[Diy_efi] GM tach RC filter

Marteney, Steve Steve_Marteney at DRSOptronics.com
Mon Sep 23 16:39:53 GMT 2002


Well, I can calculate some values in the filter.

Let's set the 3dB point of a single-pole RC filter for 10k rpm.  So, 10krpm
is 167 revs per second x 4 coil firings per rev (for a typ. V8) equals 667
Hz.  f(3db) = 1 / (2*pi*R*C).  Pick a standard resistor value, probably
somewhat low like maybe a 1k.  That makes the cap want to be about 240nF.
Okay, so maybe we choose the cap, a standard 1uF let's say.   That makes the
resistor want to be 238 ohms.  So, we'll choose a standard 237 1% resistor
value, or a 240 ohm 5% resistor.

Is it possible the 2nd RC section is meant to be a high-pass filter set-up
to create a band-pass around the rpm range?

I guess the reson for the filter is most tach signals are taken from the
negative side of the coil which has a lot of nasty inductive kickback and
ringing and overshoot and noise and....  so the filter helps tame the signal
to make the tach a little more accurate, I suppose, maybe just smoother.

I've used this filter technique in the digital realm with sampled coil
signals where I looking for edge events.  I think my 3dB cutoff was set at
15000rpm for a V16 motor (not practical, just a datapoint) and they clean up
the signal nicely.  But, I think I used a two-pass digital mean filter which
would equate to two identical RC filters cascaded.  More than you wanted to
know I'm sure.

Steve

-----Original Message-----
From: Perry Harrington [mailto:pedward at apsoft.com]
Sent: Saturday, September 21, 2002 6:24 PM
To: diy_efi at diy-efi.org
Cc: efi332 at diy-efi.org
Subject: [Diy_efi] GM tach RC filter


Hi all,

I was looking through the schematic of my Corvette wiring harness and
noticed
that GM uses a 2 section RC filter on the tach input.

I imagine there is a good reason they did this, and I wanted to experiment
with it for my own pursposes.

Does anyone know the values of the components in the filter?

I've seen an RCR tach filter schematic for the filter used on Fieros, but a
search didn't turn up the 2 section RC filter that is used in the Vette.

Any information would be greatly appreciated.

Thanks,

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation,
Inc	
perry at dainst dot com
http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary
safety
deserve neither liberty or safety. Nor, are they likely to end up with
either.
                             -- Benjamin Franklin

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