1
\$\begingroup\$

I am prototyping a NTC voltage divider circuit and I am having trouble with noise on my probe. It seems like I am constantly getting a large amount of noise (200-300mv) on my oscilloscope waveform even when all I am doing is probing a small wire. 200-300 mV is a lot isn't it? I feel like I can't really gauge the actually noisiness of an analog signal if my oscilloscope always has this minimum 200mV p2p noise on it.

enter image description here

enter image description here

Where is this noise coming from, and what can I do to remove it?

EDIT: Here are some more picture of the calibrating probe: enter image description here Zoomed in: enter image description here

There seems to be a constant 50mV p2p noise signal when calibrating...

\$\endgroup\$
  • \$\begingroup\$ When was the last time you calibrated your oscope? \$\endgroup\$ – KingDuken Jan 16 at 19:32
  • 5
    \$\begingroup\$ That's a nice antenna you've built in your second picture ;) \$\endgroup\$ – marcelm Jan 16 at 19:33
  • \$\begingroup\$ It's been a very long time since I've calibrated my oscilloscope. Could that be the issue? \$\endgroup\$ – Alex C Jan 16 at 19:36
  • \$\begingroup\$ @marcelm The noise happens even if I probe a line on my breadboard. It's like it doesn't even matter what I am probing. \$\endgroup\$ – Alex C Jan 16 at 19:37
  • 1
    \$\begingroup\$ You've made a loop antenna out of your probe in the picture, as @marcelm points out. You're picking up ambient radio noise. \$\endgroup\$ – Hearth Jan 16 at 19:42
2
\$\begingroup\$

200mV is rather large. It is possible that it is just being picked-up by the antenna formed by your probe connection, but unless you are in a very noisy environment that might be unlikely. It could be that your oscilloscope is defective.

  1. Use a BNC termination block directly at the oscilloscope input, is the noise still there? (If you don't have one you can build one by soldering a 50Ω resistor directly inside a BNC connector, alternatively you can just insert a very short piece of wire (e.g., a piece of a paperclip) to short the connector.
  2. If your oscilloscope can change the bandwidth (e.g., from 100MHz down to 20MHz) is the magnitude of the noise the same? Is the noise reduced by more than \$\sqrt{5}\$?
  3. Is the noise the same for both channels?

Those would all be indications of a noisy input stage or sampling stage to the oscilloscope channels.

If shorting the input directly at the connector removes the noise, then environmental noise is your problem. You would probably have to look into how circuits are grounded and what antenna loops are present in your measurement setup.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.