I have not read the entire thread, just the last bits, so I might have
missed something... That said, if I understand correctly there is still some
uncertainty regarding the gear ratio, correct?
It should be 29:1 , but it just
might be 28.9:1.
In the measurement in this image it's indeed rather difficult to guess if it's 29:1 or 28.9:1.
Maybe you've already done what I'm about to suggest, in which case disregard.
But if not, one possibility is this:
1 - Trigger on the positive edge of the blue signal
2 - change timebase such that you get about 2.5 full waveforms in view.
If you use 1500 RPM ==> 25 revs/sec, then a timebase of 10 ms / div
should do just that.
3 - Turn on persistence
4 - Use the right trigger type ....
And the "right" trigger type depends a bit on the capabilities of your
scope. If it can do equivalent time sampling, use that. If it doesn't
have ETS then you probably can do a divide-by-N. As in set it to trigger
on every Nth positive going edge of the blue signal. So what N to choose?
The two candidate ratios and their prime decompositions are:
29:1 --> 290/10 = 290/(2*5) = 29/1
28.9:1 --> 289/10 = 289/(2*5) = (17*17)/(2*5)
^^^--------------------------- See (*) comment below about 2's in the denominator.Triggering is always on the blue signal.
Now if you trigger on every 290th edge, with persistence on then:
If you have a 29:1 gear then for those 290 edges in the blue signal you
should get 10 edges in the orange signal. Nice integer multiples. If on the
other hand you happen to have a 28.9:1 gear then no nice integer multiples.
Possible results:
A1 - Nice clean edges for the orange signal ==> absence of bad news, probably good news (29:1 as you expect)
A2 - Orange signal smeared all over the place ==> Nope, that's no 29:1 gear ratio
And as a sanity check do it the other way around. Trigger on every 289th edge, again with persistence on.
Now the possible results are:
B1 - Nice clean edges for the orange signal ==> absence of good news, probably bad news (28.9:1, nooooo)
B2 - Orange signal smeared all over the place ==> highly unlikely it's a 28.9:1 gear ratio
What you want is results A1 and B2. Then it's pretty sure that it is a 29:1 gear, and not a 29.9:1 gear.
What you definitely do not want is results A2 and B1, because then in all likelyhood it's a 28.9:1 ratio
as per the datasheet that tatus1969 mentioned.
Is say things like "pretty sure" and "probably" because I've seen enough
funky shit with triggering on some scopes. Anyways, you will be able
to judge that since you know your scope. Case in point, on my HP boat anchor
I would trust the results and say "sure" and "certain". But doing those same
measurements on my rigol I would have to stick with "pretty sure" and "probably".
(*) If you worry about a factor of 2 because I "accidentally" used a factor of
29 instead of 58, then no worries. I did that accidentally on purpose. We can
happily ignore this factor of 2 because both ratios have a 2 in the denominator
of the prime decomposition.
So if you insist on a factor of 58 then you still get similar results. The main
difference being that now for every 290
(**) blue edges, you would get 5 orange
edges instead of 10.
(**) Well, either 290 or 289, depending on it being a 29:1 or a 28.9:1 gear.
Anyways, hope this helps in finding out what's going on.
PS: Assuming that 1500 RPM really is 1500 RPM, then based purely on the delta T for Cursor 1 the ratio is closer to 28.9:1 than 29:1.
Depends a bit on the standard deviation. Oh yeah, almost forgot to mention that. If you turn on the statistics for that
Cursor 1 then you might be able to do an educated guess too. If the stddev on that 1.154 s is high, then chances are good that it is not 29:1.
If the stddev is low, then it probably is 29:1. And this "high" and "low" being relative to the period of the blue signal. Quick guess ... stddev
larger than 1/3 * period of blue signal ==> probably not an integer multiple, so no 29:1 ratio.