[RFI] DSC Alarm Panel 75 Meters/False Alarms

[Pete Smith]

Wouldn't the bypass capacitor to ground take care of pin 1 problems by 
making a good chassis connection at RF?  I was just using ZIP cord for the 
speaker leads. Again, if this is low-speed data or DC, mightn't bypassing 
both sides to a single chassis ground help?

73, Pete N4ZR

At 10:25 AM 11/17/2004, Jim Brown wrote:

>On Wed, 17 Nov 2004 09:06:27 -0500, Pete Smith 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?
>
>It depends on how the RF is getting in. If it is a pin 1 problem, it 
>probably would not fix it.
>If it was differential mode, it probably would.
>
>To understand why, I'll review what "the pin 1 problem" is.  A cable 
>shield, if there is one,
>should go directly to the shielding enclosure, not to the circuit board. 
>If it goes to the
>circuit board first, the path from that shield to "ground" along the PCB 
>and other wiring
>will have inductance, and the received antenna current flowing through 
>that inductance
>will put an IZ drop across various points on the PCB depending on the 
>board layout.
>Thus any RF current on the cable shield is injected into the circuitry, 
>where it is likely to
>be detected.
>
>There are two practical fixes for this problem. One is to move the shield 
>to where it
>belongs -- the shielding enclosure, or, in the case of an unshielded 
>enclosure, the star
>ground point. The other is to choke off the current (or reduce its 
>strength to the point
>where detection either no longer occurs or is no longer audible/causing 
>data errors.
>
>Now, consider an unshielded cable going to an unbalanced input, with the 
>signal return
>going to "circuit common" on the circuit board. Now you have the same 
>problem, except
>that it is common mode current on the signal pair, not the shield. The 
>solution here is to
>first terminate the circuit return to the shielding enclosure. By virtue 
>of the capacitance
>between the conductors, this will reduce the common mode RF at the input. 
>The second
>part of this solution (without changing the equipment) is to use twisted 
>pair. This
>reduces the loop area, and thus the differential mode RF, and it also 
>makes the RF
>more common mode. And the common mode has been taken to the shielding
>enclosure.  Again, choking the common mode current is a good thing.
>
>There is another mechanism at play with shielded twisted pair cables. It 
>is called shield-
>current-induced noise (SCIN), and it converts RF shield current into a 
>differential-mode
>voltage on the signal pair. It happens when the shield current couples 
>asymetrically to
>the signal pair -- that is, more to the red than the black. That most 
>typically happens with
>foil/drain shields, and it occurs because the drain wire is usually 
>twisted at the same
>rate (lay) as the signal pair, and much closer to one signal conductor 
>than the other.
>SCIN is a MAJOR contributor to RFI between about 100 kHz and 10 MHz. Above 
>that
>frequency, shield current symmetry tends not to be disturbed by the drain 
>wire, flowing
>more uniformly in the foil instead.  I use the word "contributor" because 
>the victim
>equipment must contribute its inadequate filtering of the input or output 
>to which that
>cable is connected. A simple RC or LC filter (like what you did) at the 
>equipment would
>fix this, assuming that it didn't also degrade the signal.  But choking 
>the shield current
>also fixes SCIN, and it can't degrade the signal.
>
>I've published some research on both of these mechanisms in four AES 
>papers, and
>they can be downloaded from the AES website.
>
>
>Jim Brown  K9YC
>
>
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