Re: [PSES] Audio sources, class D digital amps, radiated emissions, and philosophy

2014-08-19 Thread Brent DeWitt 
Great subject Doug and one very close to my heart (or head, whatever)!  

 

As an EMC engineer for Bose Corporation, I get to see a lot of audio
amplifiers with output levels from 10s of watts to thousands of watts.  The
extension to your article that I would like to make is that not all "Class
D" amplifiers are created equal.  Various chip manufacturers have different
approaches to how the switched (and therefore Class D) signal is produced.
One of the most troublesome is a vendor whose chip family produces equal and
balanced square waves on both wires to the speaker driver when no audio is
present.  While differential mode current is minimal with no input, common
mode di/dt charging currents down the wires to the driver can be quite high
and have very high frequency components.  On the order of 80% of our
products that I have tested in the last 5 years have highest emissions
profiles when the source is muted.  For European standards this is mitigated
by a standard test method in EN55013 of using IEC Pink Noise with a 6dB
crest factor and 20Hz to 20kHz pass band at 1/8 of maximum power.  Then we
get to the US/Canada where it's "find the worst case".  You can guess which
is easier to pass!  The Pink Noise dithers the pulses on either side of the
driver leads and produces a very favorable spreading function in the higher
harmonics of the amplifier "chopping" frequency, such that radiated
emissions are significantly reduced.  Conducted emissions also benefit.

 

Bottom line is that I highly recommend that any EMC engineer first
familiarize themselves with the characteristics of the amplifier chip vendor
before launching into troubleshooting.  It will save you time in the long
run!

 

With respect,

Brent DeWitt, Senior EMC Engineer

Bose Corporation, Framingham MA

 

From: Doug Smith [mailto:d...@emcesd.com] 
Sent: Tuesday, August 19, 2014 7:13 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Audio sources, class D digital amps, radiated emissions, and
philosophy

 

Hi All,

Just finished and posted my August 2014 Technical Tidbit titled:

Radiated Emissions Can be Strongly Affected by Driving Signals, A Problem
for Emissions Testing
(An Example Using a Class D Stereo Audio Amplifier)

 

Abstract: Radiated emissions from electronic equipment can be a strong
function of how the equipment is operated. In the case of class D audio
amplifiers, the audio signal driving the amplifier can have a large effect
on emissions. Test results and implications are discussed as they relate to
the philosophy of radiated emissions testing.

The link to the article is: http://www.emcesd.com/tt2014/tt081914.htm

The link to the main page: http://www.emcesd.com/
(lots of new stuff there)

Check to make sure your computer does not corrupt the links above. They
should look just as above with no other symbols or numbers added as Windows
sometimes does at the end of the link of the form "[1]".

Doug



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[PSES] Audio sources, class D digital amps, radiated emissions, and philosophy

2014-08-19 Thread Doug Smith 

  
  
Hi All,

Just finished and posted my August 2014 Technical Tidbit titled:


Radiated Emissions
Can be Strongly Affected by Driving Signals, A Problem for
Emissions Testing
(An Example Using a Class D Stereo Audio Amplifier)
  


  

  


  


Abstract: Radiated emissions
from electronic equipment can be a strong function of how the
equipment
is operated. In the case of class D audio amplifiers, the audio
signal
driving the amplifier can have a large effect on emissions. Test
results and implications are discussed as they relate to the
philosophy
of radiated emissions testing.

The link to the article is:
http://www.emcesd.com/tt2014/tt081914.htm

The link to the main page: http://www.emcesd.com/
(lots of new stuff there)

Check to make sure your computer does not corrupt the links above.
They should look just as above with no other symbols or numbers
added as Windows sometimes does at the end of the link of the form
"[1]".

Doug
-- 
University of Oxford Tutor
Department for Continuing Education
Oxford, Oxfordshire, United Kingdom 
--
 ___  _Doug Smith
  \  / )   P.O. Box 60941
   =   Boulder City, NV 89006-0941
_ / \ / \ _TEL/FAX: 702-570-6108/570-6013
  /  /\  \ ] /  /\  \  Mobile:  408-858-4528
 |  q-( )  |  o  | Email:   d...@dsmith.org
  \ _ /]\ _ /  Web: http://www.dsmith.org
--

  

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[PSES] Tool Testing

2014-08-19 Thread jral...@productsafetyinc.com 
Hi,

Does anyone know a lab that can test to EN792 and/or ISO11148 for handheld air 
powered tools?

Thank you,

John


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