ceengland7(at)gmail.com
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 Posted: Thu Jun 26, 2014 6:17 am Post subject: COZY: Shielding of audio signal wires, ARINC 129 wires |
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Another perspective...
I can't directly address the ARINC issue, but for audio wires in
aircraft avionics, the shield is of minimal use, 99.9% of the time.
Without diving into physics, the basic idea is this: at the low (600
ohms) load impedance and even lower source impedance in avionics audio
circuits, the circuits are almost immune to electrostatic noise, and the
typical shield material will do nothing to stop electromagnetic noise.
(Some consumer audio circuits have load impedances as high as 50K ohms
and are much more susceptible to acting like an antenna to pick up
electrostatic noise.)
For both the audio and ARINC issues, consider that there are lots of
Dynon harnesses flying with Garmin radios. If there were real problems
with the lack of shields, I'm pretty sure that Dynon would have revised
their harness construction a long time ago.
I'd bet that Garmin's bean counters put the 'shield required'
phraseology in their manual. If they tell (at no cost to Garmin) all the
thousands of installers to run shielded cable, then if there is a single
installation where the shield, it has saved them a troubleshooting call.
No matter that it cost the consumers thousands (millions?) of dollars in
extra installation costs. Kinda like every caller to a company spending
20 minutes wandering through layer after layer of 'automated' call
menus, so the company doesn't have to pay one person to answer the phone
and route each call correctly in 10 seconds.
'Ground loops' (multiple ground paths) are a different issue; careful
attention to grounding techniques will do a LOT to minimize noise pickup
in the audio systems. That's the reason that mic/phone jacks should have
grounds that are isolated from the airframe, with grounds going only to
their parent device.
I think that a prowl around Bob's aeroelectric site will reveal
confirmation of the info above.
Charlie
On 6/25/2014 9:18 AM, Tim Andres wrote:
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Yes you should use shielded wire, buy 2 or 3 conductor wire as needed since you never use the shield as audio low AND you never ground the shield at both ends even accidentally as Nick pointed out the need for insulating washers on the headphone jacks if installed on a conductive panel. Typically the shields are grounded at the audio panel. 'Lectric Bob has some great pics on his web site on how to treat the shield grounds, daisy chain them and get them cleanly into D-sub pins. You may also run into different terms for "audio low" among the various mfg's.
Also, suggest you convert the shields to wire just out side the D-sub hood since the completed bundle usually grows too big to fit inside the hood on the audio panel end.
PSE once had a ppt. on their web site that that gave some good detail on how to do this. I just looked and didn't see it but I believe I have a copy of it if you want.
Tim Andres
> On Jun 25, 2014, at 6:26 AM, Alan Barnett <alansbarnett(at)verizon.net> wrote:
>
> According to the Garmin GNS430W installation manual, the audio output wires should be shielded, with the shield grounded at one end. In addition, the ARINC 129 should be shielded.
>
> I bought a dynon d10a EFIS and pre-wired harness (before I learned how easy it is to wire the connectors). In the pre-wired harness, the ARINC 129 wires are twisted together, but not shielded. Furthermore, I would expect very little crosstalk at audio frequencies, so I wouldn't expect much benefit from shielding the audio wires.
>
> Does anyone have experience with these issues? Are shielded wires really necessary?
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