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From: jsg@aero.ARPA (Jeffrey S. Gruszynski)
Newsgroups: net.auto.tech
Subject: Ignition noise
Message-ID: <506@aero.ARPA>
Date: Wed, 16-Oct-85 12:52:44 EDT
Article-I.D.: aero.506
Posted: Wed Oct 16 12:52:44 1985
Date-Received: Sat, 19-Oct-85 07:52:13 EDT
Reply-To: jsg@aero.UUCP (Jeffrey S. Gruszynski)
Organization: The Aerospace Corp., El Segundo, CA
Lines: 61


	Ignition noise on stereos:


	Ignition noise is wideband electromagnetic interference (EMI) generated
by automobile ignition systems.  It is caused by a combination of electrical
interactions in this system.  The necessary factor for this EMI is the presence
of a spark gap in the circuit.  Electrical sparks and arcs (1) all have 
negative electrical resistances, i.e. an increase in current produces a 
decrease in voltage across the gap.  This means that electrically (and 
mathematically) positive (garden variety) resistances can be reduced or 
cancelled.  Positive resistances include resistances in wires and components.
This is not to say that no power is dissapated when the resistances are
cancelled, but rather the resistance _looks_ like zero to other components
in the system.  This is the problem, the other components.  The ignition
system has stray inductances (L) and capacitances (C).  When the resistances
of a LC system are removed the system will ring at characteristic
resonance frequencies.  This is the ignition produces interfering RF.
It has formed an oscillator.  For a variety of reasons the RF is wideband.
These include:  distributed nature of the LC's and random fluctuation of 
spark length (i.e. negative resistance) over time.

	Based on this information and a little knowledge of RF we can
conclude a few things:

	Cause:	The RF is being received by the radio itself.
	Solution:
		1. Reduce RF generation.
			Use resistor plugs, suppression wire
				(introduces "+" resistance to cancel "-")
		2. Reduce RF emissions.
			Shield plugs and wire

	Cause:	The RF is being received via a ground fault.
	Solution:
		1. Reduce RF sensitivity of power lines
			Filter power supply.  Only DC should pass.
			Add a low-pass filter.

			Make sure ALL audio equipment grounds connect
			  to single point with the shortest length of
			  power cable. (2)

(1)  Sparks and arcs are members of a class of phenomena that result from
electrical breakdown of gas and subsequent conduction through the plasma.
Arcs are the low voltage/high current version.  Sparks the high voltage/
low current version.

(2)  If not done, the length of car body or extra cable will act like an
antenna.  Electrically this is the same as adding an RF voltage source
between the _equipment_ ground and the _real_ ground.  You ground is not
at ground!


	Interestingly, your ignition system works exactly the same way
as a spark-gap radio transmitter of early radio fame.


			good luck!

				jeff