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I've got strange noise while using digital oscilloscope (Auris B-424). Spectrogram of this signal presented on this image: (X time in seconds, Y frequency in Hertz, in this image case from 0 to 50 MHz) Signal spectrogram Zoomed versions: 1, 2 On oscillogram noise looks like impulse interference with amplitude approximately 10 millivolts.

Simple scheme (fiber semiconductor detector) is connected to the input of oscilloscope with short 50 Om coaxial cable. Without anything on input channel noise is significantly lower (10-15 times), although still presented. Surge protector and ground used for powering oscilloscope and notebook. (outer conductor of coaxial cable is grounded too) My main guess is impulse interference from notebook and oscilloscope itself, but I can't find way to deal with it.

What do you think? What nature has this noise and is there any way to reduce it?

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  • \$\begingroup\$ Link to full size spectrogram image: i.piccy.info/i9/4154d2bdedc0b854ba652d43faab42b3/1406231213/… Full data set if somebody will need it (~400 MB; first column is time and second is voltage): filedropper.com/noise_1 \$\endgroup\$ – user56039 Jul 25 '14 at 7:48
  • \$\begingroup\$ What are you measuring, how are you measuring it? Can you give a diagram? What are the specs of the ADC and frontend in your oscilloscope? \$\endgroup\$ – user36129 Jul 25 '14 at 8:25
  • \$\begingroup\$ The picture has no dimensions and what 1st column and what 2nd column? I see no columns. \$\endgroup\$ – Andy aka Jul 25 '14 at 11:16
  • \$\begingroup\$ I have no idea what I'm looking at. But certainly digital 'scopes and other electronics put out "stuff" that can get into your circuit. Try turning them off or moving them further away. Most DSO crud that I've seen comes from the display and that can be shielded electrostatically. \$\endgroup\$ – George Herold Jul 25 '14 at 13:11
  • \$\begingroup\$ user36129: Link to 400 MB CSV file of recorded data is given in the first comment (16M samples) Oscilloscope specs can be found here (unfortunately only Russian language) \$\endgroup\$ – user56039 Jul 25 '14 at 15:16
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Looking at the zoomed versions, and assuming the X axis is time in seconds, you can see a 10ms periodicity in the strongest component, and many of the other components show the same periodicity.

That points at full wave rectified 50Hz mains as being the underlying cause. It provides a rich source of switching spikes and current transitions at 100Hz.

If I read the first zoomed one correctly, there is something oscillating at 27MHz, frequency modulated by 100Hz spikes. If you can identify the source of that frequency, you may be a lot closer to your answer.

Where you go from here is not so clear :

  1. there could be EM coupling from high power supplies into sensitive circuitry : screening would reduce the electric field coupling, but magnetic screening (iron or mu-metal) may be required if the coupling is magnetic. (If your sensing circuit is a high impedance node, suspect electric coupling; if low impedance, suspect magnetic). Walk round the room turning everything else off while someone watches the scope and shouts when you make a difference. This includes heating, air conditioning, computers, and especially lighting since you mention fibre optics.

  2. there could be noise on your power supplies. If you have sensitive amplifiers powered from a switching PSU, run them from a linear regulator instead. Improve decoupling. Measure the noise on your power and ground lines. Isolate any high current circuitry from this system. Adopt star earthing and eliminate any ground loops.

  3. There could be noise on the scope's own power supply. Amplify the signal (without introducing any of the problems above) to make the scope's own noise less significant. Or buy a better scope...

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  • \$\begingroup\$ 27MHz is a RC frequency. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 26 '14 at 13:10
  • \$\begingroup\$ And a CB frequency. There may be nearby ISM bands (e.g. RF heating) for all I know. \$\endgroup\$ – Brian Drummond Jul 26 '14 at 13:40

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