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| Siglent SDS2000X Plus Bandwidth & Aliasing Application Note |
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| 2N3055:
--- Quote from: hpw on January 06, 2022, 08:26:37 am ---Nice measurements... Well, may show the HW setup as may you simple measure a 50E chain using a given cable... As looking for overshoots in real gear (as LVHC logic) things fails soon by the used probe.. :phew: This means using a 300Mhz probe witch has a to high cap as ~20pF will draw more overshoots as using a FET/1GHz as ~1pF. Not only the BW is important, the phase shifts will cause or show the overshoots. So phase is currently neglected. Attached a Spectrum of a 10MHz clock up to 1.5GHz using as SSA juts to see that we get or have a large fourier content. --- End quote --- 50Ohm source impedance and 20pF would form additional RC filter with -3dB point at cca. 160MHz. Good 10x probes at 300MHz and up would be more in a 12-14pF range. Good 500 MHz ones maybe even 10-11 pF. I again fail to see your point. What are you trying to say: that manufacturers should stop making any scopes with less than 2 GHz bandwidth and that passive probes shouldn't be made and used anymore and we should be allowed only to use active probes.. Or are you complaining about phase response of the scope (i.e. overshoots) when using a 200 MHz scope to look at signal that has 1.5GHz content? Or you are complaining that BW measurement of scope at 50 Ohm with coax directly from signal generator is not representative of using a scope with passive probe? It is representative of 50 Ohm signal path that has to be measured also. And that is how scopes are specified and measured. So we can compare with other manufacturers on equal footing. There are several topics where Performa did detailed characterization of how this scope behaves with several passive probes, done to industry standard too.. |
| hpw:
@2n3055: My point is, as it shows using the given probes (using shortest connections) gives false results (overshoots) for fast clocks. So using a 1GHz FET probe using shortest connection trusted me. Also the phase behavior about is an opened task. IMHO. 50E basic measurement is one task, using probes the second task... and so on. As the Topic on the given PDF: "Siglent SDS2000X Plus Bandwidth Discussion"... is IMHO a starting point. Home workers may have a limited/reduced requirements, so to deal on real digital world required BW and Phase behavior to consider. Even Siglent with the new DSO 6K model and 1..2 GHz FET probes does not provide the required information how the probes performs. While currently I am missing a clear advice about required DSO BW & probes to measure more or less exact overshoots. |
| 2N3055:
--- Quote from: hpw on January 06, 2022, 09:24:48 am --- @2n3055: My point is, as it shows using the given probes (using shortest connections) gives false results (overshoots) for fast clocks. So using a 1GHz FET probe using shortest connection trusted me. Also the phase behavior about is an opened task. IMHO. 50E basic measurement is one task, using probes the second task... and so on. As the Topic on the given PDF: "Siglent SDS2000X Plus Bandwidth Discussion"... is IMHO a starting point. Home workers may have a limited/reduced requirements, so to deal on real digital world required BW and Phase behavior to consider. Even Siglent with the new DSO 6K model and 1..2 GHz FET probes does not provide the required information how the probes performs. While currently I am missing a clear advice about required DSO BW & probes to measure more or less exact overshoots. --- End quote --- Ok thank you for clarifications.. It seems that you have missed previous few topics (they were some time ago) where it was discussed exactly that. If you want to look at signals that have 200 ps edges you need 4 GHz scope and corresponding active probe. That is only way you will get what you want: a pristine pulse response. Period Any time signal has edge that is faster than the risetime of the scope one of two things will happen: 1.) if scope has strong AA filters and aggressively cuts of (lowpass filters) any frequency that is above Nyquist by 40 dB or more (for a 8 bit scope) and it will show perfectly shaped squarewave with no overshoots but completely wrong, without any high frequency content and edges that will be as fast as a scope and not in any relation to what signal edges are. It will be complete lie, but it will look pleasing because it will look perfect. Which is complete bullshit because original signal is not perfect and 1/3 of energy content from that signal is missing. 2.) You can have response like most modern digital scopes have. That one will try to show as much of frequency content they can. If you feed that one with a signal that has slower edge than scope has, it will show it nicely without overshoots. It will show the same as scope from 1. point. But if you feed it signal that has very fast edge (like here where we feed signal with 40ps risetime to scopes with nominal risetimes from 500-800ps, 10-15 times faster), scope will show overshoots. You say that is bad. I say it is not. It's great. Scope is having problems and is not hiding the fact. Those peaks are information that my edge is too fast for my scope and that I know scope is lying to me. Scope in point 1. will show clean edges without overshoot all the time, when I have nice signal and when I have too fast signal with many thing hidden inside. I wont know a difference .. So with my digital scope, if I see that a signal has an edge that is 400 something ps (edge of what my 1GHz scopes can do) and I see overshoots, I know signal is too fast for my scope. If I see the signal has an edge that is 400 something ps and there are no overshoots, I know it is right there at the edge. And what I see is true. And if I see 500-600 ps edge and overshoots I know it is DUT, not a scope. If I routinely want to look at 100 ps edges I need to buy 4GHz+ scope... So much about that. As for probing, that has been discussed even more times. That is something that we all agree is a most important and most misunderstood topic in using scopes. Most of the times quality of measurements is directly related to probing issues. And realities of it. Many users don't understand that when you connect probe it becomes part of the circuit and changes it. Even active probes, just to a bit smaller extent (if you chose right probe for the job). As frequencies go up, it gets much, much worse. There is no perfect probe or a scope. It is all about education and knowing your instrument. Both weak and strong points. And working around those. |
| hpw:
--- Quote from: 2N3055 on January 06, 2022, 10:00:55 am --- --- Quote from: hpw on January 06, 2022, 09:24:48 am --- @2n3055: My point is, as it shows using the given probes (using shortest connections) gives false results (overshoots) for fast clocks. So using a 1GHz FET probe using shortest connection trusted me. Also the phase behavior about is an opened task. IMHO. 50E basic measurement is one task, using probes the second task... and so on. As the Topic on the given PDF: "Siglent SDS2000X Plus Bandwidth Discussion"... is IMHO a starting point. Home workers may have a limited/reduced requirements, so to deal on real digital world required BW and Phase behavior to consider. Even Siglent with the new DSO 6K model and 1..2 GHz FET probes does not provide the required information how the probes performs. While currently I am missing a clear advice about required DSO BW & probes to measure more or less exact overshoots. --- End quote --- Ok thank you for clarifications.. It seems that you have missed previous few topics (they were some time ago) where it was discussed exactly that. If you want to look at signals that have 200 ps edges you need 4 GHz scope and corresponding active probe. That is only way you will get what you want: a pristine pulse response. Period Any time signal has edge that is faster than the risetime of the scope one of two things will happen: 1.) if scope has strong AA filters and aggressively cuts of (lowpass filters) any frequency that is above Nyquist by 40 dB or more (for a 8 bit scope) and it will show perfectly shaped squarewave with no overshoots but completely wrong, without any high frequency content and edges that will be as fast as a scope and not in any relation to what signal edges are. It will be complete lie, but it will look pleasing because it will look perfect. Which is complete bullshit because original signal is not perfect and 1/3 of energy content from that signal is missing. 2.) You can have response like most modern digital scopes have. That one will try to show as much of frequency content they can. If you feed that one with a signal that has slower edge than scope has, it will show it nicely without overshoots. It will show the same as scope from 1. point. But if you feed it signal that has very fast edge (like here where we feed signal with 40ps risetime to scopes with nominal risetimes from 500-800ps, 10-15 times faster), scope will show overshoots. You say that is bad. I say it is not. It's great. Scope is having problems and is not hiding the fact. Those peaks are information that my edge is too fast for my scope and that I know scope is lying to me. Scope in point 1. will show clean edges without overshoot all the time, when I have nice signal and when I have too fast signal with many thing hidden inside. I wont know a difference .. So with my digital scope, if I see that a signal has an edge that is 400 something ps (edge of what my 1GHz scopes can do) and I see overshoots, I know signal is too fast for my scope. If I see the signal has an edge that is 400 something ps and there are no overshoots, I know it is right there at the edge. And what I see is true. And if I see 500-600 ps edge and overshoots I know it is DUT, not a scope. If I routinely want to look at 100 ps edges I need to buy 4GHz+ scope... So much about that. As for probing, that has been discussed even more times. That is something that we all agree is a most important and most misunderstood topic in using scopes. Most of the times quality of measurements is directly related to probing issues. And realities of it. Many users don't understand that when you connect probe it becomes part of the circuit and changes it. Even active probes, just to a bit smaller extent (if you chose right probe for the job). As frequencies go up, it gets much, much worse. There is no perfect probe or a scope. It is all about education and knowing your instrument. Both weak and strong points. And working around those. --- End quote --- Thank you for repeating the old good stuff :-+ may your given content should be added to the PDF??!! While was about an 20G/s Keysight DSO but the support question made the :palm: end |
| 2N3055:
--- Quote from: hpw on January 06, 2022, 10:23:43 am --- Thank you for repeating the old good stuff :-+ may your given content should be added to the PDF??!! While was about an 20G/s Keysight DSO but the support question made the :palm: end --- End quote --- Thanks! Yes unfortunately Keysight seems to have gone the way of IBM. Instead of greatest T&M company in the world, they are now cloud/synergy/solution partner for emerging synergistic 5G solution providing electronics synergy solution T&M workflow optimizing profit solutions something.... T&M instruments are not a focus anymore, they are made just to enable consulting and support sales. Basically, you used to buy a scope from them ( a primary product) and if you wanted some support you got some for free and extended support you had to pay for (additional revenue). Now support, services and consulting is the primary product. Equipment is only there so they have an object as an anchor to sell you the services for it. That will (and already has) mean that they will ignore all market segments (customers and products) that does not have potential for large volume of services and paid support. That means all low and mid range equipment, small business etc etc.. |
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