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Minimizing audio interference

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user9253

Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan

Posted: Sun Nov 10, 2013 5:59 am Post 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

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Nov 10, 2013 8:01 am Post subject: 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 . . .

mrspudandcompany(at)veriz
Guest

Posted: Sun Nov 10, 2013 8:53 am Post subject: Minimizing audio interference

 	  Quote: 	
		   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



--



Do you have a slow PC? Try a Free scan http://www.spamfighter.com/SLOW-PCfighter?cid=sigen

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Nov 10, 2013 11:17 am Post subject: Minimizing audio interference

At 10:52 AM 11/10/2013, you wrote:


<mrspudandcompany(at)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 . . .

user9253

Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan

Posted: Sun Nov 10, 2013 7:46 pm Post subject: Re: Minimizing audio interference

 	  Quote: 	
		  "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

nuckolls.bob(at)aeroelect
Guest

Posted: Sun Nov 10, 2013 8:27 pm Post subject: Minimizing audio interference

At 09:46 PM 11/10/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>

 > "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 

 [img]cid:.0[/img]

   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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user9253

Joined: 28 Mar 2008
Posts: 1944
Location: Riley TWP Michigan

Posted: Mon Nov 11, 2013 6:30 am Post subject: Re: Minimizing audio interference

I like it that Z15-3.1 shows not only the right way, but also the wrong way to ground devices.  I understand that earphones and microphones should not be grounded locally, but should only be grounded at the panel end (intercom or radio).

  I am a little confused with the engine sensors.  Grounding might be different for different types of sensors and their display (EMS or steam gauge).  Some sensors are grounded automatically by nature of their construction and mounting.  But for sensors that have isolated ground, I assume that the ground  wire should terminate at the EMS or display device and not at the engine case.  Of course always follow the manufactures installation instructions.

Joe

_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Mon Nov 11, 2013 11:07 am Post subject: Minimizing audio interference

At 08:30 AM 11/11/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com>



 I like it that Z15-3.1 shows not only the right way, but also the wrong way to ground devices.  I understand that earphones and microphones should not be grounded locally, but should only be grounded at the panel end (intercom or radio).

   I am a little confused with the engine sensors.  Grounding might be different for different types of sensors and their display (EMS or steam gauge).  Some sensors are grounded automatically by nature of their construction and mounting. 	


    Yes. Many oil pressure, oil temperature, CHT, EGT sensors are

    guilty of 'local grounding' . . . and for the manner in which they

    were originally intended to be used . . . it didnt' matter.



  	  Quote: 	
		    But for sensors that have isolated ground, I assume that the ground  wire should terminate at the EMS or display device and not at the engine case.  Of course always follow the manufactures installation instructions. 	


    Oh, absolutely. But absent a end-to-end design goal,

    one has to play the DIY integration game. One useful

    way to deal with a locally grounded signal source

    looks like this:



 [img]cid:.0[/img] 

   There are low cost integrated circuits that do the

   task of making remote voltage measurements across

   hostile environments. Alternatively, one can take all

   grounds for the instrument to the crankcase.  The same

   game is played in audio systems. I did the specification

   for design of an intercom system on the Lears that

   coupled all stations together by way of a transformer

   coupled, twisted pair . . . Intercoms scattered about

   the airplane could be grounded locally for power and

   control without the worry of noises on the ground-

   differences finding their way into the audio.



 

   Bob . . .

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bob.verwey(at)gmail.com
Guest

Posted: Mon Nov 11, 2013 11:44 pm Post subject: Minimizing audio interference

Bob N, for us neophytes, please enlighten us on the triangular device in your sketch.



Best...  

Bob Verwey



IO 470  A35 Bonanza  ZU-DLW



  

 



On 11 November 2013 21:05, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

   	  Quote: 	
		    At 08:30 AM 11/11/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "user9253" <fransew(at)gmail.com (fransew(at)gmail.com)>



 I like it that Z15-3.1 shows not only the right way, but also the wrong way to ground devices.  I understand that earphones and microphones should not be grounded locally, but should only be grounded at the panel end (intercom or radio).

   I am a little confused with the engine sensors.  Grounding might be different for different types of sensors and their display (EMS or steam gauge).  Some sensors are grounded automatically by nature of their construction and mounting. 	


    Yes. Many oil pressure, oil temperature, CHT, EGT sensors are

    guilty of 'local grounding' . . . and for the manner in which they

    were originally intended to be used . . . it didnt' matter.



  	  Quote: 	
		    But for sensors that have isolated ground, I assume that the ground  wire should terminate at the EMS or display device and not at the engine case.  Of course always follow the manufactures installation instructions. 	


    Oh, absolutely. But absent a end-to-end design goal,

    one has to play the DIY integration game. One useful

    way to deal with a locally grounded signal source

    looks like this:



 [img]cid:.0[/img] 

   There are low cost integrated circuits that do the

   task of making remote voltage measurements across

   hostile environments. Alternatively, one can take all

   grounds for the instrument to the crankcase.  The same

   game is played in audio systems. I did the specification

   for design of an intercom system on the Lears that

   coupled all stations together by way of a transformer

   coupled, twisted pair . . . Intercoms scattered about

   the airplane could be grounded locally for power and

   control without the worry of noises on the ground-

   differences finding their way into the audio.



 

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Nov 12, 2013 7:29 am Post subject: Minimizing audio interference

At 03:43 AM 11/12/2013, you wrote:

  	  Quote: 	
		  I believe that is an Op Amp – see  http://en.wikipedia.org/wiki/Operational_amplifier

  

 Jay

  

   	


    Jay is correct. The "operational amplifier" has a rich

    history in the evolution of electronics. Check out

    the articles on Wikipedia about George Philbrick,

    Bob Widlar, Jim Williams, and Bob Pease just to

    name a few of the colorful pioneers of the electronic

    arts and sciences.



    The op-amp comes in many flavors and sizes tailored

    to a constellation of tasks. I cut my op-amp teeth about

    1962 on this vacuum tube version that sold for a couple hundred

    dollars in 2013 money . . .



 

 [img]cid:.0[/img]

   

    I doubt that there were more than a few dozen

    products available.



    The last op-amp I stuck to a board looked like

    a little brick of plastic about the size of a match

    head . . .



 [img]cid:.1[/img]



   . . . and costs about $2. The Digikey site offers

   over 30,000 variations on the theme. We've come

   a long way baby . . .



 

   Bob . . .

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