This raises a question I have wondered about. Years ago, I had
interference to a stereo that originated with the speaker leads. Not
knowing any better, I went to Radio Shack and got a couple of RF chokes and
a couple of disk ceramic caps, and put them in series and shunt. Worked great.
If the sensors on this alarm system are DC, wouldn't a similar fix be the
best way to go about cleaning up the sensor lines?
73, Pete N4ZR
At 08:16 AM 11/17/2004, Jim Brown wrote:
On Wed, 17 Nov 2004 07:33:04 -0500, Brad Roope wrote:
>installed a common split core around the leads at the input to the panel.
Common split cores are optimized for 50 MHz and us, and would not be
expected to
be effective at 4 MHz.
>In addition he grounded the new panel to the common telephone/electrical
ground.
>
>If DSC does not respond to their RFI Issue, I may ask my neighbor if I
can install
>#77 beads at the input to the Alarm Panel. I may get only one
opportunity so I need
>to know if multiple beads on each lead would be better that just one?
Your tone sounds defensive -- it should not be. DSC is the guy with the
problem, and
THEY should be working much harder to resolve this. My guess is that
there is a "pin 1
problem" in their equipment, excited by common mode current on the
interconnect
wiring (receiving antenna). Choking that common mode current could fix it.
Some basic principles of beads. The series impedance is proportional to
the length of
the bead along the wire. So yes, more is better, but the series impedance
is not likely
to be large enough to make a dent, even with multiple beads.
The series resistance is also proportional to the square of the number of
turns, and it is
far more likely that a multi-turn choke of #78, #31, or #43 material would
solve your
problem.
Fair-Rite makes 2.4" toroids of #78 and #43 material. I would start with
8 turns around
either of those. Fair-Rite also makes some big clamp-ons of #31 that can
be used to
form multi-turn chokes around wiring that either can't easily be
disconnected or has a
connector on it too large to fit through a solid core or toroid.
>I may also install by-pass capacitors on each lead.
Bypass capacitors are probably inappropriate if there is data on the line
(as opposed
to a dc signal generated by the sensor). And it is far more likely to be
common mode.
Another point. Look at the interconnect cable. Is it twisted pair? It
should be. Is it tightly
twisted? It should be. Twisted pairs have solved LOTS of RFI problems when
RF was
getting into the output terminals of power amplifiers via zip cord used as
speaker wire.
My first choice would be a decent CAT5 cable, and it should NOT be
shielded. Most
cable shields can CAUSE common mode current to be coupled onto the signal
pair as
a differential mode signal.
I wouldn't rule out coupling through the power supply wiring, but I would
attack the signal
wiring first.
All of this, of course, is the responsibility of the installation company
and the equipment
manufacturer. Don't let them make THEIR problem YOUR problem. Let them
know that
this is all well established science, and that you are willing to
cooperate by firing up
your transmitter for testing on a schedule that is reasonably convenient
to you (but not
when you must take time off work). But make it clear that you know that
fixing it is
THEIR responsibility, not yours.
BTW -- you are welcome to print this email and show it to your neighbor.
And you can
tell them that I am vice-chair of the Standards Committee of the Audio
Engineering
Society's Working Group on EMC, and that I'm a consulting designing sound
systems
for both large and small installations.
Jim Brown
Audio Systems Group, Inc.
Chicago
http://audiosystemsgroup.com
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