Today's Message Index:
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1. 06:00 AM - Minimizing audio interference (user9253)
2. 08:01 AM - Re: Minimizing audio interference (Robert L. Nuckolls, III)
3. 08:53 AM - Re: Minimizing audio interference (R. curtis)
4. 11:17 AM - Re: Minimizing audio interference (Robert L. Nuckolls, III)
5. 07:48 PM - Re: Minimizing audio interference (user9253)
6. 08:27 PM - Re: Re: Minimizing audio interference (Robert L. Nuckolls, III)
Message 1
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| Subject: | Minimizing audio interference |
I was going to post on VansAirforce in a thread about audio strobe noise, but decided
to post here first to make sure that my post is accurate and correct.
Let me know if it's not.
Thanks, Joe
Whenever current from separate loads shares the same conductor (wire or metal airframe),
the current from one load can affect the current from another load.
The interference can be minimized by:
1. Keeping the shared conductors as short as possible
2. Increasing the wire size of the shared conductor
3. Do not share a conductor with more than one load.
4. Keep the positive and negative conductors of the offending circuit together
and twisted.
If the microphone jack is not isolated from ground with insulating washers, then
part of the mic current flows though the airframe which also carries current
from other aircraft loads. This violates rule 3 above. The mic circuit is
especially vulnerable because its signal gets amplified.
If the strobe uses the airframe for the negative current path, this also violates
rule 3 above. It is better to power the strobe with twisted positive and
negative wires instead of using the airframe for the negative conductor.
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=412627#412627
Message 2
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| Subject: | Re: Minimizing audio interference |
At 07:59 AM 11/10/2013, you wrote:
I was going to post on VansAirforce in a thread about audio strobe
noise, but decided to post here first to make sure that my post is
accurate and correct. Let me know if it's not.
Thanks, Joe
Whenever current from separate loads shares the same conductor (wire
or metal airframe), the current from one load can affect the current
from another load. The interference can be minimized by:
1. Keeping the shared conductors as short as possible
Generally speaking, the only conductors that
might be shared by an antagonist and potential
victim are grounds . . . and grounding a victim
onto a location already polluted with antagonistic
perturbations is easily avoided with attention to
the architecture of the ground system. See Figure Z-15
2. Increasing the wire size of the shared conductor
Better yet . . . do not share . . .
3. Do not share a conductor with more than one load.
. . . high on the list of design goals for the
elegantly crafted airframe.
4. Keep the positive and negative conductors of the offending circuit
together and twisted.
Twisting speaks to magnetic coupling between antagonistic
stimulus and potential victims. This coupling mode is
weak. Further, it's unlikely that one finds it mechanically
advantageous to route the wires for antagonists (generally
airframe bundles) along with wires belonging to potential
victims . . . generally situated on the panel.
If the microphone jack is not isolated from ground with insulating
washers, then part of the mic current flows though the airframe which
also carries current from other aircraft loads. This violates rule 3
above. The mic circuit is especially vulnerable because its signal
gets amplified.
Yes
If the strobe uses the airframe for the negative current path, this
also violates rule 3 above. It is better to power the strobe with
twisted positive and negative wires instead of using the airframe for
the negative conductor.
Not a great sin . . . the BIG guys ground nasty
loads to airframes all the time. It's easy to craft
TWO ground systems wherein the second is attentive to
risks to potential victims.
Noises from strobe systems heard on headphones and/or
transmitted signals are almost always conducted by
virtue of poor grounding choices for audio system
wiring and rarely, radiated noises from strobe tubes into
the co-located antennas. Wingtip mounted comm and vor antennas
share this risk.
Some avionics not designed in the spirit and intent
of DO160 suggestions for immunity to bus noises may
also exhibit vulnerability to strobes or other sources.
This condition calls for adding filters to the victim's
14v supply.
Bob . . .
Message 3
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| Subject: | Re: Minimizing audio interference |
> 3. Do not share a conductor with more than one load.
>
> . . . high on the list of design goals for the
> elegantly crafted airframe.
Am I right in assuming that this refers to properly
fused wires from one of the busses and does not
include the fat wires wich carry most if not all the
loads in the aircraft.
Roger
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Message 4
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| Subject: | Re: Minimizing audio interference |
At 10:52 AM 11/10/2013, you wrote:
<mrspudandcompany@verizon.net>
3. Do not share a conductor with more than one load.
. . . high on the list of design goals for the
elegantly crafted airframe.
Am I right in assuming that this refers to
properly fused wires from one of the busses and does
not include the fat wires wich carry most if not all
the loads in the aircraft.
Obviously, everything in the airplane
requiring power and ground will have
a lot of conductors in common.
It's known that some accessories tend to
be antagonists (high currents, noisy,
trashy voltage transients, strong RF
emitters, etc.) while other things tend
to be potential victims (stuff that processes
tiny signals).
We're getting off into the weeds with the
'shared conductors' terminology. When it comes
to power distribution, all things
electric 'share' conductors with each other.
DO-160 or similar qualification protocols suggest
means by which the most vulnerable of victims
can get power from the noisiest of busses with
little risk of difficulty.
99% of noise problems are founded on
a very limited range of installation issues
NONE of which have to do with twisting feeders,
co-location of coax cables in wire bundles,
or failure to include any sort of 'filter' on
either a victim or antagonist . . .
We've discussed the high order probabilities for
noise in either transmitted or received signals
when victim grounds that should be centralized
on the panel get tied down somewhere else.
A second order risk is seen when levels of
transmitted RF rise to unusual levels in the
vicinity of tha panel. This condition can present
when the antenna is too close to the equipment
installed . . . or a coax shield has detached
in a connector causing the entire feedline to
become a radiator.
"Shared conductors" is not a good way to talk
about a noise issue. You have a victim, an
antagonist and a PROPAGATION MODE. Breaking
the propagation mode is the path to noise-free
Nirvana.
Bob . . .
Message 5
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| Subject: | Re: Minimizing audio interference |
> "Shared conductors" is not a good way to talk
> about a noise issue.
What if we say that audio signals should not share conductors with other loads?
An audio signal is unlikely to share a positive 12 volt wire. But many audio
signals use ground as a common path in their circuit. A problem arises when
other loads share the same ground conductor as the audio circuit.
Joe
--------
Joe Gores
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=412669#412669
Message 6
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| Subject: | Re: Minimizing audio interference |
At 09:46 PM 11/10/2013, you wrote:
>
>
> > "Shared conductors" is not a good way to talk
> > about a noise issue.
>
>What if we say that audio signals should not share conductors with
>other loads? An audio signal is unlikely to share a positive 12
>volt wire. But many audio signals use ground as a common path in
>their circuit. A problem arises when other loads share the same
>ground conductor as the audio circuit.
>Joe
"Audio signals" are not "loads". All devices
in the airplane share a common 12v supply bus.
Eventually, ALL grounds come together too.
Study up on the 'ground loop' phenomenon illustrated
in part here.
http://tinyurl.com/6w87rvb
The design task is to re-ground those devices
marked
Emacs!
such that their shared grounds do not inject
noises. Everybody needs a ground that's
ultimately common with all other grounds.
It's WHERE the grounds are placed and
how they come together that produces the
recipe for success.
In the targets we used to build at Beech
there were three ground systems. Power,
analog and digital. Just as everything
comes to ground at the forest of tabs in
Figure Z-15, so too did all the grounds
in the target come to a common point in the
power distribution box . . . but what happened
to them along the way determines their
probability of offering in ingress point
for noise into a potential victim.
Bob . . .
Bob . . .
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