oscilloscope display explanation wanted

[txx4488]

Can someone explain me how does a high speed signal is displayed in low  refresh rate of LCD  in modern oscilloscope ?

[ataradov]

The signal is captured in the memory and then shown on the display. There are gaps between captures. That's why you get a characteristic like waveforms/second.

[xrunner]

Can someone explain me how does a high speed signal is displayed in low  refresh rate of LCD  in modern oscilloscope ?

Not quite sure exactly what you mean, but if it's set to trigger on the "high speed" signal, it's just going to display a snapshot on the screen. All the processing makes this possible given a certain bandwidth specification of the scope. If it's not triggered and the horizontal time/div is not set right, then it will look like a mess.

Maybe if that is not the answer you are looking for someone else will be along to assist you better. 

Edit: Oh Ataradov got to it first.

[txx4488]

So what might be the time delay between signal captured and signal displayed on lcd ?

 

[ataradov]

So what might be the time delay between signal captured and signal displayed on lcd ?

Much faster than your eyes reaction time. But it also depends on your time scale. You can set 10 sec/div and see the signal with your eyes in real time.

[james_s]

Many DSOs, at least older ones, capture a whole sweep before they display anything on the screen, so yes there's a delay between the capture and when it is displayed. That's not an issue though because typically the signals you are capturing will be orders of magnitude faster than you could see with your eyes, otherwise you could measure them with a meter. Using an analog scope with a very slow sweep speed you will just see a dot moving around on the screen unless you have a very long persistence phosphor or a scope camera.

[txx4488]

then the signal displayed on the screen is not real time ? just time delayed/shifted signal ?

[ataradov]

then the signal displayed on the screen is not real time ? just time delayed/shifted signal ?

Depends on your time scale setting. Fast signals are not real-time. What are you going to do with a real-time signal that you can't possibly see with your eyes? The whole point of the scope is to capture and display a signal that is impossible to observe with a naked eye.

[txx4488]

Do oscilloscope manufacturers specify such parameter(s) in their brochures?

[ataradov]

What parameters?

[txx4488]

time lag between capture and display

[ataradov]

I don't think so. It is kind of a pointless parameter. It is going to be on the order of a few ms, under 100 ms, for sure. Unless  you have some hardcore processing enabled and capturing a lot of data.

Why do you even need this?

[james_s]

If you want true realtime, get an analog scope, but what do you hope to achieve? Do you think your eyes will be faster than the scope? If you think a delay between the signal being captured and appearing on the screen is a problem then you aren't using the scope correctly.

[txx4488]

thanks for clarification !!
 i needed this info for  low cost  analog+digital  DAQ card project.

thank u very much!!

[BravoV]

As you mentioned scope with LCD display, that means a DSO.

In DSO its like taking a photo with your digital camera on the signal, and displays it at the LCD, back and forth.

What you see at the scope's screen is the freezed photo, and it gets updated NOT at realtime.

Human reactions is in order of mili seconds, are you expecting your eyes capable of detecting the change in "nano" seconds ?

[David Hess]

If the oscilloscope has an index graded display, then usually multiple acquisitions are combined into a histogram for every LCD frame.  This is what gives DSO displays that fuzzy look which resembles an analog display.  I would be nice if the oscilloscope indicated how many acquisitions are made per LCD frame but none do.

[vk6zgo]

If you want true realtime, get an analog scope, but what do you hope to achieve? Do you think your eyes will be faster than the scope? If you think a delay between the signal being captured and appearing on the screen is a problem then you aren't using the scope correctly.

Even then, when the beam lights the phosphor, what you are seeing is the result of a smaller, but still real delay.

[james_s]

Ok yes, but that's getting really nitpicky. There is always going to be some delay too as light from anything travels from the source to your eyes, but that delay is going to be many orders of magnitude less than any other delays involved. An analog scope is as close to real-time as it gets. I struggle to think of an application where that is really necessary though.

[David Hess]

There is no comparison between an LCD operating at 60 frames per second typical or 16.7 milliseconds per frame and an analog oscilloscope with a latency on the order of 200 nanoseconds.  The LCD is actually significantly worse than its frame rate would suggest because of processing delay unless the DSO is capturing directly into the frame buffer like some of the early Tektronix DPO models did.

As a practical matter however, the difference is irrelevant.  I would be quite willing to live with the latency imposed by limited LCD refresh rates if the processing software itself operated in real time which is the exception.  Most modern DSOs have horrible latency well beyond what the LCD is capable of.

Incidentally, with a fast analog oscilloscope like a Tektronix 7104, it is feasible to view the screen at a distance where the speed of light delay is greater than the delay through the oscilloscope.  The 7104's vertical delay line is only 45 nanoseconds.  The 7104 is actually fast enough to sweep the CRT beam with an apparent speed faster than light.