Ripples on oscilloscope output - Page 1
Forum section: electronics / testgear

author: giosif
author_rank: Frequent Contributor
timestamp: November 14, 2014, 12:24:06 am
content: Hi,First, apologies in advance if this has been addressed before (a search did not return anything similar, though).I am using an old DSO oscilloscope and, when setting it for calibration, I am seeing this (picture 1.jpg)?Is it normal to have those ripples on the horizontal?Also, even with the probes not connected to anything, I still see some ripples (pictures 2.jpg and 3.jpg).Again, is this normal?Thanks,George

author: tautech
author_rank: Super Contributor
timestamp: November 14, 2014, 02:18:51 am
content: Try shorting the Ch inputs to see if ripple dissapears, if not the problem is internal.Download the Tek PDF in this thread:https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909Ensure the scope PSU is to spec, ripple & voltages.Use the "Trobleshooting the Vertical Section" in the PDF.Don't be shy to disclose the brand and model, others may have had the same fault and know EXACTLY what to do.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 14, 2014, 07:02:06 am
content: Blips a few pixels tall are normal. Digitization is a noisy process (quantization noise) and necessarily produces these artifacts. If the ripples are indeed the noise you've zoomed in on, that looks like environmental noise. You can avoid that using shielded cables and proper probing techniques.Tim

author: giosif
author_rank: Frequent Contributor
timestamp: November 20, 2014, 10:08:58 pm
content: Thanks for the responses and apologies for not replying any sooner (really busy days lately)!I've downloaded the PDF and will look into it the first chance I got.In the meantime, I did another test: I disconnected the probes and, surprise-surprise, the ripples went down to really small levels.So, I am assuming it must be the cheap, eBay purchased, no name probes that are inducing the noise.About my oscilloscope brand and model... I'm not shy. :-)It's just that I can't seem to find much info about it: it's a Gould DataSYS 750.I've searched high and low for a service manual but, apart from a couple of sites asking ridiculous prices for such manual, nobody else seems to have anything similar.Cheers,George

author: AG6QR
author_rank: Frequent Contributor
timestamp: November 20, 2014, 11:09:09 pm
content: Quote from: giosif on November 20, 2014, 10:08:58 pmEven top quality probes will often show noise when not connected to anything. Nearby radio transmitters, switching power supplies, computers, motors, and various other devices will generate electromagnetic waves, and a probe is supposed to pick up changing electric fields. If the noise that's being picked up is coming from external sources, the fact that it's being picked up is not necessarily a sign of a probe malfunction, it may just be a sign that the probe is doing its job.Connect the probe across a resistor, or just short the probe's tip and shield together, and the noise should be lower. But it still won't be completely zero, because the inductive magnetic loop formed by the probe, resistor, and ground lead can still pick up changing magnetic fields.To see if noise is being generated inside the scope, remove the probes and plug a 50 ohm shielded terminator directly into the scope input jack. That should show as close to zero noise as you're going to get.

author: giosif
author_rank: Frequent Contributor
timestamp: November 21, 2014, 08:16:54 pm
content: Ah, ok.Goes to show just how "much" I know of oscilloscopes and electronics in general.I've ordered a couple of terminators but, in the meantime, I'll try it with the resistor.Thanks for the information!Regards,George