[RFI] [YCCC] Interference to CO Alarm
jim at audiosystemsgroup.com
Tue Nov 7 18:46:41 EST 2006
The CO Alarm is acting as a radio receiver, and we all know that a receiver
needs an antenna. Thus, ALWAYS begin by asking What receiving antenna(s) is
(are) connected to the device receiving the interference, including 1) power
supply leads, 2) sensor leads, 3) output leads, 4) indicator leads, and 5)
internal wiring (and circuit traces). Next, ask which of these receiving
antennas are long enough to be effective receiving antennas for the
interfering signal? Third, ask What other conductive element can sink
antenna current flowing THROUGH the device receiving interference?
Ferrite parts that surround an antenna reduce that antenna current -- IF they
significantly increase the total series equivalent impedance of the antenna.
There are several tutorials on my website that explain how ferrites work,
include graphs of measured data for some commonly available parts, and show
how to use them. The graphs of measured data will help you decide how to
maximize the impedance at any given frequency.
There are also several mechanisms that can convert that antenna current (a
common mode current) to a differential mode signal (see the tutorial and
papers on my website about SCIN (Shield-Current-Induced Noise). This means
that a simple lowpass filter may also be needed on any of this wiring, or
even at the semiconductor junction that is detecting the RF).
Note also that virtually all detection is square law, so, for example, a 3 dB
reduction in RF current results in a 6 dB reduction in detected interference.
Jim Brown K9YC
Audio Systems Group, Inc.
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