I think the big issue here is not how to fix the RFI
problem. (The RFC/cap approach might well work, or
ferrites, or twisted pair.) But it's who's
responsible.
If you start doing the alarm company's work for them,
especially for a neighbor's installation, _you_ become
potentially liable for any future fault on the system.
Ask a lawyer what that could mean. A hi-fi system is
one thing, but a security/fire alarm is another.
I suggest offering to advise their technician, but
keep hands off.
73, Martin, AA6E
--- Pete Smith <n4zr@contesting.com> wrote:
> 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
> >
> >
> >_______________________________________________
> >RFI mailing list
> >RFI@contesting.com
> >http://lists.contesting.com/mailman/listinfo/rfi
>
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