OWON Oscilloscopes

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 Post subject: Solved by Owon 2013. Was: "Under work. (About GND noise)"
PostPosted: Tue Feb 12, 2013 1:27 am 
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Joined: Wed May 16, 2012 7:54 am
Posts: 187
Under work! Not ready!






Oscilloscope SDS7102(V).
(this individual unit have noisy GND.)

(some manufacturing lots have noisy GND. There is spread spectrum noise in GND due to some undefined reason. (it is under inspection)
_____________________
UPDATE
Reason is not anymore fully undefined.
Nearly all SMPS inside oscilloscope produce its own noise peaks/parasitic oscillations.
They looks like spikes.. they are up to >200MHz oscillations. I have analyzed every single SMPS waveform inside scope. Then also looked noise so that one channel in external oscilloscope show switcher waveform and scope is also trigged with it. Other channel show "common GND noise". With this method it can regognize noise peaks just as who is who and where from this individual come) Becouse probing switchers is really difficult and also what ever you connect there it change something. Also there need care about probe ringing becouse there is very fast edges. So it need be sure these "ringings" (oscillations) are true and not made by test. This is easy to proof they are there. Running oscilloscope alone without any connections (with battery and without battery also): nearfield spectrum analyze with small magnetic loop antenna proof it fully (test made with HP8668B).


Peaks come from main power unit (there is 2 switch mode DC-DC converter) and then from "adapter card" where is 4 switch mode DC-DC converter. Both of these are new versions. Problem is related to PCB layout parasitic inductances and capasitances and selected components. (this is only speculation what is also possible: it may be so that originally tnew board versions have been acceptable in lab but then later some componets have changed before mass production.)

Of course there is more deep technical documentation about whole case but these are going first to Owon , they are not public, and it is Owon headache how they solve and handle this whole case.

____________________





Small test show how important is probe GND specially in this case.
Normally we think that probe "longwire" GND clip is ok with low frequencies and / or slow risetimes. Normally this is true.
It need know that this noise do not pollute signal pathway inside oscilloscope. So, if signal is clean in oscilloscope input BNC there is not extra noise.
Oscilloscope probe GND wire have inductance. Around 10cm GND wire inductance is ~120nH
120nH reactance at 100MHz is around 75ohm. 15cm GND wire have around 190nH and it can think as 120ohm. Yes, this straight copper wire. How about 200MHz this 15cm. Of course around 240ohm. So, for 200MHz signal it is not very tight "ground". Now if we put 200MHz signal travel over this GND wire... you know.. it produce voltage over this "inductor" what do not looks like inductor "coil" but in real world - it is.

Here you see what difference is with oscilloscope "long wire" GND and very short (effective lenght around 1cm) GND wire.

With this method you can see how it reduce signal noise.

If there is not this special noise... this is still useful specially if you look signals what have fast changes (example fast rising/falling pulses).

Image
Picture A.
Signal source 50 ohm. Probe connected to signal output using probe "long" GND wire.






Image
Picture B.
Signal source 50 ohm. Probe connected to signal output using very short GND wire.






Image
Signal output 100mVpp. Probe 10x --> real signal to oscilloscope input is 10mVpp (oscilloscope probe setting 1x). Probe connected using "long" GND wire (picture A).




Image
Signal output 100mVpp. Probe 10x --> real signal to oscilloscope input is 10mVpp (oscilloscope probe setting 1x). Probe connected using short GND wire (picture B).



Image
Signal output 100mVpp. Probe 1x --> real signal to oscilloscope input is 10mVpp (oscilloscope probe setting 1x). Probe connected using short GND wire (picture B).



Image
Signal output 10mVpp. Probe 1x --> real signal to oscilloscope input is 10mVpp (oscilloscope probe setting 1x). Probe connected using long GND wire (picture A).



Image
Signal output 10mVpp. Probe 1x --> real signal to oscilloscope input is 10mVpp (oscilloscope probe setting 1x). Probe connected using short GND wire (picture B).


As you see, in this case just so small thing and noise is "away"....

But, this is not solution to this noise problem. This is some kind of first aid for live with this issue before there come real solution.

Soon I will add some more test pictures.. also so that there is Owon and Tektronix (TDS520A) looking same things.


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 Post subject: Re: keskeneräinen työkansio
PostPosted: Tue Feb 12, 2013 1:38 am 
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Joined: Wed May 16, 2012 7:54 am
Posts: 187
Under work.
Later I will add some explanations and later also more pictures.


Here looked Owon GND. Note that both, probe tip and probe GND is connected to probe comp. output GND (all GND's are together inside scope).
Note: Probe setting is 10x and oscilloscope set for 1x (becouse Tektronix can not regognize Owon probe and it things it is 1x)
Probe is set for 10x becouse only 10x it have full frequency bandwidth. If probe is set for 1x it is like low pass filter <6MHz. Oscilloscope probe setting do not affect BW, it affect only to scale factor.


With normal "longwire" GND

Image


Image

Image




With short GND:

Image

Image

Image


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 Post subject: Re: Under work. (About GND noise)
PostPosted: Tue Feb 12, 2013 9:14 pm 
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Joined: Wed May 16, 2012 7:54 am
Posts: 187
Primary problem is here in the new adapter board version:

How to isolate noise... no...NO...NO... this is wrong road!

Do not make noise - this is right road. (and I hope Owon later select this road)



Image

Later I will explain what is what and who is who in this noise "chaos". (This kind of one shot image do not tell how chaotic it is on the live display. Also if different noise peaks randomly appear just same time it give high peak so it looks complex in live.


This is just "GND noise" - not exactly but this is perhaps ok workname for this issue
Image


There is several sources for noise "peaks". (also PSU unit but this later).
Some peaks come from one source and some other peak from some other source and sources are not in sync together.
I have made some more deep tests with "synchronizing method" for recognize separate peaks who is who and where from it come. For this tests I have run oscilloscope now first with clean linear power supply (+8.4V and -7.6V).


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 Post subject: Re: Under work. (About GND noise)
PostPosted: Sun Feb 17, 2013 7:41 pm 
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Joined: Wed May 16, 2012 7:54 am
Posts: 187
Here you can find more tests.

It is downloadable 1.17M zip file including some test pictures and explanation txt file.

Later I will add this info to formum but now too busy and also there need do some corrections.


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 Post subject: Re: Under work. (About GND noise)
PostPosted: Sun Mar 10, 2013 9:36 pm 
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Joined: Wed May 16, 2012 7:54 am
Posts: 187
Here is available DIY (version 2b) for modify it better.

Please read carefully it before open oscilloscope and start anything.
(also note that there is small mechanical changes and it need know before open so that damage can avoid)

After modification this noise is reduced.

Modification need SMD rework skills and also good tools for it.

In Finland my customers can contact me for this. If I do it to my customers also warranty continue as before. Also Owon know and accept this modification. Also they transfer this and possible other changes to production.

If you want do it yourself then factory warranty is void.


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