Groups > sci.electronics.design > #464839

Re: EMI problems

From	Jeff Liebermann <jeffl@cruzio.com>
Newsgroups	sci.electronics.design
Subject	Re: EMI problems
Date	2017-06-17 09:22 -0700
Message-ID	<oehakc58lu3p7c5rtklm73d9kfvvsrm4de@4ax.com> (permalink)
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On Fri, 16 Jun 2017 12:34:02 -0700, John Larkin
<jjlarkin@highland_snip_technology.com> wrote:

>There's also a huge amount of lower-frequency noise here, probably
>from AC-line powered things in the neighborhood.

I see the same thing even in a residential neighborhood in the middle
of nowhere.  If I unplug various AC powered gadgets around the house,
the noise level goes down a little.  I can see some of it with a scope
connected to the power line through an LISN (Line Impedance
Stabilization Network).  The really nasty stuff disappears when the
sun goes down, which suggests a grid connected solar power system. The
problem is that I live in the deep dark forest where such systems are
rare.  Nothing within a mile radius but I'll keep looking.  A line
filter usually gets rid of most such noise.  If you're located in an
industrial area, you might want to investigate what the local PV
systems are putting back into the power lines.

>This motor controller was making 20 volt spikes in the lab, until we
>hacked it.
>https://dl.dropboxusercontent.com/u/53724080/Otis/EMI/VFD.JPG

Neatly done.  No capacitors to form a low pass filter?

>Here's the general spectrum:
>https://dl.dropboxusercontent.com/u/53724080/Otis/EMI/300MHz.JPG
>-28 dBm with just a Pomona lead antenna.

30 MHz/div.  TV Ch 2 on the left.  FM broadcast band 88-108MHz seems
quite strong.  One aircraft beacon.  Starting at 150Mhz, quite a bit
of VHF commercial and public safety signals which quit at about 190MHz
with TV channel 7.  Methinks if you check the UHF TV and commercial
spectrum, the signal levels will higher.

-28dBm is 9mv rms or 25mv p-p into 50 ohms.  You should see a 25mv
"noise" level on any long wires that can act as an antenna.  If you're
working with low impedances, not a problem.  If you have bandwidth in
the FM/VHF region at high impedances, I suggest a screen room, screen
box, or differential designs.

BTW, nice Marconi/IFR/Aeroflex/Cobham 2399c spectrum analyzer.  I'm
jealous.

>>You might want to calculate the resonant frequency of the Royal Dansk
>>cookie can:
>><http://www.rfwireless-world.com/calculators/circular-cavity-resonator-calculator.html>
>>If you see a noise peak in that area, it could be something
>>oscillating or filtering using the can as a resonator.  

>I'm OK at 100 MHz. It might make a resonant cavity filter or something
>at some higher frequency.

Maybe.  I've built and seen VHF (150MHz) cavity filters in much
smaller cans.  Fruit juice cans, paint cans, beer cans, etc all can be
made to work.  
<http://vk4zxi.blogspot.com/2014/08/homebrew-cavity-resonatorduplexer-for.html>
All it takes is a center rod between the two ends with a tuning
capacitor at one end.  A length of insulated wire, soldered to one end
of the can, and leaning against the top, forming a capacitor, is all
it takes.  Small cavities have much less Q, but they still will
resonate.  I don't have any cookie cans available.  I'll get some
dimensions at the market and see what happens.  

Or, you could just frequency sweep the cookie can and see what you
get.  Just sacrifice some more Danish butter cookies to a worthy
cause.  Punch a hole in the can and solder in a BNC jack.  Make a
small wire loop across the BNC.  Connect a T-connector to the BNC
which attach to sweep (tracking) generator and spectrum analyzer.
Extra credit for using a return loss bridge.  Sweep and look for
notches on the SA.

All this is probably more of an academic exercise than anything useful
for your project.  However, it is nice to know where various
enclosures resonate in case you see a frequency dependent anomaly that
happens to coincide with the resonant frequencies.

-- 
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Thread

EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-14 16:09 -0700
  Re: EMI problems Martin Riddle <martin_ridd@verizon.net> - 2017-06-14 20:05 -0400
    Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-14 18:01 -0700
      Re: EMI problems o pere o <me@somewhere.net> - 2017-06-15 09:28 +0200
        Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-15 10:59 -0400
          Re: EMI problems Don Kuenz <g@crcomp.net> - 2017-06-21 02:53 +0000
        Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-15 07:59 -0700
          Re: EMI problems George Herold <gherold@teachspin.com> - 2017-06-15 08:07 -0700
            Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-15 11:12 -0400
              Re: EMI problems George Herold <gherold@teachspin.com> - 2017-06-15 08:42 -0700
            Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-15 09:38 -0700
          Re: EMI problems Robert Baer <robertbaer@localnet.com> - 2017-06-16 22:06 -0800
  Re: EMI problems Jon Elson <jmelson@wustl.edu> - 2017-06-15 14:14 -0500
    Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-15 13:55 -0700
      Re: EMI problems Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-06-15 14:02 -0700
      Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-16 12:43 -0400
        Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-16 10:11 -0700
          Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-16 13:24 -0400
      Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-16 10:37 -0700
      Re: EMI problems Jon Elson <jmelson@wustl.edu> - 2017-06-16 13:31 -0500
        Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-16 14:46 -0400
          Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-16 12:25 -0700
          Re: EMI problems George Herold <gherold@teachspin.com> - 2017-06-16 13:07 -0700
        Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-16 12:22 -0700
    Re: EMI problems upsidedown@downunder.com - 2017-06-16 09:35 +0300
      Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-16 06:40 -0700
  Re: EMI problems Joerg <news@analogconsultants.com> - 2017-06-15 12:27 -0700
    Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-15 12:47 -0700
      Re: EMI problems Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-06-15 12:57 -0700
        Re: EMI problems Joerg <news@analogconsultants.com> - 2017-06-15 13:12 -0700
      Re: EMI problems Joerg <news@analogconsultants.com> - 2017-06-15 13:03 -0700
    Re: EMI problems "Lund-Nielsen, Jorgen" <jorgen.lund-nielsen@xyz123desy.de> - 2017-06-16 08:58 +0200
  Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-16 10:03 -0700
    Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-16 11:09 -0700
      Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-16 12:06 -0700
        Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-16 12:34 -0700
          Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-17 09:22 -0700
            Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-17 10:08 -0700
              Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-17 11:07 -0700
                Re: EMI problems Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2017-06-17 21:30 +0300
                Re: EMI problems pcdhobbs@gmail.com - 2017-06-17 16:34 -0700
                Re: EMI problems krw@notreal.com - 2017-06-17 20:44 -0400
                Re: EMI problems George Herold <gherold@teachspin.com> - 2017-06-19 06:03 -0700
              Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-18 12:49 -0700
                Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-18 13:42 -0700
                Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-18 15:50 -0700
                Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-18 17:15 -0700
                Re: EMI problems upsidedown@downunder.com - 2017-06-19 09:39 +0300
                Re: EMI problems upsidedown@downunder.com - 2017-06-19 09:48 +0300
                Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-19 12:45 -0400
    Re: EMI problems "John Miles, KE5FX" <jmiles@gmail.com> - 2017-06-16 15:30 -0700
      Re: EMI problems John Larkin <jjlarkin@highland_snip_technology.com> - 2017-06-16 18:10 -0700
        Re: EMI problems Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-06-17 11:08 -0400
      Re: EMI problems upsidedown@downunder.com - 2017-06-17 14:25 +0300
      Re: EMI problems Jeff Liebermann <jeffl@cruzio.com> - 2017-06-17 09:35 -0700
        Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-17 10:42 -0700
  Re: EMI problems upsidedown@downunder.com - 2017-06-17 14:45 +0300
    Re: EMI problems John Larkin <jjlarkin@highlandtechnology.com> - 2017-06-17 08:28 -0700

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