[RFI] DSC Alarm Panel 75 Meters/False Alarms

[Pete Smith]

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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