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| pdenisowski:
Lots of really great discussion and comments here about measuring scope bandwidth! We get so many questions about this topic, I actually made a video explaining the basic concepts of "bandwidth" in oscilloscopes. Probably a bit too introductory for most of the people posting here, but it does cover a few things discussed in this thread such as calculating BW from rise time, the different frequency responses (Gaussian vs. flat), etc. One thing I didn't cover is how to measure the bandwidth of an oscilloscope (i.e. procedures, techniques), but it seems like this might be a good topic for a future video. I've already learned a few things from this thread myself :) |
| 2N3055:
--- Quote from: MrAl on December 02, 2022, 07:14:55 am --- Oh so i guess we see some differences AGAIN between CRT and DSO scopes, that's a pain in the neck. The settling time is the time to settle to within 99 percent of maximum, and that is an important spec of any ADC. I think the percentage used can be different though, such as 95 percent or something. So if you shoot a 1v high square wave into the front end and the scope display starts to rise, it's the time it takes to get to 99 percent of the full max which in this case would be 0.99 volts. This could be a lot longer than the rise time. This was a big concern for me when i was designing a 100MS/s scope way back in the early 1990's. --- End quote --- I will politely comment on that.. In my opinion it is not "between CRT and DSO scope" but how industry was making and looking at scope usage (and design that stems from that) 30 years ago as opposed to today. And also technology gap. Perfect impulse response (that was industry standard many years ago) proved to be suboptimal for many other things people do with scopes. For example my Keysight MSO3104T has practically flat BW until it's 1GHz BW. If I'm looking at signal in frequency domain (FFT) or verifying amplitude of 868 Mhz oscillator it will show excellent amplitude accuracy. All while having "only" 450ps risetime. If I wanted to do the same on scope with Gaussian response (that would yield perfect impulse response) i would need at least 2GHz scope for same amplitude flatness up to 1 GHz. There are digital scopes that have Gaussian response. They are just not common anymore. |
| nctnico:
--- Quote from: 2N3055 on December 02, 2022, 10:59:47 am --- --- Quote from: MrAl on December 02, 2022, 07:14:55 am --- Oh so i guess we see some differences AGAIN between CRT and DSO scopes, that's a pain in the neck. The settling time is the time to settle to within 99 percent of maximum, and that is an important spec of any ADC. I think the percentage used can be different though, such as 95 percent or something. So if you shoot a 1v high square wave into the front end and the scope display starts to rise, it's the time it takes to get to 99 percent of the full max which in this case would be 0.99 volts. This could be a lot longer than the rise time. This was a big concern for me when i was designing a 100MS/s scope way back in the early 1990's. --- End quote --- I will politely comment on that.. In my opinion it is not "between CRT and DSO scope" but how industry was making and looking at scope usage (and design that stems from that) 30 years ago as opposed to today. And also technology gap. Perfect impulse response (that was industry standard many years ago) proved to be suboptimal for many other things people do with scopes. For example my Keysight MSO3104T has practically flat BW until it's 1GHz BW. If I'm looking at signal in frequency domain (FFT) or verifying amplitude of 868 Mhz oscillator it will show excellent amplitude accuracy. All while having "only" 450ps risetime. If I wanted to do the same on scope with Gaussian response (that would yield perfect impulse response) i would need at least 2GHz scope for same amplitude flatness up to 1 GHz. There are digital scopes that have Gaussian response. They are just not common anymore. --- End quote --- Actually you can make the oscilloscope Gaussian again by turning the bandwidth limit on. Higher frequency oscilloscopes typically have various bandwidth limits (with 1st order roll off) so you can make the tradeoff yourself. |
| 2N3055:
--- Quote from: nctnico on December 02, 2022, 11:12:19 am --- --- Quote from: 2N3055 on December 02, 2022, 10:59:47 am --- --- Quote from: MrAl on December 02, 2022, 07:14:55 am --- Oh so i guess we see some differences AGAIN between CRT and DSO scopes, that's a pain in the neck. The settling time is the time to settle to within 99 percent of maximum, and that is an important spec of any ADC. I think the percentage used can be different though, such as 95 percent or something. So if you shoot a 1v high square wave into the front end and the scope display starts to rise, it's the time it takes to get to 99 percent of the full max which in this case would be 0.99 volts. This could be a lot longer than the rise time. This was a big concern for me when i was designing a 100MS/s scope way back in the early 1990's. --- End quote --- I will politely comment on that.. In my opinion it is not "between CRT and DSO scope" but how industry was making and looking at scope usage (and design that stems from that) 30 years ago as opposed to today. And also technology gap. Perfect impulse response (that was industry standard many years ago) proved to be suboptimal for many other things people do with scopes. For example my Keysight MSO3104T has practically flat BW until it's 1GHz BW. If I'm looking at signal in frequency domain (FFT) or verifying amplitude of 868 Mhz oscillator it will show excellent amplitude accuracy. All while having "only" 450ps risetime. If I wanted to do the same on scope with Gaussian response (that would yield perfect impulse response) i would need at least 2GHz scope for same amplitude flatness up to 1 GHz. There are digital scopes that have Gaussian response. They are just not common anymore. --- End quote --- Actually you can make the oscilloscope Gaussian again by turning the bandwidth limit on. Higher frequency oscilloscopes typically have various bandwidth limits (with 1st order roll off) so you can make the tradeoff yourself. --- End quote --- That is very true, it converts 1GHz scope to 200 or 500MHz scope and user have control... I actually use 200Mhz BW limit on Siglents most of the time I work with passive probes. Or whenever I don't need full BW. I wish MSOX3104T had more BW limits in addition to 20Mhz. |
| MrAl:
--- Quote from: gf on December 02, 2022, 08:16:16 am ---A suitable ADC driver can charge the S&H capacitor to settling within the sample interval. At the end, the settling time of the overall system is still predominantly determined by the impulse/step response of the analog frontent. --- End quote --- Yes i am sorry i misspoke, of course it was the front end i was talking about the the ADC 'system'. The front end settling time is the thing i looked at most when designing the front end. |
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