Post by thread:Oscilloscope Question

[snip] > Analog storage scopes used a very-high-persistence phosphor which > allowed the image to last for a number of seconds... As far as I > know, no one makes them anymore. > > -Andy > > === Andrew Warren - .....fastfwdKILLspam.....ix.netcom.com > === Fast Forward Engineering - Vista, California > === http://www.geocities.com/SiliconValley/2499 > er,not quite. If the phosphor was long persistance every sweep would remain on the screen. There is some sort of mesh inside the tube which can preserve the scan when energised. Gradually the electrons leak away and the trace fades. When storage mode is de-energised, the scope reverts to normal display.

> Analog storage scopes used a very-high-persistence phosphor which > allowed the image to last for a number of seconds... As far as I > know, no one makes them anymore. With a good tube, an image would last for hours. A replacement tube cost $5k 10 years ago. It's not suprising that nobody makes any more. Second hand ones are pretty rare and still expensive. The advantage is that you can set it on 1 Sec/div and still have the full 100MHz bandwidth. It's one of those things like an analog multimeter. Fine for general use, no where near as many "bells and whistles" but occasionally, the best tool for the job. Steve. ====================================================== Very funny Scotty. Now beam down my clothes. ====================================================== Steve Baldwin Electronic Product Design TLA Microsystems Ltd Microcontroller Specialists PO Box 15-680 email: EraseMEstevebspam_OUTTakeThisOuTkcbbs.gen.nz New Lynn, Auckland ph +64 9 820-2221 New Zealand fax +64 9 820-1929 ======================================================

> er,not quite. If the phosphor was long persistance every sweep would > remain on the screen. There is some sort of mesh inside the tube which > can preserve the scan when energised. Gradually the electrons leak away > and the trace fades. When storage mode is de-energised, the scope > reverts to normal display. I think the theory is that a very high voltage screen is placed near the phosphor, and that if a photon leaves the phosphor and hits the high volt- age screen it will knock off an electron (which in effect "recharges" that spot on the phosphor). I have actually used an analog storage scope once, but it was a bit trick- y to adjust. Basically, there was a persistance knob which would set how fast the trace would fade. If the knob was set too high, however, then the trace would start to "bloom" until eventually the whole screen was a solid glow. I think the thing must have had two screens, though, one inside the other since if memory serves it had a "Save" switch which would blank the display without disturbing the image on it; when you turned off the "save" switch the last-saved image would reappear. I think the idea there was that the image would last longer if it didn't have to be swapping enough electrons for photons and vice versa to produce a visible picture. Kinda a fun gizmo, being able to set the persistence. Personally, I sorta wish people could make digital scopes that did a better job of emulating analog ones. For example, I'd like to see a scope that could (assuming moderate resolution, e.g., 10uS/div), store all of the following values for each pixel: [1] The minimum and maximum values observed during that time period, as sampled. [2] The minimum and maximum values observed during that time period, if run through a moderately low-pass filter (e.g. if each pixel is 500ns, I'd like the filter's "effective" RC time constant to be about 100ns. if the minima and maxima are logged every 10ns. [3] The average value observed during that time period, Gaussian-weighted into the preceding and following ones. If the scope logged these five values, it could use them to show a waveform that was much more informative than that given by any mode on a normal DSO. Peaks would still be shown, but they wouldn't drown out the rest of the wave- form. Also, since peaks would be stored in filtered as well as unfiltered form, it would be possible to estimate how much energy they actually cont- ained. Does anyone know if any scopes do anything like that?

Yeah. While we're at it, how about being able to do a one-shot triggered sweep, highlight a piece of the trace and say 'that's the trigger code - every time you see this pattern, trigger'. Wonderful for looking at stuff like composite video. {Quote hidden}> ---------- > From: John Payson[SMTP:@spam@supercatKILLspamMCS.NET] > Reply To: pic microcontroller discussion list > Sent: Friday, December 12, 1997 4:30 PM > To: KILLspamPICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: Oscilloscope Question > > > er,not quite. If the phosphor was long persistance every sweep would > > remain on the screen. There is some sort of mesh inside the tube > which > > can preserve the scan when energised. Gradually the electrons leak > away > > and the trace fades. When storage mode is de-energised, the scope > > reverts to normal display. > > > I think the theory is that a very high voltage screen is placed near > the > phosphor, and that if a photon leaves the phosphor and hits the high > volt- > age screen it will knock off an electron (which in effect "recharges" > that > spot on the phosphor). > > I have actually used an analog storage scope once, but it was a bit > trick- > y to adjust. Basically, there was a persistance knob which would set > how > fast the trace would fade. If the knob was set too high, however, > then > the trace would start to "bloom" until eventually the whole screen was > a > solid glow. > > I think the thing must have had two screens, though, one inside the > other > since if memory serves it had a "Save" switch which would blank the > display > without disturbing the image on it; when you turned off the "save" > switch > the last-saved image would reappear. I think the idea there was that > the > image would last longer if it didn't have to be swapping enough > electrons > for photons and vice versa to produce a visible picture. > > Kinda a fun gizmo, being able to set the persistence. Personally, I > sorta > wish people could make digital scopes that did a better job of > emulating > analog ones. For example, I'd like to see a scope that could > (assuming > moderate resolution, e.g., 10uS/div), store all of the following > values > for each pixel: > > [1] The minimum and maximum values observed during that time period, > as > sampled. > [2] The minimum and maximum values observed during that time period, > if > run through a moderately low-pass filter (e.g. if each pixel is > 500ns, > I'd like the filter's "effective" RC time constant to be about > 100ns. > if the minima and maxima are logged every 10ns. > [3] The average value observed during that time period, > Gaussian-weighted > into the preceding and following ones. > > If the scope logged these five values, it could use them to show a > waveform > that was much more informative than that given by any mode on a normal > DSO. > Peaks would still be shown, but they wouldn't drown out the rest of > the wave- > form. Also, since peaks would be stored in filtered as well as > unfiltered > form, it would be possible to estimate how much energy they actually > cont- > ained. > > Does anyone know if any scopes do anything like that? >

> My understanding was that it was the phosphor, but I could be wrong. I > recall someone (Iwatsu, perhaps) came up with some sort of hybrid. It had a > tube that worked like an analog scope but they could read the grid/phosphor > back. That was digitised with slower A/D's and then put back up on the > screen. I have seen such a beast. It was an analog storage tube with screen dump capability. The act of printing would clear the display, however; if memory serves, a bright green line would sweep from left to right, wiping the dis- play. I think what was happening is that the unit was sensing how many electrons/photons/whatever got knocked off as it did the bright green scan. If memory serves, the printout was that sort of icky stuff that a lot of microform printers use; I would not be at all surprised if the device had a 1D scanning tube and photo-imaged the data it was reading off the main view screen as the paper rolled through. The thing that still amazes me, though--all that old technology with tubes, mercury delay lines, punched cards, etc. actually WORKED. Nowadays it would probably be unthinkable to build such things with the quality required for good operation but back then that's what people had to work with. Amazing how times change.

John Payson <RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU> wrote: > I sorta wish people could make digital scopes that did a better job > of emulating analog ones. For example, I'd like to see a scope > that could (assuming moderate resolution, e.g., 10uS/div), store > all of the following values for each pixel: > > [1] The minimum and maximum values observed during that time period, > as sampled. > [2] The minimum and maximum values observed during that time period, > if run through a moderately low-pass filter (e.g. if each pixel > is 500ns, I'd like the filter's "effective" RC time constant to > be about 100ns. if the minima and maxima are logged every 10ns. > [3] The average value observed during that time period, > Gaussian-weighted into the preceding and following ones. John: My Tek 2230 comes pretty close to this... It does #1 just as you describe, and you can fake #2 by using the "compress display by a factor of four" option. It doesn't really do #3, although it does have a "smooth" option that does something similar. The best thing about the 2230, though, is that it can be switched from digital storage to a true (non-digitized) analog mode... So when you get tired of being lied to, you can flip the switch and see what the signal's REALLY doing. To be fair, though, the 2230's digital mode rarely lies to me... Its aliasing problems are nowhere near as bad as what I've seen in my more-or-less limited experience with the Tek TDS220 and the HP 54645D. -Andy === Andrew Warren - spamBeGonefastfwdspamBeGoneix.netcom.com === Fast Forward Engineering - Vista, California === http://www.geocities.com/SiliconValley/2499

Yo, Andy! Analog storage scopes use a variety of methods to achieve virtually infinite persistence, allowing capture of single shot very high speed events. They are still in use (mainly using microchannel plate tubes) in many labs looking at stuff in the sub-nanosecond range. Where there are really high speed single shot requirements you can use various types of streak cameras and their relatives, where the persistence may also be provided by photographic means, electronic or traditional. This is the standard method of characterizing the pulses of picosecond class lasers. You've just been hanging around digital stuff too long... {Original Message removed}

Andrew Warren wrote: >The best thing about the 2230, though, is that it can be switched >from digital storage to a true (non-digitized) analog mode... So when >you get tired of being lied to, you can flip the switch and see what >the signal's REALLY doing. >To be fair, though, the 2230's digital mode rarely lies to me... Its >aliasing problems are nowhere near as bad as what I've seen in my >more-or-less limited experience with the Tek TDS220 and the HP >54645D. I use a Tek TDS220 at work, and I also have a Tek TDS 210 at home. You're talking about aliasing problems. I'm not sure what you mean. These scopes have a sample rate of 1 GS/s and an analog bandwidth of 100 MHz (60 MHz for the TDS 210). Aliasing should not occur with frequencies less than 500 MHz. Since the bandwith limitation would attenuate such a signal to virtually zero, I cannot see how you would be able to experience aliasing problems. Care to explain? BTW, I'm really pleased with the performance of these scopes. I think they have an unbeatable performance/price factor. -Oyvind

> You're talking about aliasing problems. > I'm not sure what you mean. These scopes have a sample rate of 1 GS/s and > an analog bandwidth of 100 MHz (60 MHz for the TDS 210). > > Aliasing should not occur with frequencies less than 500 MHz. > Since the bandwith limitation would attenuate such a signal to > virtually zero, I cannot see how you would be able to experience > aliasing problems. > > Care to explain? The problem comes when a scope is used on a slower timebase; most digital scopes, if they have 20 pixels/division, will sample at a rate of only 2MHz if set for 10us/div; at 1ms/div they'll only sample at 20KHz. This behavior makes it very difficult to use such scopes for looking at certain types of signals; if I'm looking for short pulses widely spaced, the scope may not see them AT ALL if it's not set for peak-detect mode. If it IS set for peak-detect mode, then it may be highly succeptible to noise pickup.

Tom Rogers <TakeThisOuTtrogersEraseMEspam_OUTtimetech.com> wrote: > Yo, Andy! Analog storage scopes use a variety of methods to achieve > virtually infinite persistence, allowing capture of single shot very > high speed events. They are still in use (mainly using microchannel > plate tubes) in many labs looking at stuff in the sub-nanosecond > range. Thanks for the info, Tom... I guess if I'd thought about it for a bit, I would've realized that there are lots of measurement applications that require sweep speeds faster than digitizing scopes can provide. > You've just been hanging around digital stuff too long... Yeah... Every time I leave the office after working a 20-hour day, I think the same thing. -Andy === Andrew Warren - RemoveMEfastfwdTakeThisOuTix.netcom.com === Fast Forward Engineering - Vista, California === http://www.geocities.com/SiliconValley/2499

I wrote: > To be fair, though, the 2230's digital mode rarely lies to me... > Its aliasing problems are nowhere near as bad as what I've seen in > my more-or-less limited experience with the Tek TDS220 and the HP > 54645D. and Oyvind Kaurstad <PICLISTEraseME.....MITVMA.MIT.EDU> replied: > I'm not sure what you mean. The [TDS220] scopes have a sample rate > of 1 GS/s and an analog bandwidth of 100 MHz (60 MHz for the TDS > 210). > > Aliasing should not occur with frequencies less than 500 MHz. > Since the bandwith limitation would attenuate such a signal to > virtually zero, I cannot see how you would be able to experience > aliasing problems. Oyvind: John Payson explained it well: The problem has to do with the fixed-number-of-samples-per-graticule-division sampling speed; just because a scope CAN sample at 1 GS/sec doesn't mean that it ALWAYS samples at that speed. If a scope takes 100 samples per division, signal pulses shorter than 1/100-division can occur BETWEEN samples, where they'll be completely missed. To test your scope for this behavior, program a PIC to generate 1-microsecond pulses spaced 5 milliseconds apart, then set your scope to 20 milliseconds per division and see if it always shows exactly four pulses in each division... Unless it has an "envelope" mode which samples at a very high rate and displays the minimum and maximum sampled values for each column of pixels on the screen, it'll miss pulses. If your scope passes the above test, you can move on to testing for the noise-susceptibility problem to which John alluded... Modify your PIC program to repeatedly generate a 2-millisecond pulse followed by a string of 47 1-microsecond pulses spaced at 1-millisecond intervals... The idea is that you're simulating a signal (a 2-millisecond pulse once every 50 milliseconds) in the presence of noise (a 1-microsecond pulse every millisecond). Set your scope to 200 milliseconds per division and see what it displays. It's a rare digital scope that passes both these tests... > BTW, I'm really pleased with the performance of these scopes. I > think they have an unbeatable performance/price factor. I agree that they're great tools for the money... My only point was that there's still a need, even when dealing with relatively-slow signals, for an true analog scope. -Andy === Andrew Warren - EraseMEfastfwdix.netcom.com === Fast Forward Engineering - Vista, California === http://www.geocities.com/SiliconValley/2499

-----Original Message----- From: Andrew Warren <RemoveMEfastfwdEraseMEEraseMEIX.NETCOM.COM> I agree that they're great tools for the money... My only point was that there's still a need, even when dealing with relatively-slow signals, for an true analog scope. ----------------------------------- Oh, yeah, amen to that. There's an even greater need to know what is actually going on behind the instruments' display, i.e., through the measurement chain. I remember the surprise I had when first using a digital scope (a Nicolet) that some of the guys didn't expect aliasing. Jeeze, I wanted to get my first actual look at it. The same understanding should be behind any measurement important enough to write down. Come to think of it, this thread might have been the sleeper of the year in terms of actually imparting some of what it takes to the less experienced... --Tom Rogers