Author Topic: Instek GDS2000E EMI reduction  (Read 565 times)

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Offline rhb

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Instek GDS2000E EMI reduction
« on: November 21, 2018, 01:55:26 pm »
The GDS2000E design has *no* shielding except for the AFE and the front panel.  As a consequence it emits a terrific amount of SMPS EMI.    I'll post some photos later.  At the moment I can't find my probe.

If you want to see for your self,  make up a small loop that slips over a scope probe tip with the grabber removed.  Make the loop from tip to ground 1/4-3/8"  (6-9 mm).  The smaller the loop the better the localization.  Its a directional loop antenna.  Rotating the loop will indicate the axis and moving along the axis will show you the source

DO NOT USE THE PROBE AS A FORM!   MEASURE THE DIAMETERS AND USE DRILL BIT SHANKS TO WIND THE COILS TO SLIP OVER THE TIP AND GROUND.   

The probe tip will not withstand the force of winding copper wire around it.

Has anyone attempted to retrofit shielding into one of these?  Obviously the first place to start is the PSU, but the main board is also pretty noisy.

While putting a box around the PSU is pretty straight forward, I'm concerned about the cooling.  The fan arrangement is less than impressive.  There's only 3/8" between the fan intake and the AFE shield.
 

Online nctnico

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Re: Instek GDS2000E EMI reduction
« Reply #1 on: November 21, 2018, 08:08:28 pm »
The simplest solution is to spray the inside of the casing with conductive spray paint. But then again I'm not quite sure what the gain is. I have done some measurements in the past myself and if you are more than a few centimeters away you can't see the noise from the oscilloscope in your measurements.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline rhb

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Re: Instek GDS2000E EMI reduction
« Reply #2 on: November 21, 2018, 08:28:42 pm »
Check around the back and sides.
 

Online nctnico

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Re: Instek GDS2000E EMI reduction
« Reply #3 on: November 21, 2018, 08:34:49 pm »
Check around the back and sides.
That is only relevant if you have something sensitive sitting directly behind the oscilloscope. The fields are magnetic anyway so they get lower exponentially over distance. At 20cm away you can't pick up anything with the oscilloscope itself using a H field probe.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Carrington

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Re: Instek GDS2000E EMI reduction
« Reply #4 on: November 21, 2018, 08:45:17 pm »
The simplest solution is to spray the inside of the casing with conductive spray paint. But then again I'm not quite sure what the gain is. I have done some measurements in the past myself and if you are more than a few centimeters away you can't see the noise from the oscilloscope in your measurements.
It is a good idea, but please don't do that.
The conductive paint can crack, so you will end with two problems instead of one:
https://www.eevblog.com/forum/testgear/review-of-owon-sds7102/msg293991/#msg293991
https://www.eevblog.com/forum/testgear/review-of-owon-sds7102/msg294630/#msg294630
https://www.eevblog.com/forum/testgear/review-of-owon-sds7102/msg295049/#msg295049
https://www.eevblog.com/forum/testgear/review-of-owon-sds7102/msg295159/#msg295159
https://www.eevblog.com/forum/testgear/review-of-owon-sds7102/msg302126/#msg302126
My English can be pretty bad, so suggestions are welcome. ;)
Space Weather.
Lightning & Thunderstorms in Real Time.
 

Offline rhb

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Re: Instek GDS2000E EMI reduction
« Reply #5 on: November 22, 2018, 12:22:34 am »
The only remedy I would consider is a steel enclosures.  The SMPS has a steel  barrier near the front.  There is both a strong E field and a strong H field from the transformer.

My E field probe was a regular scope probe with the ground and grabber removed.  I was amazed at how well it worked.  When I removed the pointed tip there was no E field pickup.  Just that 1/4" sticking past the shield works remarkable well.  My H field probe is equally elaborate, a 1/2" diameter loop of copper wire.  But it goes from strong signal to almost no signal when rotated 90 degrees. I've got a noise source which is not in my bench area.  Unless it was coming from the DSO.  I turned off everything and the noise was still there.  I plan to test next using the 485.

I wasted a bunch of time trying to make a durable H field probe, but finally concluded it's better to make them out of 24 AWG as needed.  A #60 drill was too big, so I switched to a paper clip for the form for the probe tip coil.

I put a regular 4" electrical box cover in a heavy plastic bag and put it between the transformer and the H probe.  It was very effective.  I feel confident that 18 to 20 gauge steel sheet will provide proper shielding.

I only discovered the Intsek EMI problem bu accident when a probe got shoved under the scope while I was working amid a lot of clutter.
 


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