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Help needed: continued strobe noise & weak radio
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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Wed Jul 19, 2006 6:20 am Post subject: Help needed: continued strobe noise & weak radio

On Jul 19, 2006, at 6:49 AM, FLYaDIVE(at)aol.com wrote:



 	  Quote: 	
		   Barry, you keep making statements like this but I am not sure you

 have really thought through what you are saying.



 When you run two wires in parallel there are two coupling modes:

 capacitive and inductive. When impedances are very high (it is

 difficult to get current to flow) then capacitive coupling dominates.

 When impedances are very low, as in power circuits, coupling is

 inductive, with the current in one wire inducing a current in the

 other wire by magnetic (inductive) coupling.



 Brian:



 I make these statements because I know they work.


	


Copper braid or foil shield does not work to reduce inductive  

coupling between wires in low-impedance circuits at audio frequencies.



 	  Quote: 	
		   As for Inductive / capacitive coupling.  We are talking DC circuits  

 with Noise in the AUDIO frequency range.


	


Right.



 	  Quote: 	
		   As the saying goes: K.I.S.S. M.E. All this stuff about capacitive  

 and inductive coupling is not an issue in a simple noise problem as  

 we are discussing here.


	


Actually, it is.



 	  Quote: 	
		   AND what the heck is "NON-MAGNETIC BRAID-TYPE SHIELDING"?  Do you  

 really expect people to go out looking for "mu-metal"?


	


Yes, if that is the problem they are trying to solve.



 	  Quote: 	
		   ALL the braid used on any electrical circuit on our planes is  

 NONMAGNETIC ... It is made of COPPER WIRE.  Copper is non magnetic  

 the last time I checked.  Again K.I.S.S. M.E. is the word of the day.


	


But Barry, it isn't going to work in this application. There are  

magnetic shielding braids that are available. They look a bit like  

stainless steel braid and they work WONDERS for eliminating DC and  

audio effects on conductors carrying significant currents.



 	  Quote: 	
		   Electronics is simple, it is really!


	


Well, like most things, it is simple if you understand it and have  

worked with it. It is not simple if you are ignorant of the  

principles. (And ignorance is not a bad thing as it can be remedied  

by education. It is only a bad thing if one chooses to remain ignorant.)



 	  Quote: 	
		   All these boog-a-boo statements only confuses the non-electrical  

 types amongst us.


	


Confusion is eliminated through education. I happen to believe that  

anyone smart enough to successfully build an airplane is smart enough  

to learn and understand some basic electrical principles.



 	  Quote: 	
		   The only thing they need to know is how to remove the noise.  Again  

 that goes back to the basics of noise, and breaking it down into DC  

 and AC ... AC goes one step further (Audio Frequencies [AF] and  

 Radio Frequencies [RF]).

 Basics:

 AF Noise - Shield and Ground at the source [This is why the Mic &  

 Phone lines use insulating washers at the jacks and ground at the  

 audio panel or radios.]

 RF Noise - Shield and Ground at both ends.  [Here is where  

 exceptions come into play - Some times the grounding is done  

 through a capacitor. BUT we are dealing with AF Noise not RF so I'm  

 not going to try to confuse people out there.]


	


Einstein once made a very profound statement. He said, "Everything  

should be made as simple as possible, but not simpler." The point I  

was making is trying to explain *WHY* we use different kinds of  

shielding and why some shielding won't work at all. If you understand  

WHY you can make educated guesses about what will work and what won't.



And your "Basics" above are not 100% correct. So let's get down to a  

real analysis.



Let's go back to the problem. The problem is that noise from the  

strobe power supply is getting into the mic audio circuit. (I made a  

mistake assuming that it was the strobe's inverter, i.e. audio whine  

that changes pitch, as opposed to the "pop pop pop" noise of the  

strobe ignition coil or the discharge of the cap through the  

discharge tube but I digress.) The question then becomes, "how can  

the signal get from the strobe power supply to the mic audio circuit  

and this is where I started from.



First, how can the noise get out of the strobe power supply? It is  

probably not RF as the rise times of the signals do not fall into the  

RF spectrum. (Actually the "pop pop" noise could be RF borne but that  

is not what we are talking about.) So it is either conducted or  

radiated magnetically.



Now, how can the noise get into the audio panel or radio? As I see  

it, there are two paths:



1. it can get in through the power lead;



2. it can get in through the mic input.



Let us treat #1. How can it get into the power lead? There are two ways:



1. it can be conducted by the bus in the form of voltage variations;



2. it can be induced inductively by the changing current in the power  

or ground leads of the strobe power supply.



If the problem is #1 we will be able to see it with a scope on the  

bus. Also, our battery should filter that out. I am betting that is  

not the problem if the builder has been following Bob's  

recommendations for wiring his airplane. (This is, after all, the  

Aeroelectric list and you are here because Bob has been doing a good  

job teaching you how to do a good job of wiring.)



If it is #2 we can attack the problem by minimizing inductive  

coupling. There are four ways to reduce the inductive coupling:



1. Increase separation between the power wires going to the audio  

panel/radio and the strobe power supply.



2. Reduce radiated magnetic field by binding the strobes power and  

ground leads together or, better still, twisting them together. This  

works because the current the the hot and ground leads are equal and  

opposite so their magnetic fields will be equal and opposite. This  

causes them to cancel out. This is one of the big reasons for running  

a separate ground to a device rather than using the airframe as the  

ground.



3. Reduce susceptance to magnetic fields in the audio panel by  

running its power and ground leads together. (Again, twisting them is  

even better.) Any varying magnetic field would induce equal and  

opposite currents in the conductors and again they would cancel out.



4. You can use a magnetic shielding material like mu-metal around the  

offending conductors to "trap" the lines of magnetic flux and keep  

them from reaching (and influencing) other conductors.



(See: http://www.magnetic-shield.com/ for information on these kinds  

of products.)



But if you haven't run your power and ground leads together you can  

still "break the loop" and minimize the effect of inductive coupling  

by putting a brute-force audio filter at or near the audio panel. (It  

must be near the audio panel to eliminate the problem of inductive  

coupling between the filter and the audio panel.) How does this work?  

Well, the inductor acts as a high resistance to the AC part of the  

signal (noise) while acting as a low resistance to the DC (power). It  

therefore resists the transmission of the audio noise. The capacitor  

acts as a low resistance to the AC part of the signal (noise),  

shunting it to ground while not impeding the flow of power to the radio.



Shielding the power leads with some kind of copper braid or foil is  

not going to do anything. The impedances of the circuits are just too  

low.



OK, we have done the things I have mentioned above because they are  

just good practice. But the noise is still present. This means it is  

most likely getting in through the mic input.



Mic inputs in civil aircraft are low impedance, on the order of 200  

ohms. Again, the noise is getting in either by conduction or  

inductive coupling. It isn't getting in by capacitive coupling so a  

braid/foil shield isn't going to do much. (Braid and foil shielding  

work well at RF frequencies but that is not what we are dealing with  

here.)



What we want to do is break any possibility for noise to get in as  

conducted in either the mic signal lead or the mic ground lead.



If you ground the mic jack to the airframe there is a chance that the  

strobe power supply will have its noise impressed on the ground of  

the mic circuit. The airframe is like a big wire with all these  

different currents flowing through it. But the airframe is like other  

conductors and has some amount of voltage drop across it. If the  

current in the strobe power supply is varying (it is) that same  

variation appears in its ground wire. If that ground is the airframe  

we have a varying current flowing through the airframe. That means  

there will be different ground voltages at different points on the  

airframe. If we also use the airframe as part of our ground circuit  

for the mic by grounding the mic jack to the airframe (remember that  

your audio panel is already grounded to the airframe through its  

power ground), that voltage variation caused by the strobe ground  

current now appears as part of the signal for the mic. Poof! Noise!  

We get rid of this by insulating the mic jack from the airframe thus  

eliminating the airframe as a possible source of noise in the mic  

circuit.



Once we do that there is only one other way for noise to get into the  

mic circuit. You got it right -- magnetic induction. How do we deal  

with that? The same way we did with the power leads. We run the  

signal and ground wires together, preferably twisting them. You don't  

need a shield.



Now some of the people out there are asking, "But we were told to use  

shielded wire for the mic jack. What gives?" Well, the shielding is  

not doing anything for our audio noise pick up but it does have a  

good effect to reduce RF noise pickup. So while the shielded wire  

won't help to reduce our strobe noise problem it may help eliminate  

any problem stemming from RF from our comm radio transmitter getting  

into our mic audio circuit and causing a problem there. So we shield  

our mic audio wiring using shielded wire just in case. Belt and  

suspenders.



 	  Quote: 	
		   Let the manufacture of the wire worry about resistance of 1000 Ft  

 and the capacitive coupling / inductive coupling at AF & RF.  The  

 ONLY thing that would be of interest to our noise plagued brevien  

 is the Percentage of Braid. You did not say anything about that!


	


Of course not. This is not an RF problem where braid effectiveness is  

an issue. This is an AF problem and we just don't have high enough  

impedances for electrostatic shielding to be of any consequence.



 	  Quote: 	
		   Yet again as long as you use Mil-Spec wire the percentage of braid  

 is acceptable and again our brevien does not have to be concerned.


	


Well, they have to be concerned in that you are trying to solve a  

different problem from the one presented and your efforts are not  

likely to be effective.



 	  Quote: 	
		   As for the Separation of wires .... I did address that with my very  

 first statement to Lucky; read above.



 I would suggest that all the AF noise (strobe noise) we are  

 discussing be approached as I described AND then if the noise still  

 exists just add another ground t=at the other end of the shield.   

 It is MUCH easier to ADD a ground than to remove a ground.


	


As soon as you ground your shield in two places you have created a  

loop of wire consisting of the braid and the airframe. Poof! Instant  

ground loop. If I have a varying magnetic field near one conductor,  

maybe your shield wire because you have routed it near the power lead  

for some device, but that magnetic field isn't near my airframe  

return so it doesn't get an equal and opposite current induced in it,  

I am going to have a noise current conducted in that loop. You will  

be able to tap off that noise from the airframe just about  

anywhere ... such as with a mic jack that someone forgot to insulate.



There are a lot of things we just do because we were told we should.  

But I think it makes a lot more sense to understand *WHY* we do it so  

that we can guess at what might work and what might not. There are  

many paths for unwanted signal (noise) to get from one device to  

another. Understanding them and treating them separately will be much  

more likely to rid us of a problem than will blindly shotgunning a  

problem using rules-of-thumb that may or may not apply.



Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

trigo(at)mail.telepac.pt
Guest

Posted: Wed Jul 19, 2006 8:55 am Post subject: Help needed: continued strobe noise & weak radio

Another excellent piece of education from Brian Lloyd !

It's crystal clear and simple to understand and learn.

I don't know how you can find the time for this, but please keep up the good 

work. I just love to read your postings, even the longest, because that's 

the way why electric and electronic stuff is no longer voodoo to me!!



Thanks

Carlos

---

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Jerry Grimmonpre'

Joined: 09 Jan 2006
Posts: 144
Location: Huntley, Illinois 60142

Posted: Wed Jul 19, 2006 9:29 am Post subject: Help needed: continued strobe noise & weak radio

Brian ...

Your work on this subject has produced excellent information and 

understanding.  Your knowledge and genius are all telling.  Is it now 

possible you could boil all this down, into one simple, short, document? 

Something we could post on our shop wall for quick reference.   A single 

piece of paper as to what's twisted, what's grounded and where, what's 

grounded on one end and where and the other do's and don'ts?  I've seen this 

stuff scattered all over the electric list but I think it would help 

everyone to have a consolidated list.  Would you please consider doing this 

for the list?

Many thanks ...

Jerry Grimmonpre'

RV4 Flying

RV8A Wire






On Jul 19, 2006, at 6:49 AM, FLYaDIVE(at)aol.com wrote:

Copper braid or foil shield does not work to reduce inductive

coupling between wires in low-impedance circuits at audio frequencies.



Well, like most things, it is simple if you understand it and have

worked with it. It is not simple if you are ignorant of the

principles. (And ignorance is not a bad thing as it can be remedied

by education. It is only a bad thing if one chooses to remain ignorant.)



Confusion is eliminated through education. I happen to believe that

anyone smart enough to successfully build an airplane is smart enough

to learn and understand some basic electrical principles.

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Wed Jul 19, 2006 11:00 am Post subject: Help needed: continued strobe noise & weak radio

On Jul 19, 2006, at 12:48 PM, Carlos Trigo wrote:



 	  Quote: 	
		   

 <trigo(at)mail.telepac.pt>



 Another excellent piece of education from Brian Lloyd !

 It's crystal clear and simple to understand and learn.

 I don't know how you can find the time for this, but please keep up  

 the good work. I just love to read your postings, even the longest,  

 because that's the way why electric and electronic stuff is no  

 longer voodoo to me!!


	


Thank you! I make these postings in the hope that it will help others  

solve problems and understand how this stuff works. I am glad to hear  

that I am successful.



They also give me the opportunity to exercise my grey matter on  

problem solving. Use it or lose it! 



Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Wed Jul 19, 2006 11:03 am Post subject: Help needed: continued strobe noise & weak radio

On Jul 19, 2006, at 1:07 PM, Jerry Grimmonpre wrote:



 	  Quote: 	
		   

 <jerry(at)mc.net>



 Brian ...

 Your work on this subject has produced excellent information and  

 understanding.  Your knowledge and genius are all telling.  Is it  

 now possible you could boil all this down, into one simple, short,  

 document? Something we could post on our shop wall for quick  

 reference.   A single piece of paper as to what's twisted, what's  

 grounded and where, what's grounded on one end and where and the  

 other do's and don'ts?  I've seen this stuff scattered all over the  

 electric list but I think it would help everyone to have a  

 consolidated list.  Would you please consider doing this for the list?

 Many thanks ...


	


Huh. Interesting idea. Isn't there a chapter in the Aeroelectric  

Connection that covers this? It has been a long time since I read  

Bob's book but it struck me as a pretty good primer on this stuff.



Regardless, if it seems like a one-pager would be good to have around  

I will try to generate one. Bob's book is clearly the seminal work on  

the power wiring for an airplane. My experience is a bit more toward  

audio and RF.



But right now I have to fix two bilge pumps. <sigh> The only place  

nastier than the bilge on a boat is perhaps a septic tank.



Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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