>> If you measure period (using a hole or strip of dot of
>> paint) then the slope widths can reach 10% of the dot width very fast.
>
>Which doesn't matter if you just want frequency.  You merely end up with a
>phase shift in the frequency, but it's still the same frequency.

Olin, I said period. For slow spin you need to measure period to get a
usable realtime rpm readout (rpm is obtained by F=K*1/T where K is related
to t/T of the dot or stripe you use - and to the slope fudge factor I was
talking about). Under about 300 rpm there is no other way (because of
averaging etc).

>> If you happen to be using something like this to adjust engine firing
>> points etc then you want to bother about this probably.
>
>OK, I agree it matters in this application since you are trying to
>measure phase.

Worse, you usually want instanetaneous rpm (with any phase cancelled) if
this is used in a control loop. I leave it to your imagination as to what
happens if the I constant in PID needs to be tuned with frequency because
of the slopes.

When frequency-or-period measurement is used, it is necessary to make sure
that the transition if monotonous in both directions (if not smooth).
Otherwise again control systems based on this may go 'crazy' in the
transition area.

Peter

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