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| Oscillo-confusion MHz GSa/s wfm/s Mpts |
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| tggzzz:
--- Quote from: james_s on January 02, 2019, 08:03:59 pm ---Yes but on the same note even a state of the art scope is not a silver bullet. You could have a 10 GHz scope and still if you are not using it properly it will not tell you what you need to know. Throwing money at the problem beyond a point is usually not the best solution, there is no substitute for understanding what you are trying to do, the limitations of your test gear and how to use it properly. This is where most hobbyists fall short anyway, and why I say that learning to properly use whatever test gear you have, and understanding its limitations is far more beneficial than obsessing over the specs. --- End quote --- Those are strawman arguments in this context. I frequently explicitly make the same points, and implicity in my .sig. |
| james_s:
--- Quote from: ebastler on January 02, 2019, 08:00:41 pm ---A lot of good info in the above posts. But I don't think the following point has been made: For me, 4 channels are much more helpful in a digital scope than in an analog one. If you have repetitive signals (for which an analog scope works well), you can always work around the 2-channel limitation by using one channel as the "reference", then looking at various other signals in the second channel, one after the other. In contrast, for on-repeating signals or rare events involving more than two signals, you do not have the opportunity to look at the signals successively. That's where a digital scope shines, and that's where only a 4-channel digital scope will help if you need to look at more than two signals. --- End quote --- Agreed, I never felt any need for 4 channels on an analog scope. I have a 4 channel DSO and once in a while the extra channels are handy but 99% of the time I use only one or two. |
| rstofer:
Back when we were using 20 MHz scopes, it was rare to have anything running over a couple of MHz and the external bus signals would be some fraction of that. I agree, that 10 MHz Heathkit I had helped me build an 8" floppy controller (based on WD1771) back in '77. It was awkward, it was ugly, but it got the job done. Scoping the PLL was a challenge. Today those data rates are laughable. We could probably carry them on calibrated tin cans with impedance matched strings. We get into things like SPI running at 12.5 MHz and I have no idea what the rise time is. I know that I want to see at least the 7th harmonic so somewhere around 100 MHz is good. It turns out that the DS1054Z far exceeds its specs (measured 3 dB point is around 130 MHz) so I can even see the 9th harmonic. That's a pretty good square wave. But if I want to see the DDR buses, I'm probably out of luck other than to see if they wiggle around. But I don't use DDR (at the moment) so it's not very important. Internally, my FPGA projects run at 100 MHz (I could push most of them to 200 MHz). Signals I can scope external to the FPGA are seldom running this fast. On my projects... It really gets back to my stake in the ground in terms of money. How much can you afford to spend and what can you get for the money? Unlocking the DS1054Z to get 100 MHz is done all the time. There is a process to unlock the Siglent SDS1104X-E to get 200 MHz (identical to the more expensive SDS1204X-E) and this is probably the new sweet spot in the market. 4 Channels, 200 MHz, upgrade a $500 scope to an $800 scope, what's not to like? Caveat: I don't know anything about the Siglent unlocking. Check the Test Equipment forum. Even buying the SDS1204X-E outright is still a bargain! |
| rstofer:
--- Quote from: hamiltont on January 02, 2019, 06:01:31 pm ---For example, USB 1.0 in full speed mode runs at 12Mbit/s. It's digital, so a 1 followed by a 0 would look like a square wave e.g |▔▔|___| so 12/2 (two bits per entire "wave") means it runs at about 6MHz. So if you want to play with USB 1.0, you need a scope that can reliably handle a wave of 6MHz - typical advice is to get a scope with more bandwidth than you need due to attenuation(see image belwo). Once you know what MHz your signal of interest will require, other posters can give you lots of guidance on how to choose the best scope. --- End quote --- It would seem to me that you need a lot more than 6 MHz of bandwidth to display a 6 MHz signal. At a minimum I would want to see the 5th harmonic (30 MHz) and preferably more (like 7th, 9th, 11th, 13th and 15th) with a 100 MHz scope. If you were to view a 6 MHz square wave on a 6 MHz scope (or 10 MHz scope) all you would see is a sine wave at the fundamental frequency. |
| james_s:
--- Quote from: rstofer on January 02, 2019, 08:19:44 pm --- It would seem to me that you need a lot more than 6 MHz of bandwidth to display a 6 MHz signal. At a minimum I would want to see the 5th harmonic (30 MHz) and preferably more (like 7th, 9th, 11th, 13th and 15th) with a 100 MHz scope. If you were to view a 6 MHz square wave on a 6 MHz scope (or 10 MHz scope) all you would see is a sine wave at the fundamental frequency. --- End quote --- But does that really matter? It depends on what you're trying to see here, whether it is the data/timing or signal integrity which is going to need more bandwidth. I actually do not recall ever using a scope to look at USB, signal integrity is rarely the issue there, normally USB problems are down to drivers and software issues. |
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