Smoothing motion of a Lissajous pattern on a Siglent SDS1202X-E scope? - Page 1
Forum section: electronics / beginners

author: elroy
author_rank: Regular Contributor
timestamp: September 23, 2019, 08:41:21 pm
content: I'm displaying a Lissajous pattern on my Siglent SDS1202X-E scope in order to zero-beat a reference signal from a waveform generator to a signal under study.My mentor who is advising me noticed that the motion of the Lissajous pattern as it rotated seemed jerky. He felt this made it more difficult to get the frequency dialed-in quickly, and recommended either using an analog scope (as he does), or finding a way to smooth the motion of the Siglent's display.He suggested taking a look at the sample rate on the Siglent. The manual indicates this is controlled by the Horizontal Scale Knob. Turning this, however, didn't resolve the problem; at finer time intervals the Lissajous circle changed to a line and then a dot.Any suggestions on a way to accomplish what we're trying to do? If it makes a difference, the frequency involved is about 15 kHz.

author: Tom45
author_rank: Frequent Contributor
timestamp: September 23, 2019, 08:45:00 pm
content: If you are trying to match the two signals why not just connect one signal to each channel and trigger off of one of them. Set the horizontal time base to show one or two cycles.The channel you trigger on will be stationery, while the other channel will drift left or right if the frequencies don't match, and will be stable when they do match.

author: elroy
author_rank: Regular Contributor
timestamp: September 23, 2019, 10:47:35 pm
content: That's certainly possible -- in fact, when I took the scope out of XY mode, it was already set up with the display triggering from the X channel input. In this view, the channel X waveform appears rock-solid dead-still, whereas the Y channel waveform shakes back and forth a fair amount as it very gradually drifts one way or the other. This is a little surprising, since the Y channel input is from a Siglent SDG2042X generator. I assume the jiggle doesn't necessarily mean that the generator's signal is unstable, but maybe it appears to be in relation to the X channel signal -- which could mean that it's actually the X channel signal that is wobbly (this being RF transmitted at a distance).Overall I can't honestly say which view makes it easier to zero-beat the signals as quickly as possible. I think my advisor just likes working with the Lissajous pattern, but is used to seeing it render more smoothly.

author: tautech
author_rank: Super Contributor
timestamp: September 23, 2019, 11:36:42 pm
content: Quote from: elroy on September 23, 2019, 10:47:35 pmA combination of both methods IMO.In 2ch mode you can get info on the frequency of each channel with frequency measurements and in XY mode fine tune frequencies for a stable circle.Last time I looked at this with SDS1202X-E was with a much earlier firmware and I found a 10Hz difference was about as much as they liked for a smooth XY pattern. Get the frequencies close then switch to XY is my recommendation.

author: Tom45
author_rank: Frequent Contributor
timestamp: September 24, 2019, 02:46:39 am
content: I would expect that the SDG2042X is very stable. If it wasn't there would long threads about it in these forums.So, you should be triggering on the stable SDG2042X signal. Any jitter you see on the other channel is due to the signal coming in on that channel. You can also more clearly see the signal quality of your test signal, or lack of quality.For what you are apparently trying to do, I see no reason to use Lissajous curves in XY mode. Triggering on the reference signal and watching the other (untriggered) channel for a drifting and jittering trace works equally well for analog and digital scopes. Lissajous curves work well for analog scopes and generally suck with digital scopes.