Lissajous Figures: Indulge your Inner Geek!

[bob p]

I bought my second function generator the other day, so I decided to indulge my inner geek by drawing some lissajous figures on the oscilloscope. It was actually harder to take good photos than I had expected it to be. Although some of the snapshots are interesting, seeing the moving waveforms on the scope is alot more fun.

[bob p]

And just in case you're hungry:

[Rob Mercure]

Hey Bob,

Really nice pics and I still remember my frustrated and feeble attempts at recording 'scope images from the late 1960s - early 1970s. With sufficient tweaking - and a little slop from the mid-grade early '60s EICO tube 'scope I had then (not perzactly a "flat faced" CRT) - I could get the images to slowly "weave" and rotate providing a nice cheapo light show for all of the town freaks gathered into my bedroom, work room, "laboratory" (use your best Boris Karloff accent as you pronounce the word) - clear still photos were beyond the 16 year-old me and I eventually gave up. But now I notice others others posting snapshots on websites that aren't that great an improvement of the ones I rejected 37 years ago.

Yeah, just what I needed, a "cup of Lissajous" to start the morning.

Rob

[bob p]

... I could get the images to slowly "weave" and rotate providing a nice cheapo light show for all of the town freaks gathered into my bedroom, work room, "laboratory" (use your best Boris Karloff accent as you pronounce the word) ...

Glad you enjoyed it, Rob. You know, getting those photos turned out to be a lot easier in theory than in practice. The biggest problem is that you have to have the frequency ratios precisely correct to see the lissajous figures, and that's harder to accomplish than it sounds. Even when my HP frequency counter told me that the ratios of the two function generator frequencies was spot on (to a number of decimal places), there was still enough frequency variation to screw up the lissajous figures. It was virtually impossible to get them to hold still, so most of the time I had to settle for slow rolling of the displays.

Its really amazing how much an infinitesmal difference in the frequency can screw-up the lissajous figures. It took a very steady hand just to get close to displaying recognizable figures on-screen.

Then there's the photography problem. I have a flat-screen scope, but it was still difficult to get the photos. I had to darken the room and shoot a whole bunch of photos with my digital camera on a tripod. Then once I had a bunch of photos, I ended up throwing out most of them.

It was a fun experience. Every tech geek should try it at least once so they can fully appreciate how hard it was to measure unknown frequencies back in the old days.

[bob p]

I copied this quote from one of the oscilloscope threads and put it here to keep things on topic.

BTW - bob those Lissajous figures look very cool. I was gonna make a 'spirograph' joke (remember those?) but nah...they look tres cool.

I did some very light reading on them...I guess they appear to 'rotate' (in a 3D sense) and that the direction of rotation is...what's the word...merely a 'perception' and could be in either direction (or both...or it might switch directions arbitrarily).

If any of you remember the TV show "The Outer Limits," they displayed a lissajous figure on-screen during the opening credits for the show. The voiceover said, "Don't adjust your TV -- we control the horizontal and the vertical!" That was the utmost in high-tech in the 50s!

I always thought that the lissajous figures were cool ever since the time that I first played with an oscilloscope in my college physics lab. Its taken me 25 years or so since then to get my own gear, and as soon as I got the second function generator I didn't waste any time.

Yes, the static photos that I've posted here really don't do the Lissajous Figures justice. The figures do move about, as its damned near impossible to generate frequencies in the exact ratios to make the figures stand still. I took lots of photos and posted the best ones.

If you do try to draw these on your own, you'll find that the figures appear to rotate in space as 3-D designs. Whether they rotate left and right is really an optical illusion -- your brain can interpret them as moving either way.

If you don't have a scope, you can google for lissajous and find a java applet to draw them in a browser. Its a good tutorial for learning how they work. As Enzo said in that other thread, they're defined by the phase relationships and the ratio of the frequencies of the horizontal and vertical oscillators, and they can be used to determine the frequency of an unknown signal if you don't have a frequency counter. You just need to know the frequency of one signal to measure the other.