Products > Test Equipment
12 bit Keysight... when?
2N3055:
--- Quote from: David Hess on January 17, 2024, 07:02:49 pm ---
--- Quote from: 2N3055 on January 17, 2024, 02:45:01 pm ---Generally speaking marketing always speaks only off things that they can leverage as perceived advantage. They emphasize what they have...
While Keysight is talking it's ass off how important wfms/s is when talking about Megazoom scopes, they don't even mention it on Infinium-S series. Cause it has really low wfms/s because it is long memory analytical scopes. In datasheets for those it is all about how large memory is great..
So which one is true?
--- End quote ---
Marketing may also drive engineering.
During the Tektronix 22xx and 24xx era of DSOs, engineering came up with a way for the DSO to indicate that aliasing was likely present; just compare the trigger frequency with the sample rate. So they had a prototype with an indicator to show when aliasing was present. Leaving aside how useful this might be to inexperienced users, and keep in mind that everybody was an inexperienced user as far as DSOs back then, this feature was obviously dropped because it implied that Tektronix DSOs suffered from aliasing when their competitors did not.
Is there any actual use for the ENOB specification? I would rather know the SFDR (spurious free dynamic range) which is directly observable with an FFT, and very likely where they are calculating the less useful ENOB from anyway.
Oscilloscopes used to specify aberrations, which is roughly equivalent to settling time. I imagine these specifications would look especially bad on 12-bit and higher resolution instruments, some of which will never settle. One reason sampling oscilloscopes are useful at lower frequencies is overload immunity and instant settling time.
--- End quote ---
Thank you for interesting story!
I believe it was dropped because it was not very useful, except in case of sinusoidal signals. And sounded bad like you said.
As for specifications, all the specifications are useful for somebody... ENOB was mentioned because of bits mentioned, because not all bits are same.
I agree with you that it would be nice if more of critical data would be specified. SFDR, SINAD, results ot two tone test (IMD) etc..
But I guess that gets complicated fast. With attenuators, VGA etc. you get many combinations of signal path to ADC, so that would be complicated tables of all the specs in all combinations...
electr_peter:
--- Quote from: David Hess on January 17, 2024, 07:02:49 pm ---During the Tektronix 22xx and 24xx era of DSOs, engineering came up with a way for the DSO to indicate that aliasing was likely present; just compare the trigger frequency with the sample rate. So they had a prototype with an indicator to show when aliasing was present. Leaving aside how useful this might be to inexperienced users, and keep in mind that everybody was an inexperienced user as far as DSOs back then, this feature was obviously dropped because it implied that Tektronix DSOs suffered from aliasing when their competitors did not.
--- End quote ---
Interesting story. Need to think how useful (or not) this feature would be.
I think that current DSOs could show something related on the screen. A popup of some sort can light up whenever sampling rate is <2x of scope nominal BW (or with selected 20MHz BW filter 40MS/s or less). This is left to the user AFAIK - actual sampling rate is shown, but it is not immediately obvious if it is sufficient.
Such popup would at least indicate that some signals may alias and sinc interpolation may not be appropriate. DSOX3k scopes solve these problems behind the curtains so to speak (via random decimation and by disabling sinc interpolation with lower sample rate).
David Hess:
--- Quote from: Wolfgang on January 17, 2024, 08:45:43 pm ---- There are never more bits than an ADC can provide. Everything else is done by boxcar averaging and oversampling. While this can reduce noise, it will only make the inherent nonlinearities of the ADC more visible >:D
These nonlinearities create spikes in the spectrum, automatically reducing dynamic range and also the resulting ENOB.
--- End quote ---
Nothing prevents having a converter with greater linearity than its resolution will support, but they are rare and except as describes below, I am not aware of any fast ones.
Modern fast ADCs all seem to be pipelined subranging types with 3 or 4 stages in series, with the earlier stages having a linearity much better than their low 3 to 5 bit resolution will support because it directly affects the later stages. Error correction then discards some of the generated bits to produce the specified resolution at the interface. Multiples of these subranging converters are interleaved for even higher sample rates and their self calibration encompasses this as well.
--- Quote from: electr_peter on January 17, 2024, 09:53:47 pm ---I think that current DSOs could show something related on the screen. A popup of some sort can light up whenever sampling rate is <2x of scope nominal BW (or with selected 20MHz BW filter 40MS/s or less). This is left to the user AFAIK - actual sampling rate is shown, but it is not immediately obvious if it is sufficient.
--- End quote ---
Some of the appeal of what the Tektronix designers did was because it was so easy to do on these early DSOs. They all included high speed level triggering which operates very much like a data slicer, so comparing the processed trigger signal to 1/2 of the sample rate will immediately reveal aliasing. The only thing required was a simplified frequency counter.
Wolfgang:
--- Quote from: David Hess on January 18, 2024, 01:22:23 am ---
--- Quote from: Wolfgang on January 17, 2024, 08:45:43 pm ---- There are never more bits than an ADC can provide. Everything else is done by boxcar averaging and oversampling. While this can reduce noise, it will only make the inherent nonlinearities of the ADC more visible >:D
These nonlinearities create spikes in the spectrum, automatically reducing dynamic range and also the resulting ENOB.
--- End quote ---
Nothing prevents having a converter with greater linearity than its resolution will support, but they are rare and except as describes below, I am not aware of any fast ones.
Modern fast ADCs all seem to be pipelined subranging types with 3 or 4 stages in series, with the earlier stages having a linearity much better than their low 3 to 5 bit resolution will support because it directly affects the later stages. Error correction then discards some of the generated bits to produce the specified resolution at the interface. Multiples of these subranging converters are interleaved for even higher sample rates and their self calibration encompasses this as well.
--- Quote from: electr_peter on January 17, 2024, 09:53:47 pm ---I think that current DSOs could show something related on the screen. A popup of some sort can light up whenever sampling rate is <2x of scope nominal BW (or with selected 20MHz BW filter 40MS/s or less). This is left to the user AFAIK - actual sampling rate is shown, but it is not immediately obvious if it is sufficient.
--- End quote ---
Some of the appeal of what the Tektronix designers did was because it was so easy to do on these early DSOs. They all included high speed level triggering which operates very much like a data slicer, so comparing the processed trigger signal to 1/2 of the sample rate will immediately reveal aliasing. The only thing required was a simplified frequency counter.
--- End quote ---
Agreed, but none of these tricks can be used in scope front ends with high sampling speeds. The "naked" linearity is what counts there, and there seems to be no way around that, AFAIK.
nctnico:
--- Quote from: electr_peter on January 17, 2024, 09:53:47 pm ---
--- Quote from: David Hess on January 17, 2024, 07:02:49 pm ---During the Tektronix 22xx and 24xx era of DSOs, engineering came up with a way for the DSO to indicate that aliasing was likely present; just compare the trigger frequency with the sample rate. So they had a prototype with an indicator to show when aliasing was present. Leaving aside how useful this might be to inexperienced users, and keep in mind that everybody was an inexperienced user as far as DSOs back then, this feature was obviously dropped because it implied that Tektronix DSOs suffered from aliasing when their competitors did not.
--- End quote ---
Interesting story. Need to think how useful (or not) this feature would be.
--- End quote ---
I agree. A light goes on saying what you see is not what you got. And then??? :) IMHO peak-detect is a much more useful feature to prevent aliasing. At least you can see the extremes of the signal.
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