Analog vs digital X-Y mode

[pdenisowski]

digital scope do poorly with XY because they inherently have slow update rate.   R&S has the fastest update rate with MXO4 and MXO5....but they don't yet support XY mode...and maybe their XY mode, when released, will be in SW or maybe in HW.   All digital scopes so far have XY done in SW.

I guess you'll just have to wait and see

[wolfy007]

Digital scopes dont do so bad these days, even some of the cheap ones, below are a few examples from various age groups and price range using the Dutchtronix clock (my favourite XY test), all of which show the clock fairly smoothly once you get the setting right.

[KungFuJosh]

I couldn't say how legit, but the Batronix demo video for the Magnova oscilloscope had what looked like X-Y mode that impressed me more than any other modern scope so far.

[Phil1977]

Does anyone know how much dead time the current digital scopes have in xy mode?

AFAIK modern xy mode is a spin-off of the digital phosphor technology. Many digital scopes even keep the trigger activated in XY-mode. This means, each time the trigger is activated, a snapshot of the signal on X and Y is taken and summarized to the display matrix.

But this also means that the time between the snapshots is just plotted to the screen.

In the end we have (at least) three main differences comparing analogue to digital:
- Analogue is without any dead time. XY-mode does not even need an electron beam return blank.
- Analogue has no latency or display update rate
- Analogue has no quantisation effects reducing the available resolution

Is there any modern digital scope that really addresses all these issues? At least for lower bandwidths I could imagine a continuos, triggerless data acquisition like in roll-mode. Display update rate is a matter of computing power and the relevance of quantisation is a matter of available memory - so very theoretically it may be possible, but did anyone do it?

[MathWizard]

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.

So what could happen if you took some scope, and tried to get it working with a nice gaming PC. Surely if it's possible, someone with nothing better to do, has tried it.

So with say a 1440p, 144Hz monitor, and some top of the line CPU and GPU ?? How good of an X-Y scope or regular scope could you make then ??

What about years ago, when all CRT scopes had tiny screens, did people try to hack them together with nice big CRT TV's ??

[Phil1977]

So what could happen if you took some scope, and tried to get it working with a nice gaming PC. Surely if it's possible, someone with nothing better to do, has tried it.

So with say a 1440p, 144Hz monitor, and some top of the line CPU and GPU ?? How good of an X-Y scope or regular scope could you make then ??

What about years ago, when all CRT scopes had tiny screens, did people try to hack them together with nice big CRT TV's ??

This both doesn't work. You can't use a TV CRT for scope purpose and vice versa. Scope CRTs have electrostatic deflection, while TV CRTs have magnetic deflection. Both have their advantages/disadvantages and can mostly not be exchanged.

Using a gaming PC? Yes, such a GPU is powerful but the system is everything but not almighty. A state of the art scope samples e.g. with 12bits and 2GS/s. A 12bit value is usually transferred as a 16bit value into a PC. That means we have 4GB/s (16bit = 2byte) per channel, meaning 8GB/s for two channels. We would at least need some specialized hardware that directly moves the data to the GPU memory. I´m not aware anything like that would exist.

This data processing needs to be done in the FGPA of the scope (it has direct access to the samplers and to fast memory).

[Someone]

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.

So what could happen if you took some scope, and tried to get it working with a nice gaming PC. Surely if it's possible, someone with nothing better to do, has tried it.

So with say a 1440p, 144Hz monitor, and some top of the line CPU and GPU ?? How good of an X-Y scope or regular scope could you make then ??

What about years ago, when all CRT scopes had tiny screens, did people try to hack them together with nice big CRT TV's ??

Oh dear, another one of those threads: https://www.eevblog.com/forum/testgear/oscilliscope-memory-type-and-why-so-small/?all

The video and audio world have fancy CRT emulating diagnostic displays including x-y traces. It's possible to do if there is a budget/demand for it. General purpose scopes have more important things to improve first.

[David Hess]

AFAIK modern xy mode is a spin-off of the digital phosphor technology. Many digital scopes even keep the trigger activated in XY-mode. This means, each time the trigger is activated, a snapshot of the signal on X and Y is taken and summarized to the display matrix.

Old DSOs also work that way in XY mode.

Is there any modern digital scope that really addresses all these issues? At least for lower bandwidths I could imagine a continuos, triggerless data acquisition like in roll-mode. Display update rate is a matter of computing power and the relevance of quantisation is a matter of available memory - so very theoretically it may be possible, but did anyone do it?

Bandwidth has nothing to do with sample rate, especially in XY mode.  One of the odder uses of those ancient Tektronix sampling mainframes was to produce an XY display with GHz of bandwidth, all at a sample rate of about 50 kS/s.  With one exception, analog oscilloscopes otherwise have limited XY bandwidth because they have low horizontal bandwidth and the vertical delay line adds phase shift to the Y input.  The one exception that I know of is the Tektronix 7104 because it has a high bandwidth vertical amplifier design for its horizontal deflection plates, achieving like 350 MHz on its X axis.

But you bring up an interesting question.  I just tested my 30 year old 2232 DSO, and in roll mode at slow sweep speeds, it generates its digital XY display continuously without any pause for triggering, just like the roll mode display is produced.