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Noise filter noise

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james(at)berkut13.com
Guest

Posted: Fri Aug 25, 2006 10:20 am Post subject: Noise filter noise

Hi all,



I'm wondering if someone could educate me a little on something I 

encountered while trying to eliminate electrical noise coming from an 

autopilot servo.  The story goes like this:



I recently installed a new autopilot servo in my Berkut (all composite).  It 

worked perfectly, but for some reason, the servo makes a loud squealing 

noise in the intercom audio when it's activated.  I did all the usual 

things - changed ground points, increased grounding, double checked 

shielding on cables, un-bundled wires, ferrite beads on the cables, etc. 

The only thing that really worked was to power the servo from a separate 

battery - indicating electrical propagation, not RF.   I spoke with the 

manufacturer, and they did mention that there was a 2.5KHz PWM signal that 

is used to control the servo, and that the "noise" I hear is likely that. 

So, on to trying to isolate the servo from everything else...right.



I first tried (at the manufacturer's request) a pair of capacitors (.1uf and 

4700uf) in parallel, grounded off the servo power lead itself.  This did 

slightly dampen the noise, but certainly not enough.  So, I also went out 

and purchased a common 10-amp car audio power filter - just like those sold 

by Radio Shack and documented by Bob on his site - 2 caps with a choke coil 

potted in a case although I can not see the ratings on the caps.  I placed 

the filter in an orientation that isolated the power source from the servo, 

instead of servo from power source as IT was the verified source of the 

noise.



The good news is, that filter eliminated about 99% of the noise - and that's 

good to go.  I have not yet done extensive tests to see if the AP now 

operates properly in flight with the filter in-line (possible I/O errors, 

proper servo feedback, etc)...that's coming soon.  But, I want to explore 

the "weirdness" first.



Now, the weird (to me anyway) part is that the filter itself now audibly 

"sings" with the same frequency that I had heard before on the intercom.  I 

mean, it literally makes the noise like a speaker would.   Is this normal? 

What would be causing that?  Is this symptomatic of some additional problem 

that might effect servo operation?  Something else I should do or try?



All information and education is welcome...but remember, I'm not an EE.  



Eager to learn,



James Redmon

Berkut #013  N97TX

http://www.berkut13.com

By the way, this is not a common situation as the manufacturer has not seen 

this type of issue before...although they are doing everything they can to 

get this under control - under warranty, to their credit!   I have no issues 

with the way they are handling this..so please don't pester me about that 

part.

FLYaDIVE(at)aol.com
Guest

Posted: Fri Aug 25, 2006 2:23 pm Post subject: Noise filter noise

In a message dated 8/25/06 2:26:09 PM Eastern Daylight Time, 

james(at)berkut13.com writes:



 	  Quote: 	
		   Hi all,

  

  I'm wondering if someone could educate me a little on something I 

  encountered while trying to eliminate electrical noise coming from an 

  autopilot servo.  The story goes like this:

  

  I recently installed a new autopilot servo in my Berkut (all composite).  

It 
	


 	  Quote: 	
		    worked perfectly, but for some reason, the servo makes a loud squealing 

  noise in the intercom audio when it's activated. 


	


[Barry] When what is activated, the intercom or the servo?

 	  Quote: 	
		    I did all the usual 

  things - changed ground points, increased grounding, double checked 

  shielding on cables, un-bundled wires, ferrite beads on the cables, etc. 

  The only thing that really worked was to power the servo from a separate 

  battery - indicating electrical propagation, not RF.   I spoke with the 

  manufacturer, and they did mention that there was a 2.5KHz PWM signal that 

  is used to control the servo, and that the "noise" I hear is likely that. 

  So, on to trying to isolate the servo from everything else...right.

  

  I first tried (at the manufacturer's request) a pair of capacitors (.1uf 

and 
	


 	  Quote: 	
		    4700uf) in parallel, grounded off the servo power lead itself.  This did 

  slightly dampen the noise, but certainly not enough.  


	


[Barry] I gather the 47000uf cap was an electrolytic and the other was a 

ceramic disc?



 	  Quote: 	
		    So, I also went out 

  and purchased a common 10-amp car audio power filter - just like those 

sold 
	


 	  Quote: 	
		    by Radio Shack and documented by Bob on his site - 2 caps with a choke 

coil 
	


 	  Quote: 	
		    potted in a case although I can not see the ratings on the caps.  


	


 	  Quote: 	
		   I placed the filter in an orientation that isolated the power source from 

the servo, 
	


 	  Quote: 	
		    instead of servo from power source as IT was the verified source of the 

  noise.


	


[Barry] I'm not sure what you mean my that statement.  

Is the filter mounted as close to the servo as posable, short leads and a 

clean ground?  That is how it should be installed.



 	  Quote: 	
		    

  The good news is, that filter eliminated about 99% of the noise - and 

that's 
	


 	  Quote: 	
		    good to go.  I have not yet done extensive tests to see if the AP now 

  operates properly in flight with the filter in-line (possible I/O errors, 

  proper servo feedback, etc)...that's coming soon.  But, I want to explore 

  the "weirdness" first.

  

  Now, the weird (to me anyway) part is that the filter itself now audibly 

  "sings" with the same frequency that I had heard before on the intercom.  

I 
	


 	  Quote: 	
		    mean, it literally makes the noise like a speaker would.   Is this normal? 


	


[Barry] Weird to me too?  Are you saying that the filter has a noise coming 

from it?  If so then your GROUND is poor and maybe your leads too long.  This 

of course is assuming that the filter is made well and the coils of the filter 

are mounted / potted securely.  But, the first thing I would check is the 

GROUND.



Other thing to check would be your crimps.  I would rather have you tell me 

that you soldered these connection.



Barry

"Chop'd Liver"



"Show them the first time, correct them the second time, kick them the third 

time." 

Yamashiada

brian-av(at)lloyd.com
Guest

Posted: Fri Aug 25, 2006 4:36 pm Post subject: Noise filter noise

On Aug 25, 2006, at 11:17 AM, James Redmon wrote:



 	  Quote: 	
		   Now, the weird (to me anyway) part is that the filter itself now  

 audibly "sings" with the same frequency that I had heard before on  

 the intercom.  I mean, it literally makes the noise like a speaker  

 would.   Is this normal? What would be causing that?  Is this  

 symptomatic of some additional problem that might effect servo  

 operation?  Something else I should do or try?


	


You are putting current pulses through an inductor so it has a  

magnetic field that is varying with the current pulses. Any steel  

object nearby is going to be alternately pulled and released by the  

varying magnetic field. It is probably the case of the inductor or  

the inductor's core that is making the noise.



OTOH, there shouldn't be that much current change. Put the capacitor  

combo back at the input to the servo.



Brian Lloyd                         361 Catterline Way

brian HYPHEN av 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

N6030X(at)DaveMorris.com
Guest

Posted: Fri Aug 25, 2006 4:57 pm Post subject: Noise filter noise

Well, 2.5 KHz is within the audible frequency range.  The A above 

Middle C is 440 Hz.  So you're only talking about less than 6 octaves 

above it.  All it would take is having the coil inside the filter 

that has this frequency applied to it being physically in contact 

with the case or other structure.  I can envision that real 

easily.  We usually call that "headphones" or "loud speaker".  So, 

you might have to take it apart and make sure the filter is 

physically isolated from the box around it using RTV or rubber 

gaskets or something.



Be careful about filtering the PWM signal, though.  The whole purpose 

is to feed that signal to the servo, and if you are applying a filter 

that changes the shape, the servo might start functioning incorrectly.



Dave Morris

At 01:17 PM 8/25/2006, you wrote:

 	  Quote: 	
		  



Hi all,



I'm wondering if someone could educate me a little on something I 

encountered while trying to eliminate electrical noise coming from 

an autopilot servo.  The story goes like this:



I recently installed a new autopilot servo in my Berkut (all 

composite).  It worked perfectly, but for some reason, the servo 

makes a loud squealing noise in the intercom audio when it's 

activated.  I did all the usual things - changed ground points, 

increased grounding, double checked shielding on cables, un-bundled 

wires, ferrite beads on the cables, etc. The only thing that really 

worked was to power the servo from a separate battery - indicating 

electrical propagation, not RF.   I spoke with the manufacturer, and 

they did mention that there was a 2.5KHz PWM signal that is used to 

control the servo, and that the "noise" I hear is likely that. So, 

on to trying to isolate the servo from everything else...right.



I first tried (at the manufacturer's request) a pair of capacitors 

(.1uf and 4700uf) in parallel, grounded off the servo power lead 

itself.  This did slightly dampen the noise, but certainly not 

enough.  So, I also went out and purchased a common 10-amp car audio 

power filter - just like those sold by Radio Shack and documented by 

Bob on his site - 2 caps with a choke coil potted in a case although 

I can not see the ratings on the caps.  I placed the filter in an 

orientation that isolated the power source from the servo, instead 

of servo from power source as IT was the verified source of the noise.



The good news is, that filter eliminated about 99% of the noise - 

and that's good to go.  I have not yet done extensive tests to see 

if the AP now operates properly in flight with the filter in-line 

(possible I/O errors, proper servo feedback, etc)...that's coming 

soon.  But, I want to explore the "weirdness" first.



Now, the weird (to me anyway) part is that the filter itself now 

audibly "sings" with the same frequency that I had heard before on 

the intercom.  I mean, it literally makes the noise like a speaker 

would.   Is this normal? What would be causing that?  Is this 

symptomatic of some additional problem that might effect servo 

operation?  Something else I should do or try?



All information and education is welcome...but remember, I'm not an EE.  



Eager to learn,



James Redmon

Berkut #013  N97TX

http://www.berkut13.com

By the way, this is not a common situation as the manufacturer has 

not seen this type of issue before...although they are doing 

everything they can to get this under control - under warranty, to 

their credit!   I have no issues with the way they are handling 

this..so please don't pester me about that part.

james(at)berkut13.com
Guest

Posted: Fri Aug 25, 2006 8:55 pm Post subject: Noise filter noise

 	  Quote: 	
		  >  I recently installed a new autopilot servo in my Berkut (all composite).

 It

>  worked perfectly, but for some reason, the servo makes a loud squealing

>  noise in the intercom audio when it's activated.



 [Barry] When what is activated, the intercom or the servo?


	


When the servo is active.  The squeal is itermittent but coupled with the 

motion of the servo.  As in when pressure is applied against the stick and 

the servo is commanded to move the other direction.  This was without the 

filter installed, of course.



 	  Quote: 	
		  >  I did all the usual

>  things - changed ground points, increased grounding, double checked

>  shielding on cables, un-bundled wires, ferrite beads on the cables, etc.

>  The only thing that really worked was to power the servo from a separate

>  battery - indicating electrical propagation, not RF.   I spoke with the

>  manufacturer, and they did mention that there was a 2.5KHz PWM signal 

> that

>  is used to control the servo, and that the "noise" I hear is likely 

> that.

>  So, on to trying to isolate the servo from everything else...right.

>

>  I first tried (at the manufacturer's request) a pair of capacitors (.1uf

 and

>  4700uf) in parallel, grounded off the servo power lead itself.  This did

>  slightly dampen the noise, but certainly not enough.



 [Barry] I gather the 47000uf cap was an electrolytic and the other was a

 ceramic disc?


	


Yup.  The cap combo didn't do much to squelch the noise.  The filter cuts it 

by 99%.



 	  Quote: 	
		  >  So, I also went out

>  and purchased a common 10-amp car audio power filter - just like those

 sold

>  by Radio Shack and documented by Bob on his site - 2 caps with a choke

 coil

>  potted in a case although I can not see the ratings on the caps.



> I placed the filter in an orientation that isolated the power source from

 the servo,

>  instead of servo from power source as IT was the verified source of the

>  noise.



 [Barry] I'm not sure what you mean my that statement.

 Is the filter mounted as close to the servo as posable, short leads and a

 clean ground?  That is how it should be installed.


	




Yes, they are short - about 3" from servo.  I rigged up a short pig-tail so 

I could attach it and detach it for testing.   I just mean that the filter 

is a stimple choke coil with a two caps in parallel like Bob has documented:

http://www.aeroelectric.com/articles/filter/RS_Noise_Filters.pdf#search=%22filter%22



 	  Quote: 	
		  >  The good news is, that filter eliminated about 99% of the noise - and

 that's

>  good to go.  I have not yet done extensive tests to see if the AP now

>  operates properly in flight with the filter in-line (possible I/O 

> errors,

>  proper servo feedback, etc)...that's coming soon.  But, I want to 

> explore

>  the "weirdness" first.

>

>  Now, the weird (to me anyway) part is that the filter itself now audibly

>  "sings" with the same frequency that I had heard before on the intercom.

 I

>  mean, it literally makes the noise like a speaker would.   Is this 

> normal?



 [Barry] Weird to me too?  Are you saying that the filter has a noise 

 coming

 from it?  If so then your GROUND is poor and maybe your leads too long. 

 This

 of course is assuming that the filter is made well and the coils of the 

 filter

 are mounted / potted securely.  But, the first thing I would check is the

 GROUND.


	




Yup. the filter itself is the source of the audible noise.  It is connected 

to the common "nose" ground block.  That block is fat wired striaght back to 

the battery. I'm not sure what else to "check" or how.  I have opened the 

filter's case up, but it's components are potted with glue and I can't read 

the cap values.  But I can see that it's just a coil, and two caps in a 

plastic case.

 	  Quote: 	
		   Other thing to check would be your crimps.  I would rather have you tell 

 me

 that you soldered these connection.


	




All PIDG crimps.  All have been checked as secure.



Filter still singing like a canary.



James Redmon

Berkut #013  N97TX

http://www.berkut13.com

james(at)berkut13.com
Guest

Posted: Fri Aug 25, 2006 9:01 pm Post subject: Noise filter noise

 	  Quote: 	
		   You are putting current pulses through an inductor so it has a  magnetic 

 field that is varying with the current pulses. Any steel  object nearby is 

 going to be alternately pulled and released by the  varying magnetic 

 field. It is probably the case of the inductor or  the inductor's core 

 that is making the noise.


	


Humm...might be the inductor core..it's not mounted near anything else (6"+ 

minimum).  The noise is definately coming from inside the filter case 

(plastic) and I can feel it vibrating with the sound.

 	  Quote: 	
		   OTOH, there shouldn't be that much current change. Put the capacitor 

 combo back at the input to the servo.


	


The cap combo didn't do much at all to help the noise.  Maybe a 5% 

reduction.



Thanks for the input!



James Redmon

Berkut #013  N97TX

http://www.berkut13.com

james(at)berkut13.com
Guest

Posted: Fri Aug 25, 2006 9:06 pm Post subject: Noise filter noise

 	  Quote: 	
		   Well, 2.5 KHz is within the audible frequency range.  The A above Middle C 

 is 440 Hz.  So you're only talking about less than 6 octaves above it. 

 All it would take is having the coil inside the filter that has this 

 frequency applied to it being physically in contact with the case or other 

 structure.  I can envision that real easily.  We usually call that 

 "headphones" or "loud speaker".  So, you might have to take it apart and 

 make sure the filter is physically isolated from the box around it using 

 RTV or rubber gaskets or something.


	


Well, the caps and coil are in a plastic case, and are installed with some 

kind of clear gel glue - like hot glue.  I can not see that anything is 

touching the case without the goop inbetween.



 	  Quote: 	
		   Be careful about filtering the PWM signal, though.  The whole purpose is 

 to feed that signal to the servo, and if you are applying a filter that 

 changes the shape, the servo might start functioning incorrectly.


	




Yeah.  There does not seem to be any issues with the servo operation, but 

then I have not flown it either.  I will do some flight testing tomorrow.



I also plan to disconnect the filter, setup the AP servo, get it working in 

flight (if I can bear the noise in the headset), and then re-connect the 

filter and see if there are any servo problems.



I'll know more tomorrow.  One thing - I can live with the singing filter if 

the AP works properly.  It's not loud enough to hear over any engine 

noise....but the noise over the headsets is LOUD!

James Redmon

Berkut #013  N97TX

http://www.berkut13.com

jindoguy(at)gmail.com
Guest

Posted: Fri Aug 25, 2006 11:11 pm Post subject: Noise filter noise

I've had singing capacitors in audio equipment before, it means it's a bad capacitor. Try replacing it or the whole filter assembly.



Rick



On 8/25/06,  James Redmon <james(at)berkut13.com (james(at)berkut13.com)> wrote:[quote] --> AeroElectric-List message posted by: "James Redmon" <james(at)berkut13.com (james(at)berkut13.com)>



 	  Quote: 	
		   Well, 2.5 KHz is within the audible frequency range.  The A above Middle C 

 is 440 Hz.  So you're only talking about less than 6 octaves above it.

 All it would take is having the coil inside the filter that has this

 frequency applied to it being physically in contact with the case or other 

 structure.  I can envision that real easily.  We usually call that

 "headphones" or "loud speaker".  So, you might have to take it apart and

 make sure the filter is physically isolated from the box around it using 

 RTV or rubber gaskets or something.


	


Well, the caps and coil are in a plastic case, and are installed with some

kind of clear gel glue - like hot glue.  I can not see that anything is

touching the case without the goop inbetween. 



 	  Quote: 	
		   Be careful about filtering the PWM signal, though.  The whole purpose is

 to feed that signal to the servo, and if you are applying a filter that

 changes the shape, the servo might start functioning incorrectly. 


	




Yeah.  There does not seem to be any issues with the servo operation, but

then I have not flown it either.  I will do some flight testing tomorrow.



I also plan to disconnect the filter, setup the AP servo, get it working in 

flight (if I can bear the noise in the headset), and then re-connect the

filter and see if there are any servo problems.



I'll know more tomorrow.  One thing - I can live with the singing filter if

the AP works properly.  It's not loud enough to hear over any engine 

when you live at the airport.  [quote][b]

brian-av(at)lloyd.com
Guest

Posted: Sat Aug 26, 2006 6:27 am Post subject: Noise filter noise

On Aug 25, 2006, at 9:58 PM, James Redmon wrote:

 	  Quote: 	

 <james(at)berkut13.com>

> You are putting current pulses through an inductor so it has a   

> magnetic field that is varying with the current pulses. Any steel   

> object nearby is going to be alternately pulled and released by  

> the  varying magnetic field. It is probably the case of the  

> inductor or  the inductor's core that is making the noise.

 Humm...might be the inductor core..it's not mounted near anything  

 else (6"+ minimum).  The noise is definately coming from inside the  

 filter case (plastic) and I can feel it vibrating with the sound.

> OTOH, there shouldn't be that much current change. Put the  

> capacitor combo back at the input to the servo.

 The cap combo didn't do much at all to help the noise.  Maybe a 5%  

 reduction.

I know that but it might help with the noise coming from the filter.

Brian Lloyd                         361 Catterline Way

brian HYPHEN av 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

james(at)berkut13.com
Guest

Posted: Sat Aug 26, 2006 6:45 am Post subject: Noise filter noise

Humm...given the potting inside the filter, I think I'd end up  destroying it in the process of dis-assembly.  I might try replacing the  whole filter.

  

 Attached is a picture of the unit - I have not been able  to track down any electrical specifics.

  

 Curious, if one of the two capacitors went bad, wouldn't it  fail (at least partially) to squelch the noise and no longer "filter" as  well?  Or is there a partial failure mode of a capacitor that I'm not aware  of?

  

 Thank you all for the help and ideas - it's very  educational.

  

 I'll fly it today, and see if the servo is operating properly  with and without the filter attached.  (it's rigged as a small jumper cable  so I can easily plug and un-plug it)

  

 I think if the AP servo works properly, I'll just let the  filter audibly "sing".  At least there's no appreciable noise in the  headsets.

 

James Redmon

Berkut #013  N97TX

http://www.berkut13.com



 [quote]   ---

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Description:

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james(at)berkut13.com
Guest

Posted: Sun Aug 27, 2006 7:39 pm Post subject: Noise filter noise

An update to the noise issues:



Well, in short, the noise problems are solved.  The test flights showed that 

the filter disrupted the PWM signal to the extent that it was un-flyable. 

So, the quick fix "filter" solution was not possible.



I tried various combinations of grounds and power and the only combination 

that eliminated the noise was feeding the unit directly from the battery and 

grounding it only to the battery as well.  This meant isolating the unit's 

mounting so that it did not touch a ground in the front of the plane 

(battery is in the back).



So, with this combination required tearing the plane apart to install 

dedicated lines (all out of spares), but at least all systems are functional 

and quite happy with one another.  The one combination I didn't try was to 

install the filter on the intercom or audio circuit itself.  I figured I'd 

take the simplest route to solve the problem as protecting the audio may 

have opened the door to at least 6 different sources (2 coms, audio panel, 

intercom, engine monitor, etc.).  Yikes!



Moral:  if you are still building, do yourself a favor and pre-installed 

triple the number of fore-aft wires that you think you need - you'll run our 

of spares before you know it.  



Anyway, the AP tracks dead-nuts-straight-on...and no noise to boot.  All 

smiles here...despite the bloody knuckles and pricked fingers.



Thanks to those offering explanations and ideas.  I learned a lot on this 

one.



James Redmon

Berkut #013  N97TX

http://www.berkut13.com

phillip.johnson(at)lmco.c
Guest

Posted: Mon Aug 28, 2006 5:13 am Post subject: Noise filter noise

The core of the  inductor is probably lightly magnetostrictive which will cause the core to  vibrate under electrical excitation. This is normal, however if the mechanical  resonance of the core is close to the excitation frequency then it will howl  quite loudly and may induce premature failure of adjacent components. If this is  the case then you can damp the vibration by adding some silicone or epoxy to the  inductor. This will move the natural frequency and damp the  vibration.

  

    Phillip  Johnson

 L

 <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office"  /> 

 Chief Engineer &  Manager Underwater Systems Group

 Tel (613) 599 3280 ext.  3232

 Cell (613) 799  1644

 FAX (613) 599  3282

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