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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Apr 23, 2011 5:47 am Post subject: shields

At 09:29 AM 4/23/2011, you wrote:

 	  Quote: 	
		  



When do you hook up both ends of the shield vs just one end?


	


    When the installation instructions for the

    system's components call for it.



 	  Quote: 	
		    Can you run a left and right audio channel in a two-wire shielded 

 cable, or do they need to be separated?


	


   They can share the bundle.



 	  Quote: 	
		    Does the Garmin lo go to the shield of the high, or do both get 

 their own shields?  Two wires in the same shield, or two separate singles?


	


   Shielding of wires has a VERY limited utility

   in fabricating our airplanes. The effect of a

   shield is to break the potential for CAPACITIVE

   coupling of a high-intensity, fast-rise time

   antagonistic stimulus into a potential victim

   small signal pathway.



   A pair of small wires tightly twisted together

   have an inter-wire capacitance on the order of

   1 PICOFARAD per inch. A very tiny capacitance.

   It takes a butt-ugly signal of very large magnitude

   to push across that a few pico-farads of

   inter-wire capacitance.



   Those kinds of antagonists were rare behind the

   panel in years past (magneto p-leads which were

   always shielded). With the advent of electronic

   ignition systems, those leads have become less

   to non-antagonistic.



   You could wire your entire system with unshielded,

   twisted trios or pairs with a high probability

   for success.



   In some, rare instances, the shield is ALSO a

   signal or ground pathway between black boxes

   and the installation wiring diagram will be

   explicit in the termination of the shield

   at both ends. In all other cases, connection

   at one end only is quite sufficient . . . and

   should you choose NOT to shield the wire, the

   probability of unwelcome behaviors from the

   system is very, very low.

   Bob . . .

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email(at)jaredyates.com
Guest

Posted: Sun Apr 24, 2011 4:44 pm Post subject: shields

Thanks Bob!  I'm connecting the 9009 Audio Iso Amp, so I'm sort of on the manufacturing team myself, and thus partly responsible for the manufacturer's instructions.  I'm at the phase of system integration, with systems that don't necessarily have harmonious instructions.  It has been a fun detective project so far, but I'm hoping that I can get some advice from more experienced folks on a couple of key points.   



The impression that I get from your reply is that unless I'm counting on using the shield as a specific conductor like a signal ground, it doesn't really matter very much how I hook up the shields in the audio system.  The 9009 documentation shows the Nav 1 audio input as a single conductor shielded cable with the audio signal on the center conductor and the signal ground on the shield.  The Garmin wiring diagram that I have shows the audio output hi and lo as a pair in a shielded cable, which is why I'm confused about terminology.  When they say lo, do they also mean signal ground, or are the two something different?

 

If you have time to answer any more questions on this topic, I have a few listed in my messages from a few days back.  Here's a link to the message board version:

http://forums.matronics.com/viewtopic.php?t=81010

 [url=http://forums.matronics.com/viewtopic.php?t=81010][/url]

I can also paste the other message again, but it's a long one, and since some folks must have to pay by the line, I'll hold off unless you can't see at the link.

 

  [quote][b]

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