15ps and falling

Yow! Most optical sensors are capacitive output impedance, with photomultipliers and maybe avalanche diodes the exception. Signal to noise is gonna kill it at small timescale.

The classic gear-tooth-sensor with a ferromagnetic gear tooth also has field-diffusion-rate limits, BUT a brass gear variant using diamagnetism should hold up to fast toggling. That'd be what I'd be looking for. If the gear teeth were uniform enough, you can phaselock and get to serious frequencies that way. There was a brief time (NeXT anyone?) when magnetooptic disks seemed like a good idea. But, the optic readout couldn't keep up with magnetic readout technologies, and all fast disks now are magnetic.

yeah but the dominant noise seems to be of mechanical origin. bearing play, torsional vibrations, general flexing etc. so just lots of samples to overcome this.

even the interval is only measured with a 40mhz clock too, the time is averaged with 4000 pulses per rev.

the faster the speed the lower the time error for the same mechanical error in angle.

Colin =^.^=

There's some state-of-the-art bearings (hydraulic bearings; google for "moglice") that might do some good, but the ultimate is levitation. Rogers Ritter and Jesse Beams (at U. Va) have made working systems, should be easy to find in the physics literature.

Yeah I was thinking about magnetic bearings too, probably making some.

its hard to be sure what proportion of the error is originating where.

Colin =^.^=

On Jun 24, 1:54 pm, whit3rd wrote: [....]

I PMT plate looks like a very high resistance in parallel with a capacitor. The capacitance limits the frequency responce.

I have 5.76e9 samples per day so I get a lot of the resolution from averaging.

the mechanical noise is a lot worse than this atm. the noise from comparing the two channels from one encoder is quite low.

Colin =^.^=

Yes, that sort of method is quite a good way to improve the measurement. I have had a few troubles when doing this so I will suggest them as things you need to watch out for.

(1) If the experiment and the measurement system can crosstalk, the really high frequencies from the experiment can sneek into the clock circuit of the measurement system. This makes the two try to lock together and modulates the time base.

(2) The measurement assumes that the timing of the pulse slides to all posible times in the cycle of the timebase. This is almost always true but sometimes gremlins will get in and make it not true. These gremlins only seem to act when the lights are off over night.

thanks,

fortunatly there is only one clock. I have considered using an atomic clock. although the electronics is actually rotating, atomic clocks are affected by magnetic field, crystal clocks are affected by gravity/tilt, although the clock error would have to be quite large to give a significant error in the reading that wasnt canceled out by other means.

The BLDC motor is driven by the same clock that measures the pulse, however the phase is not fixed, and the speed varies by a small error, so this should hold true, plus the optical discs have a small but detectable unintended eccentircity, so this gaurantees a reasonable slide, eqv to several pulses.

I did have a problem with day light though, a small amount geting through to the optos was enough to introduce a ~1ns error, especialy when the sun shone in through the window at dawn.

I have come accross all sorts of problems on the way so far ...

Colin =^.^=