Electronics > Beginners
Read RPM's directly from DSO math functions.
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tautech:

--- Quote from: james_s on September 25, 2019, 05:29:42 am ---This is not really the sort of thing a scope is designed for, I have never found myself wishing I could display RPM directly on my scope.

--- End quote ---
I have and any DSO with a frequency counter is perfectly capable of use as an RPM counter with just a RF loop, near field or even a traditional probe with the ground lead clipped into the grabber.

Use case:
Tune a chainsaw to max permissible factory RPM of 11,000.
11000/60 = ~183 Hz reading required on the DSO.
Circlotron:
^^ Advantage over a normal frequency counter is you can adjust the trigger level to a stable trace and then be sure you have a valid reading.
james_s:

--- Quote from: tautech on September 26, 2019, 01:48:47 am ---Use case:
Tune a chainsaw to max permissible factory RPM of 11,000.
11000/60 = ~183 Hz reading required on the DSO.

--- End quote ---



Well yes, it's possible to do, it's also possible to haul a load of groceries in a Ferrari but it would be a bit silly to complain that the Ferrari lacks adequate cargo space to easily carry a load of groceries since that's not really what it was designed to do.

For that same use case I would whip out my $15 handheld laser tachometer and get a direct reading.
tautech:

--- Quote from: james_s on September 26, 2019, 02:46:31 am ---For that same use case I would whip out my $15 handheld laser tachometer and get a direct reading.

--- End quote ---
And you'd have trouble finding somewhere safe to get an optical reading.

For single cylinder motors there are a few tacho tools the most basic of which is a Sirometer that works reasonably accurately with engine vibration. I've had one for 40+ years.

http://www.treysit.com/english/

They are quite accurate at low RPM however when tuning your $1000+ chainsaw running on a 40:1 or 50:1 mix they just don't give the precision required for long engine life at the revs and lean oil mixes the modern chainsaw requires.

For decades the most valued tachometer for high performance single cylinder engines has been any number of wireless tachs of which probably the earliest was one from Brigg & Stratton.


Lately digital tachos are preferred however there are few that offer fast update rates so to not have a engine screaming at full revs for too long.
Several models from PET have been the industry leading tacho's and resold by several of the big chainsaw manufacturers as their preferred maintenance tool.


So yes if you have a portable DSO and no other means to tune a small motor use of a RF pickup for the ignition pulse is certainly viable, safe and effective. Done it a few times since I left the small engine shop some 35 yrs ago.
james_s:
Ok well I also have a $10 pocket frequency counter I could use with that same RF pickup, again the DSO could do it but would not be the first instrument I drag out to the garage. Or the second.
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