Scope blurred trace - Page 1
Forum section: electronics / testgear

author: gaminn
author_rank: Frequent Contributor
timestamp: October 14, 2015, 02:37:54 pm
content: Hi,on my LeCroy WaveJet 354 (500 MHz) I get "blurred trace" for 500 MHz sine wave. Also, the scope reads 31 mV RMS voltage but there is 100 mV set on the generator. It is the same on all channes. On LeCroy wavesurfer 104mxs the trace and voltage is OK. Please look at the pictures.Is it normal?

author: mij59
author_rank: Frequent Contributor
timestamp: October 14, 2015, 02:48:35 pm
content: Yes this is normal, most oscilloscopes don't trigger very well on a AM modulated signal.

author: gaminn
author_rank: Frequent Contributor
timestamp: October 14, 2015, 03:18:22 pm
content: Thanks. What is typical attenuation scopes have at their "margin" frequency (for this scope at 500 MHz)?

author: kfitch42
author_rank: Frequent Contributor
timestamp: October 14, 2015, 03:21:11 pm
content: Quote from: gaminn on October 14, 2015, 03:18:22 pmThe rated bandwidth of a scope is usually specified at the -3dB point, i.e. you should only expect about 70mV on the scope from a 100mV pure sine wave at the rated frequency of the scope.

author: nfmax
author_rank: Super Contributor
timestamp: October 14, 2015, 03:38:53 pm
content: Quote from: mij59 on October 14, 2015, 02:48:35 pmBut the generator is clearly showing that AM is OFF (I have an 8647A too)

author: gaminn
author_rank: Frequent Contributor
timestamp: October 14, 2015, 03:58:39 pm
content: Ups, I didn't notice the "AM modulated" on mij59's reply.The signal is harmonic, 500 MHz sine wave. So?

author: kfitch42
author_rank: Frequent Contributor
timestamp: October 14, 2015, 04:38:16 pm
content: OK, now it's time to be systematic:1) Go through scope settings and make sure you don't have some weird mode turned on by accident (e.g. b/w limit, trigger hold-off, etc...).2) Play with trigger level, if it is close to the top or bottom of the waveform the trigger could be unstable.3) Play with different frequencies. When does the problem become visible? 1MHz? 10MHz? 100MHz? 200MHz? 400MHz?4) Play with different amplitudes. You see problems at 100mV RMS, what about 50mV? or 1000mV?You are pushing this scope pretty hard. It only has (at best) 2GS/s*. That is only 4 samples per cycle. If you enable all the channels (or probably just the 'wrong' two channels) you are at 1GS/s... which is giving Nyquist a bit of a close shave.*http://cdn.teledynelecroy.com/files/pdf/lecroy_wavejet-a_oscilloscope_datasheet.pdf

author: Kalvin
author_rank: Super Contributor
timestamp: October 14, 2015, 04:57:25 pm
content: What happens if you set the frequency to 400MHz, for example? At what frequency the problem appears? The coax is 50 ohm? Have you checked with another coax?

author: gaminn
author_rank: Frequent Contributor
timestamp: October 14, 2015, 05:10:28 pm
content: I don't have access to the scope right now so I will perform all the tests later.I have also picture of 100 MHz wave with 500 ps horizontal resolution. From that we can guess the issue is not related to the frequency of the signal.I haven't tested with another 50ohm coax, but I mesuared the signal with another scope (same generator, same coax) and the trace is clear on that scope.

author: mij59
author_rank: Frequent Contributor
timestamp: October 14, 2015, 05:14:20 pm
content: Quote from: gaminn on October 14, 2015, 03:58:39 pmYour right, its not modulated, it looks more like a shift in the vertical axis.Same problem with the sensitivity set to 20 mV/div ?

author: kfitch42
author_rank: Frequent Contributor
timestamp: October 14, 2015, 05:41:13 pm
content: Quote from: mij59 on October 14, 2015, 05:14:20 pmHmm, I think you are right that this is a vertical problem, not horizontal. I was thinking horizontal/trigger jitter, but I think you are right about vertical. The trace appears to be just as thick at the top as it is in the middle. That made me take a closer look at pics and I noticed they were all DC coupled. Does it go away with AC coupling? Was the scope with the clean trace AC coupled? If I am reading the first pic(s) correctly they show a DC offset of 600uV. Does this number jump around? Could we be seeing common mode noise and/or grounding issue? We are seeing it on all channels, so maybe it's a bad connection to the mains ground/earth from the scope? Try swapping power cords with the "good" scope?CAVEAT: I am a software guy who plays an EE at home and occasionally at work, so I apologize if I have butchered anything (terminology/physics) here.

author: Bud
author_rank: Super Contributor
timestamp: October 14, 2015, 06:30:21 pm
content: Turn roll mode off.Do not overload the scope, make the complete waveform fit vertically.

author: nfmax
author_rank: Super Contributor
timestamp: October 14, 2015, 09:47:47 pm
content: Try selecting RF off on the generator, leaving everything else in place. You should get a horizontal zero volt trace. If not, suspect ground problems somewhere.