Analog vs digital X-Y mode

[Miti]

Just for fun, I guess no digital can beat an analog scope at X-Y.

[tautech]

Link to file please so we can all play. 

[james_s]

File? Isn't it a piece of hardware? I have a scope clock that I built from a kit years ago, the particular one I have is a standalone device but I've seen boards meant to connect to an existing scope.

As much as I love my Tek DSO, the XY mode is pretty hopeless, that is one of the few things where analog wins hands down.

[Miti]

It's a kit.

http://www.dutchtronix.com/ScopeClockH3-1-Enhanced.htm

[Tom45]

So why do digital scopes do so poorly in X-Y mode?

[ataradov]

So why do digital scopes do so poorly in X-Y mode?

Pixels are much more visible. I'm sure some of those $5000 R&S scopes will look much nicer.

And I'm pretty sure scope manufacturers simply don't care, just like with FFT function. I personally never had to use XY mode for anything in my entire life.

[egonotto]

Hello,

I think most DSO are good in streaming. They capture many samples and after they postprocess the samples. In x-t-mode they "know" the best samplerate.
But in x-y-mode they dont know the best  sample rate. If you have 10 Hz than a sample rate of 20kHz would be good, but if you have  10Mhz than you need a much higher samplerate.
Perhaps they take 1 sample and process this sample and so on, but this slows the whole.

Best regards
egonotto

[Performa01]

Setting up X/Y mode is different for analog vs. digital.

For analog, we only need to take care of the channel gain.

For digital, we also need to set the timebase to make sure at least one full signal period is captured (but also not too many more). This also determines the sample rate which is usually plenty high enough, hence not a problem.

What is a problem though is the low amplitude resolution of an 8-bit digital scope.

8 bits means 256 steps, but most scopes allow some overscan and e.g. only 200 steps are visible on the screen. On a 400 pixel high display, this means two pixel per amplitude step. In X/Y mode, the same is true for the x-direction. A picture composed of 2x2 pixel clusters on a low resolution 400x400 screen area just cannot look pretty, full stop.

A DSO with at least 10 bits and high-res display should be able to produce reasonably nice X/Y figures.

[tautech]

We'll all know when DSO's have really come of age when they can handle Jerobeam Fenderson's oscilloscope music.
https://www.youtube.com/user/jerobeamfenderson1

Yummy stuff.   

[rf-loop]

Also with digital scopes there is one thing what also may affect result. There is some different methods to do X-Y
Some DSO's may use direcft XY plot method (read X and Y ADC and plot point and repeat)
Some DSO may use normal acquisition to sampling buffer (just as in YT mode) and  then process it for X-Y "image")
This last method make possible to do many automatic measurements also in XY mode. Rise times, pos and negative width, periods - whole buch of measurements including full sample resolution rise and fall times, pulse widths etc etc..


I do not know any reason to plot analog clock to oscilloscope display - exept fun. Oscilloscope is tool mainly for inspect more or less unknown signal. But of course if want entertainment box there is lot of many kind goods for this purpose. Much better than oscilloscope. I do not even know where XY is useful today. For what?

Yes I have played with clock and other images with  my previously owned couple of Tek7000 series and some Tek portable tool as 2465 etc.

[Fungus]

We'll all know when DSO's have really come of age when they can handle Jerobeam Fenderson's oscilloscope music.
https://www.youtube.com/user/jerobeamfenderson1

Yummy stuff.   

My cat loved it. 

[Tom45]

Also with digital scopes there is one thing what also may affect result. There is some different methods to do X-Y
Some DSO's may use direcft XY plot method (read X and Y ADC and plot point and repeat)

Or instead of plotting points,  draw a vector from X_n-1, Y_n-1  to X_n, Y_n

There is still the question of what sample rate to use. But I don't see any downside to oversampling. So why not sample as fast as possible.

But after thinking about it, perhaps the tricky part is how to emulate phosphor persistence.

[David Hess]

So why do digital scopes do so poorly in X-Y mode?

DSOs have six problems with X-Y mode:

1. Non-index graded displays cannot be anti-aliased and most DSOs with index graded displays do not anti-alias anyway or anti-alias poorly; this becomes an additional source of quantization noise.  Note that the analog examples above show a raster displayed on a vector display device producing further aliasing on the DSOs.  A true vector implementation would look better on both types of oscilloscopes.
2. DSOs suffer from quantization noise from the digitizer which is usually handled poorly in X-Y mode.  High resolution mode should solve this.  Average acquisition mode should also solve this unless it lowers the update rate.
3. Most DSOs produce display updates by batching record length groups of samples.  If the vector generation time does not fill the record length evenly, then the display updates either miss or overlap sections.  "Phosphor" type DSOs still update the display in batches determined by the number of samples between display updates.
4. DSO display refresh and update rate is orders of magnitude slower than the continuous update rate of an analog oscilloscope.  This should not matter above about 60 Hz but combined with 3 above it becomes a problem.
5. Almost all DSO displays have a much lower resolution per area than an analog CRT.  Many old CRT DSOs operated at 254dpi (100dpc) and could produce a pretty good vector display in X-Y mode except for the record length and update rate issues described above despite lacking anti-aliasing.
6. Most DSOs do not support a Z-axis (intensity) input so all of the vectors run together.  The Rigol example above shows this.

These issues with DSO XY mode could be fixed to produce a display limited only by the display device itself but it is hardly a high priority.

[T3sl4co1l]

Hey, I'm impressed those did as well as they did.

X-Y is a notoriously hard problem, and low on the list of priorities to address.

Tim

[james_s]

I use XY when working on vector arcade boards, a scope is more convenient than having one of the (hard to find) 19" vector monitors and a power supply producing the multiple oddball voltages they require on the bench. At least I did before I built a 5" vector monitor for one of my other projects.

[JohnnyMalaria]

I have to monitor an I-Q pair of a demodulated signal while the signals are also being captured. My Rigol DSO is great for Y-T plots and I love the compact size and weight. But the X-Y function absolutely sucks (the FFT isn't all that great, either.) I use my 1974 vintage Tektronix dual 100MHz just for the X-Y plots and they are gorgeous. I can tell what's going right/wrong with my instrument. With both 'scopes, I get to look at 6 signals which happens to be how many I have

[Scratch.HTF]

Despite the retrace lines on the digital scope, it still looks impressive.

[vk6zgo]

Just for fun, I guess no digital can beat an analog scope at X-Y.

Interesting that the old "tedious"210 does better than the much more capable DS1054Z.

[ataradov]

Interesting that the old "tedious"210 does better than the much more capable DS1054Z.

210 has a horribly slow display, so it probably contributes quite a bit to the "analog" feel.

But it seem like DS1054Z does not even attempt to blank the transitions, so may be some configuration of intensity will improve things.

[JohnnyMalaria]

Hopefully the linked videos will convince the nay-sayers that analog X-Y rocks  (They are too big to attach. The links are to OneDrive).

https://1drv.ms/v/s!AhmR5if7W0HCjcJG8SPwrOfTWF-grg

https://1drv.ms/f/s!AhmR5if7W0HCjcJFalJzpym6hx4C_A

A bit of description is probably necessary. There are two videos of a real/imaginary pair of signals from a photodetector (after demodulation) for light scattered by electrically charged nanoparticles moving in a liquid. A laser illuminates the sample. The first video shows just random Brownian motion from the particles. In the second video, an electrical field is turned on and off every five seconds or so. When it is on, the nanoparticles move toward the electrode of opposite charge. Movement of the particles causes a Doppler shift of the laser light. The X-Y trace indicates the amplitude and the optical phase difference of the light. The phase is proportional to the position of the particles. When the field is on, you can see "circles". One complete circle is one hertz. In terms of position, it is approximately 1 micron. The frequency of the circles is a perhaps 5 to 10Hz. That's a change of 5 to 10 parts in 100 trillion.

Rock on, 48-year old 475 scope!!!

(Sorry the videos are upside and grainy).

[nctnico]

Are there any loss-less wav files available for the clock so people can try without needing the actual hardware?

[CopperCone]

for constallation diagrams

[rstofer]

Over in a distant corner of the electronics sandbox, a few of us play with analog computing.  Not many people over here...

X-Y mode is critical.  We usually use one integrator to provide the X axis and the output drives the Y axis.  There are cross-coupled differential equations that draw a kind of egg-shaped pattern.  Not a circle, more like one that has been stomped on.  It's interesting to print two channels of Y(t) but more interesting to plot the simultaneous values of X and Y.

FWIW, the classic example is the Predator Prey problem.  As the rabbit population increases, eventually there will be more foxes.  More foxes will drive down the rabbit population which will reduce the fox population allowing the rabbit population to increase, and so on.

When I want X-Y done well, I use my Analog Discovery 2.  Among other things, I get a better screen shot.

[mtdoc]

FWIW, the classic example is the Predator Prey problem.  As the rabbit population increases, eventually there will be more foxes.  More foxes will drive down the rabbit population which will reduce the fox population allowing the rabbit population to increase, and so on.

Lotka–Volterra equations

[rstofer]

FWIW, the classic example is the Predator Prey problem.  As the rabbit population increases, eventually there will be more foxes.  More foxes will drive down the rabbit population which will reduce the fox population allowing the rabbit population to increase, and so on.

Lotka–Volterra equations

Exactly!  I need to read through that document.  About half way down they give the X-Y plot that I was discussing above.  Both populations are displayed on a single graph.

These equations don't have to be put into an actual analog computer.  MATLAB does a fantastic job of modeling the required integrators (2) and gain stages.  Simulink  gives the same answers without all the op amps.  But I like the old-school approach.  There's just a warm fuzzy feeling as the integrators integrate, the multiplier multiplies and the gain stages do their thing.

Attached is the Simulink model

Note that the model is pretty superficial.  As the article points out, the assumptions are probably not realizable.  Notably that the Prey can grow without bound because there is always adequate food.  The model could be improved to account for an exponential growth asymptotic as some logistic limit.

In my view, the X-Y mode more appropriately displays the interaction between the populations.  The Y(t) mode is useful but the X-Y mode drives the point home.