Why does my oscilloscope show different fluctuations depending on volts/div - Page 1
Forum section: electronics / beginners

author: AQUAMAN
author_rank: Regular Contributor
timestamp: December 17, 2014, 02:29:11 pm
content: HelloI have a Lecroy 12 bit oscilloscopeIt is very good. However one thing massively bothers me about it and has caused a lot of hassle.Basically it is to do with the volts per division. For some reason, as soon as you go below 1.02V/div the probes begin to show completely different values.For example, I am looking at how a signal changes with frequency from 1kHz-30kHz:With my probe set at 1.02V/div, approximately:1kHz = -9.990V30khZ = -9.800VWith my probe set at 1.00V/div:1kHz = -9.940V30kHz = -9.550VThis is not only an offset, which wouldn't be a problem, but the oscilliscope is ACTUALLY SHOWING DIFFERENT FLUCTUATIONS FOR THE SAME SIGNAL. It seems like the 1.00V is completely wrongWhy is this? What is the difference between 1.02V/div and 1.00V/divI just took an entire setup apart to work out what was wrong and I think that now it is actually the probes.One thing is that I can hear the oscilloscope 'click' when i move from 1.02V to 1.00V. So there is something inside it that is changing.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2014, 03:31:31 pm
content: What do you mean by "probes set at 1.02V"?What is the stated accuracy of the scope? 9.94V is within 5% of 9.99V.

author: ExtremHybrid
author_rank: Newbie
timestamp: December 17, 2014, 03:35:12 pm
content: I dont know the details of your scope since I dont own that model. But keep in mind oscilloscopes need calibration ever so often. Just put that option on your list of things to check/do, in case you dont find any other solutions.

author: KedasProbe
author_rank: Frequent Contributor
timestamp: December 17, 2014, 03:40:13 pm
content: Nice scope, I wouldn't expect such error for this not cheap scope, can you give some screenshots (or Video showing changing the volt/div.)If you don't want to use it anymore and buy a new one due to this problem you can always send it to me, I will take it of your hands for free

author: macboy
author_rank: Super Contributor
timestamp: December 17, 2014, 03:42:32 pm
content: Quote from: tggzzz on December 17, 2014, 03:31:31 pmActually it is 0.5%.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2014, 04:29:19 pm
content: Quote from: macboy on December 17, 2014, 03:42:32 pmHow very annoying. Sometimes I really dislike facts.

author: w2aew
author_rank: Super Contributor
timestamp: December 17, 2014, 04:38:43 pm
content: Scopes are typically calibrated on their "major" step settings, such as 0.1V/div, 0.5V/div, 1V/div, etc. The "fine" settings between the major steps are often simply mathmatically scaled values from one of the "major" step settings, thus may be subject to a larger error.The "click" you hear is likely an attenuator change. Most scope front ends are built with a channel preamplifier that is either fixed gain, or made with just a few gain steps. This is done so that the channel response can be carefully controlled and calibrated (which is much more difficult and expensive to do with a variable gain amplifier). The various volts/div settings are achieved by using a passive attenuator ahead of the preamp - thus changing the signal level into the "fixed gain" amplifier. At the "fine" settings which lie in-between the attenuator steps, the results are often mathmatically scaled down from measurements taken from the next higher attenuator setting.

author: AQUAMAN
author_rank: Regular Contributor
timestamp: December 17, 2014, 07:21:37 pm
content: Quote from: macboy on December 17, 2014, 03:42:32 pmFurthermore the offset doesnt actually matterIts the fact that the oscilloscope shows 200mV or 400mV difference from 1khz to 30khz depending on the volts per division

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2014, 07:49:14 pm
content: Quote from: AQUAMAN on December 17, 2014, 07:21:37 pmSo,What do you mean by "probes set at 1.02V"?What is the stated accuracy of the scope?

author: AQUAMAN
author_rank: Regular Contributor
timestamp: December 17, 2014, 10:46:59 pm
content: Its clear what I mean, I said 1.02V/div

author: ivan747
author_rank: Super Contributor
timestamp: December 17, 2014, 11:47:46 pm
content: He set the vertical volts per division in vernier mode, and adjusted it down to 1.02V, in my interpretation.Check if the accuracy specifications of your oscilloscope matches the error that you see.Have you tried self-calibration?

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2014, 11:56:14 pm
content: Quote from: AQUAMAN on December 17, 2014, 10:46:59 pmI've never seen a "probe set to" whereas I've seen "channels set to".And what's the stated accuracy of the scope? Are the measurements that are within the stated accuracy? If so, stop wasting our time.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2014, 11:57:47 pm
content: The voltage changing when the frequency is adjusted also implies the probe isn't adjusted properly or the signal generator's amplitude doesn't remain constant.