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| Gw Instek GDS2204E (200MHz 4 channel DSO) review |
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| marmad:
--- Quote from: nctnico on September 16, 2016, 08:41:45 pm ---Again: I wrote 'margin' so don't insist on reading something different! --- End quote --- Sheesh... I'm not reading anything other than your English - which, admittedly, is poor. You can't "overkill" based on a theory, and by "practical situations", you actually mean "real-world applications". It's ridiculous to write posts which might make some people believe that sampling above the Nyquist Rate is irrelevant (i.e. "overkill"). |
| Wuerstchenhund:
--- Quote from: marmad on September 16, 2016, 09:01:46 pm --- --- Quote from: nctnico on September 16, 2016, 08:41:45 pm ---Again: I wrote 'margin' so don't insist on reading something different! --- End quote --- Sheesh... I'm not reading anything other than your English - which, admittedly, is poor. You can't "overkill" based on a theory, and by "practical situations", you actually mean "real-world applications". It's ridiculous to write posts which might make some people believe that sampling above the Nyquist Rate is irrelevant (i.e. "overkill"). --- End quote --- I understood nctnico's post so that he's saying that, while in general anything over Nyquist-Shannon doesn't provide any benefit, some provisions have to be made for signals above the rated analog BW beccause the true BW might be higher and because of the filter properties which isn't infinite steep. And that is absolutely correct, and a sample rate in excess of what's required to cover that frequency range is, indeed, "overkill" (or do you want to say a 20GSa/s sample rate would make sense for a 100MHz scope?). On a side note, I have to say nctnico's English is absolutely fine and much better than what I've seen from many 'native' speakers, and on a personal note I find your (repeated) rather condescending attitude towards non-native English speakers pretty poor. Especially when your own grasp of the use of words hasn't been exactly stellar. |
| eeguy:
--- Quote from: nctnico on September 16, 2016, 07:41:34 pm ---I did some testing with the GDS2204E and aliasing is minimal because it has anti-aliasing filters in place. With a 200MHz bandwidth the 500Ms/s is more than enough to show the signal. You have to keep in mind that in theory the minimum sampling frequency is twice the bandwidth. Everything more is overkill but in practical situations you need some margin for the anti-aliasing filters. --- End quote --- Thanks. Do you mean it should be OK but if I could get an oscilloscope (100MHz-200MHz) with 1GS/s per channel, that would be better? |
| marmad:
--- Quote from: Wuerstchenhund on September 17, 2016, 10:55:52 am ---I understood nctnico's post...or do you want to say a 20GSa/s sample rate would make sense for a 100MHz scope? --- End quote --- I understood his post, and although you may accept at face value that "with a 200MHz bandwidth, the 500Ms/s is more than enough to show the signal" on a low-cost DSO, I'm a little more skeptical. In any case, there have been repeated discussions here (and much talk by Dave during some of his reviews) on this subject, and whether or not you feel that a sampling rate that is 2.5x the bandwidth is "overkill" to reliably reconstruct complex, high-frequency signals with sin(x)/x, I don't. In that regard, I feel it's worth debating whether it's the correct term in this context for prospective GW-Instek (or any DSO) buyers. --- Quote ---...or do you want to say a 20GSa/s sample rate would make sense for a 100MHz scope? --- End quote --- You believe there's an equivalency between a sampling rate 2.5x higher than the BW and one that's 200x higher than the BW? --- Quote ---...and on a personal note I find your (repeated) rather condescending attitude towards non-native English speakers pretty poor. --- End quote --- On a personal note, I find your use of the word "condescending" to describe any other poster on this blog rather humorous, and while I'll fully admit that I'm sometimes (either intentionally or unintentionally) condescending, somehow I doubt you'll follow suit. And I've certainly never claimed my English is "stellar", and I know I've made plenty of incorrect or awkward word choices while dashing off posts. And yes, nctnico's English is normally fine and I was being overly harsh, but I believe "overkill" to be a misleading description in this particular example. |
| jjoonathan:
I don't think it reflects negatively on peoples' English skills, but I do think that many of the remarks in this thread -- "rolloff is not infinitely steep" and "some margin for the anti-alias filters" understate the role of rolloff. It's typical for modern scopes to devote half (or more!) of their Nyquist band to rolloff rather than sticker bandwidth (i.e. samp rate >= 4x BW). Gentle rolloffs make for better step responses, so Oscilloscope designers prefer them when possible, but the tradeoff is that a huge part of your Nyquist band must then be dedicated to rolloff in order to promise decent attenuation of aliases. Not 10% or 20% of your Nyquist band, 50% or 60% of your Nyquist band, and even then the alias attenuation isn't spectacular. For the front end on my personal scope, a Rigol DS4014 hacked to 500MHz (see "Rigol MSO4024-500" line on the graph below), here is the relationship between sample rate and the alias attenuation they can promise given the gentle rolloff of their front end filter: SampleRateAliasAttenuation2BW-0.5dB (!)2.5BW-2dB (!)4BW-13dB (actual sample rate, 4 channels)8BW~30dB (actual sample rate, 2 channels) I am quite pleased they did not try to sample at 2.5*BW. The benefit of going from 4*BW to 8*BW sampling (-13dB aliases to -30dB aliases) is more debatable, but -13dB attenuation isn't actually all that great -- it's still 20% of the original voltage amplitude, which is a considerable amount of worst case waveform fuzziness. It's comforting to have the option to bump up to 8*BW sampling (aliases at most ~3% of original amplitude). With credit to Altemir: Of course, we are discussing the GDS2204E, not the Rigol DS4000 series, but I could not find frequency response plots for the GDS2204E so it would be difficult to comment on the tradeoffs made by GW Instek. Here is the Bode plot for the DS1054Z (credit to ankerwolf) which might be more representative: |
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