Checking.

Since my last post I had performed a further four trials with the average of all of them being -0.47% the accepted speed in air. It seems I'm hitting a wall in terms of the achievable accuracy with this setup.

Below is a capture of the noise present. Around the trigger point on Ch1 / Ds the timing is accurate, although I'll admit to not knowing at this stage why the apparent trigger is offset by about a nano second or less from the actual trigger point.

Anyway I used the cursors to mark the sweet spot on Ch1 and the expected 60.8ns ToF. From the Cursor B point we see a -4.8 and +8.8ns error. If we take the average of those two numbers we arrive at a potential +2ns bias in our results.



However if I adjust all the results by 2ns it goes way off to about +2.62%. That's to quick.

A bias adjustment number that does work perfectly though is about 0.5ns, hey wait a minute that's close to that weird offset between the trigger and the stable part of the wave? I wonder.


One strange thing I'll need to think about though is why does Ds always see a smaller signal then Df. The Start detector is on the reflect face of our beamsplitter but I brought a 50R/50T plate specifically so I would not see any difference?

I spent most of the afternoon swapping cables, scope channels, detectors around to reassure myself that they were all good and they are or at least within my ability to measure any difference. So I'm confident the signal is effected by the beamsplitter itself, I have it at the 45 degrees so what gives?