Noise filter, off subject

[Tom Cloud]

>I used a 470mfd capacitor in the power cord to my radar detector. This took
>care of it for me. My problem was that the Laser would get pegged when I
>hit the horn..
>I think it would work for your problem too..... Just a suggestion.
>
>Jesse.
>
>> I have an interference problem with a radar detector, Uniden 2100W.  I
>have
>> HEI ignition w/Accel supercoil.  Problem is, if I rev the motor above
>2000
>> rpm the laser goes off full blast.  Major problem.
>> 
>> Supposedly I need a noise filter.  Are there any cheap fixes or
>> recommendations for an inexpensive noise filter.  I trust the opinions of
>> the list members for more that my local, pimple faced Radio Shack
>saleperson.

I'm rusty on my auto install noise-suppression but I will tell you
the basics

 - there's two primary ways noise can get into your equipment (well,
actually three 8^)
  1 - radiated -- like noise from the ignition
  2 - through the power line or other wiring
  3 - could actually be picking it up on the "antenna" -- i.e.
it's coming in at the same frequency as what you're looking for
(that'd be tough to get rid of) or at the i.f. (intermediate
frequency) amp frequency.

If it's radiated, then shielding the source and/or the equipment
can help.  This involves metallic shielding -- not necessarily
grounded to the car, just completely enveloping the item being
shielded (and there's the rub, you gotta have wires going in and
out so that's a place for stuff to leak through)

If the noise is due to radiated magnetic fields (not likely in your
car) you're probably dead cause there ain't much'll keep out magnetism
(actually nothing will -- can only encase it in something that
the magnetic fields would prefer to pass through).

If it's getting in through the power lines or other connections (like
speaker cables) then a simple capacitor across the wires or from each
wire to ground (or both) *might* do the trick.  This is the simplest
solution and, if it works, it works  8^)   If that doesn't get it,
then you'll need an L-C filter, which is a choke in series with
the line and a capacitor to ground.  I think RatShack has these.
I'm certain that Crutchfield has them as I would guess Mouser or
Digi-Key.  I think RatShack probably even has a book on this (look
for literature on radio/stereo installs).

Let me suggest that if you use a "large" value capacitor (anything
over 1 microfarad) that you put a "small" capacitor across it in parallel
(something on the order of .01 uF or smaller).  "Large" caps don't
do much to frequencies above 5 or 10 MHz and the small cap in shunt
will take care of them whilst the "big" one will take care of the
noise lower in frequency.

Your wiring can be shielded by being careful not to run it through
noisy environs, by running it behind or under sheet metal and
crossing wires that have noise on them at right angles.  Also,
you can get metal braided sleeving to put over your wires or just
use coaxial cable or shielded wiring (the wrapped foil is usually
best).

Tom Cloud