Two Oscilloscopes Connected to the Same Input Source Showing Differing Traces - Page 1
Forum section: electronics / beginners

author: rman
author_rank: Newbie
timestamp: April 04, 2019, 01:29:37 am
content: Hello. My first posting. I'm very much a novice and I don't know how to interpret some results. I recently purchased a Tektronix 2215 60MHZ 2 channel scope and a Tektronix 2246 100MHZ 4 channel scope. I don't know when either scope was calibrated last, but both seem to function reasonably well. I'm only going to keep one of the scopes. I connected each scope to the same function generator set to input a sine wave at the same frequency and amplitude to compare the resulting trace on each scope. The Vertical and Horizontal parameters were set the same so I expected to see the same looking trace on each scope. However, the 2215 was showing 4 complete sine waves while the 2246 showed only 3 complete waves. The peak to peak voltage was the same. So, don't know if I have a problem with one of the scopes. The peak voltage was a bit over 1V, but I didn't check what the frequency was set to, but seems to me both scopes should be showing the same trace. Right?

author: Circlotron
author_rank: Super Contributor
timestamp: April 04, 2019, 01:37:24 am
content: Most scopes have a knob to adjust the horizontal sweep rate to any desired in-between rate besides the 1-2-5 stepped selection knob. Maybe this first knob is not right at the "cal" position?

author: ArthurDent
author_rank: Super Contributor
timestamp: April 04, 2019, 01:38:28 am
content: Have you checked to see that the timebase on both scopes are set to the same sweep rate and both are in the calibrated position?

author: Brumby
author_rank: Unknown
timestamp: April 04, 2019, 01:40:51 am
content: SNAP!The two posters above beat me to saying the same thing.Check the variable time base control is in the "CAL" position - on both scopes.

author: DDunfield
author_rank: Regular Contributor
timestamp: April 04, 2019, 01:43:28 am
content: Make sure both scopes:- Have exactly the same number of graph divisions across the screen.- Are set to exactly the same horizontal sweep rate- If there is a variable sweep control that it is set to "Calibrate" which means exactly the rate on the horizontal sweep control.- Are set to trigger in exactly the same position on the waveform.Any of the above can make the traces look different.If the distance between the peaks is the same on both scopes (as measured on the graph) then they are both showing the same horizontal timebase and most likely the problem is that the scope is being triggered at a different point in the waveform.If the peaks are NOT the same distance apart, then the horizontal sweep rate is different.If you know the precise frequency of the signal, then you know the expected time between peaks and you can work out which scope is "wrong" ... then it's just a matter of seeing how it's set differently (or possibly if there is an actual fault).Dave

author: rman
author_rank: Newbie
timestamp: April 04, 2019, 02:16:25 am
content: Thanks for all the quick replies. I have egg on my face. I know about the cal buttons. I placed the 2215 scope on top of the 2246 scope and must have somehow touched the cal button time base on the 2215. It was not on cal, slightly off. Should have checked that. I checked everything else. That's why I'm a novice. Now have to figure out why the new to me 2246 shows flickering of the trace and some ghost sweeping compared to the 2215 trace. This should keep me busy. I do have the full manuals for both scopes. Thanks again for helping me out.

author: Berni
author_rank: Super Contributor
timestamp: April 04, 2019, 05:10:53 am
content: That could simply be due to a longer persistence phosphor in the screen.

author: David Hess
author_rank: Super Contributor
timestamp: April 04, 2019, 08:18:49 am
content: Quote from: rman on April 04, 2019, 02:16:25 amThe CRT readout on the 2246 is time multiplexed with the signal which causes flicker at some sweep speeds. The 2215 has no readout.Disabling the readout using the readout intensity control should stop the flicker on the 2246.

author: rman
author_rank: Newbie
timestamp: April 04, 2019, 12:04:22 pm
content: Thanks. I will try the readout intensity.