be carefull with grounding schemes
I think I'm heading this way, but still unsure why Garmin insists on braid instead of just wire.
Just a word of caution about changing up whatever grounding details (shields or power connections) an avionics manufacturer specs out in their installation instructions. Not all grounds are equal in an electrical system, especially those relating to audio circuits. (Yea, I know it seems like they should be equal, especially to my mechanical engineering mind.) And, some times you really do need to do it EXACTLY like the manufacturer tells you to. Yes, a solder sleeve with a drain wire to a ground shield termiation may work just fine in most cases. But I ran into one that did not work very well, and it caused a very annoying buzz in my intercom audio.
Here are the details: The installation instructions on my previously installed uAvionix AV-30-E attitude indicator called for an 8 inch long (maximum) x 1/4 inch wide ground braid from the 15 pin d-sub connector housing to an airframe ground. This was in addition to the normal ground connection from the AV-30 to the forrest of tabs "electrical system" ground. Thinking "ground braid....really?", I installed an 8 inch 20 awg wire from the d-sub to the airframe. Worked just fine.....until months later when I connected the AV-30 audio output into my intercom, so I could make use of the AV-30 AOA headset warning tones. The intercom had a nasty annoying buzz/hum in the audio....clearly audible on the ground and in the air. NOT acceptable.
Process of elimination determined it was not the radio, was not the audio mix board, was not the intercom itself.....it was the AV-30 causing the buzz. After making sure I had not introduced any ground loops when installing the audio mix board between the AV-30 and radio audio signals, and the intercom system....I changed the AV-30 d-sub to airframe ground from the earlier installed 8 inch long 20 awg wire to the 8 inch long 1/4 inch wide ground braid. Problem solved....the buzz/hum dissapeared almost entirely. On the ground, in a quiet hangar, with the engine not running, a very slight buzz can be heard. But once the in the air, the buzz is undetectable....drowned out by the engine noise.
I was thinking the braid provided better grounding for high frequency noise (AC current skin effect....anyone?), but my EE analog circuit design engineer friend says it probably is the higher inductance of the ground braid. OK...that's outside of my pay grade, so I'll take his word for it.
So be careful with grounding, especially around an audio system. The intercom is where you are going to hear the results of injected noise and grounds that may not be elictrically "good enough" to minimize or eliminate the audio noise. If the avionics manufacturer calls out a ground braid, there is likely a good reason to spec that type of connection.
