6e-12 ^ 2
+ 8e-12 ^ 2 ).
When you measure B vs. C you should get 1.28e-11, since that is sqrt( 8e-12 ^ 2
+ 10e-12 ^ 2 ).
When you measure C vs. A you should get 1.17e-11, since that is sqrt( 10e-12 ^
2 + 6e-12 ^ 2 ).
So given your three ADEV measurement pairs (AB=1.00e-11, BC=1.28e-11,
AC=1.17e-11) you just work backwards to compute ADEV for A, B, and C, as in:
A = 0.707 * sqrt(+ AB*AB - BC*BC + AC*AC)
B = 0.707 * sqrt(+ AB*AB + BC*BC - AC*AC)
C = 0.707 * sqrt(- AB*AB + BC*BC + AC*AC)
Depending on how clean your measurement system is, and how well-behaved and
modestly different the oscillators are, the 3-hat technique can work pretty
well.
/tvb
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