Nature of noise gotten from digital oscilloscope

Question

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) 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?

Answer 1

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...