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From: Chris Matthaei
Subject: Re: Automotive EMI
X-Newsreader: NWReader V 0.9.9.10 BETA (c) 2001 by Stefan Reck
NNTP-Posting-Date: Tue, 24 Sep 2002 12:10:49 CDT
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Date: Tue, 24 Sep 2002 17:10:49 GMT
Tom Faloon wrote:
> First you need to establish how the noise is getting onto the amplifie=
> The ignition is certainly a possible source, but so is the car electri=
> You can expect a very 'dirty' supply on automobile electrics. (HF nois=
> very large amplitude spikes.)
> **** Other points ****
> What exactly did you mean by 'when the load on the engine increases'
> Are the sensors connected to ground at the point where they are physic=
> i.e. could they form earth loops.
> Does the amplifier have a solid earth.
> Is the amplifier in a screened box. (Not suggesting that is essential,=
> wondering about the construction.)
> Why 10 Hz cutoff. What decides that?
> ( The lower the better.)
> Tom Faloon
> EMI / EMC
First, thanks to all who responded. What I meant by "load on the engine"=
basically the throttle position. As I approach full throttle, the noise =
worse. The signal is actually pretty clean at idle and part throttle. It=
's at WOT
when the spark voltage has to overcome a significant fuel-air charge tha=
really see the noise. At least that's what I'm guessing is happening. Th=
sensor I'm attempting to monitor is the Mass Air Flow sensor. I figured =
would be adequate for this signal since the fastest I'd expect it to cha=
around 1/10th of a second. I don't have any amplifier at all yet. I have=
MAF signal connected directly to the A/D input.
From the responses I got, I think I'm going to try to make a passive fil=
instead. I'll also power the whole thing from a 12v battery. I'll still =
common ground for the MAF sensor return. I really don't need a perfectly
noise-free signal. A small amount of noise can be averaged out in my
analysis. But I'm currently seeing close to 1v P-P of noise on my 5V sig=
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