DI-194 (was Re: Data Acquisition newbie question)

[Phil Hunter]

"Don DRI05 Ricciardiello" wrote:

> My single cylinder engine sparks at TDC every
> revolution. Max is 9000 so lets say 10K upper limit
> to round it up. That would require a 166.6 samples
> per second which doesn't give me room for much else.

Don, me thinks you misunderstood, you need an absolute
minimum of 2 samples per revolution (cycle), so you'll
need 333.3 S/s to measure 10K RPM, not 166.6, and as
Bob pointed out, that's assuming a perfect sine or
square wave signal and all you want to know is the
frequency. It's clear that several people on this list
do not understand this important point, it's not
intuitive, following Bob's advice should keep you out
of trouble, 5X to 10X "oversampling" as a minimum for
virtually any waveform you want to record, also the
"anti-aliasing" filter to limit the input frequency
to no more than 1/5 to 1/10 the max S/s if the input
freq is close to the upper limits of the DataAcq
system you choose.

> If I went with Brian's LM2907 suggestion can that
> method be scaled so that 0 to 10K rpm=0 to 10Volts?

I'd assume so. Going the Freq-Volt converter route
is the way I'd do it, considering RPM can't change
instantaneously, the sample rate is far less important
and lets you use the additional A/D channels. The only
other choices you have in this situation is to divide
the RPMs down mechanically or electronically, or you
could go w/ a much more expensive DataAcq system.

One last point, as Brian surmised, power for the unit
comes thru the RS-232 connector, you'll need it hooked
up to a PC while you're recording data.

regards,
phil
(digest)


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