Author Topic: Oscillo-confusion MHz GSa/s wfm/s Mpts  (Read 9930 times)

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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #50 on: January 02, 2019, 09:03:07 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.

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.

Yes, it does matter, all too often.

What you are trying to see is of key importance with any tool. Often it indicates that a different type of tool is possible or preferable.

Of course you don't look at a USB line with a scope to verify signal integrity. If you haven't designed the hardware, if there are problems then it shouldn't have escaped the factory. Alternatively if you are the hardware designer, then you ought to know what the data sheets and app notes indicate. If you don't then signal integrity is the least of your problems!
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Offline james_s

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #51 on: January 02, 2019, 10:03:36 pm »
I just don't fully agree. I got by for many years with a 100MHz scope, before that I was using a 20MHz scope. It worked just fine for the stuff I was doing, microcontrollers and analog circuits. It simply did not matter that higher frequency square waves had the corners rounded off, I knew that was a limitation of the scope and it didn't matter, it was an irrelevant detail. I needed to see the data, and I could read rounded square waves just as easily as I could read nice square ones. It's just funny to see people going on like a hobbyist absolutely NEEDS things that no hobbyist could ever hope to afford until very recently. High bandwidth is a nice luxury, but outside of certain specialized use cases you don't NEED high bandwidth unless it's just to satisfy some kind of OCD. You bring up edge cases like looking at USB, why would someone even be looking at USB with a scope? That's using a hammer to drive a screw, it may work, sort of, but it's not the right tool for the job.

When it comes to digital circuits, signal integrity is exactly what a scope is ideal for, otherwise a logic analyzer is often a more appropriate tool. The logic analyzer doesn't show you the true shape of the waveform either but that doesn't matter because unless you're looking at signal integrity the precise shape of the digital waveform is irrelevant so long as it is read as 1s and 0s and there is not something waaaaaaaaaaay out of whack.
 

Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #52 on: January 02, 2019, 10:14:44 pm »
I just don't fully agree. I got by for many years with a 100MHz scope, before that I was using a 20MHz scope. It worked just fine for the stuff I was doing, microcontrollers and analog circuits. It simply did not matter that higher frequency square waves had the corners rounded off, I knew that was a limitation of the scope and it didn't matter, it was an irrelevant detail. I needed to see the data, and I could read rounded square waves just as easily as I could read nice square ones. It's just funny to see people going on like a hobbyist absolutely NEEDS things that no hobbyist could ever hope to afford until very recently. High bandwidth is a nice luxury, but outside of certain specialized use cases you don't NEED high bandwidth unless it's just to satisfy some kind of OCD. You bring up edge cases like looking at USB, why would someone even be looking at USB with a scope? That's using a hammer to drive a screw, it may work, sort of, but it's not the right tool for the job.

When it comes to digital circuits, signal integrity is exactly what a scope is ideal for, otherwise a logic analyzer is often a more appropriate tool. The logic analyzer doesn't show you the true shape of the waveform either but that doesn't matter because unless you're looking at signal integrity the precise shape of the digital waveform is irrelevant so long as it is read as 1s and 0s and there is not something waaaaaaaaaaay out of whack.

I didn't bring up USB; I was responding to it in your post - which you have conveniently snipped.

Your contentions are mutually contradictory and/or claiming the same points I have repeatedly advocated - without acknowledging that.

For example, you claim a scope is ideal for signal integrity, but then claim that 20MHz scopes are adequate - which they demonstrably are not.
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Offline FriedMuleTopic starter

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #53 on: January 03, 2019, 07:59:03 pm »
Right now I am thinking that better data always give higher resolution and accuracy while more channels only are useful the times I do need to measure 3+ places, is that stupid thinking or good thinking?
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Offline ebastler

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #54 on: January 03, 2019, 08:48:28 pm »
Right now I am thinking that better data always give higher resolution and accuracy while more channels only are useful the times I do need to measure 3+ places, is that stupid thinking or good thinking?

Yes, that's right. Requiring more than two channels is relatively rare.

But on the other hand: In most cases, having a lower bandwidth will only give you gradually reduced information. (The bandwidth rolls off at 3dB/octave or such, so you can still see signal above, although at limited resolution and maybe noticeably distorted.) In contrast, not having the two extra channels when you need them will often mean a "hard stop"; you just can't observe the required connection between the signals.

Sorry, that argument doesn't make your decision any easier... I had it easier when the DS1054Z was a breakthrough in bandwidth and number of channels a few years ago.  :P
 

Offline rstofer

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #55 on: January 03, 2019, 09:31:16 pm »
Right now I am thinking that better data always give higher resolution and accuracy while more channels only are useful the times I do need to measure 3+ places, is that stupid thinking or good thinking?

If the ADC had many more bits than log base 2 of vertical pixels on the screen, what would be the purpose?  Many very high dollar scopes are still using 8 bit ADCs.  Oh, I guess you could download the conversion data to a PC and produce a much higher resolution display but before going to all that work, check the specified accuracy for the scope itself.  It's not going to be a 0.001% device.

Scopes are 'about' kinds of measuring tools, you don't get many digits of precision eyeballing the trace.  What you're looking for is "does it look 'about' right?".  The measurement tools can bring along more precision than you get from the display but I wouldn't substitute a voltage reading on a scope for a reading on a DMM.  At best, the DS1054Z is a 3% kind of tool.

Watch the transistor video linked below.  Notice how the author doesn't even pretend to try for a high degree of accuracy in his measurements.  He just picks out the 'about' value and moves on.  More often than not, this is exactly how a scope is used.



The problem with the 4 vs 2 channel debate is that it is binary.  If you need 4 channels and don't have them, you have to come up with some other solution.  If you need 2 channels and have 4 channels, everything is great.

Bandwidth is the same argument on a sliding scale.  Yes, if you need 500 MHz and you only have 100 MHz, you're stuck.  But if you need 110 MHz and you have 100 MHz, you're probably still in the game.

It's like cars:  You never have too much horsepower.  OTOH, speed costs money, how fast can you afford to go?


 

Offline FriedMuleTopic starter

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #56 on: January 03, 2019, 10:09:08 pm »
Maybe a new question can make my choice easier to make:-)

If you forget about RF and high speed digital logic. What scope would you choose for less then 1,000$
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Offline maginnovision

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #57 on: January 03, 2019, 10:12:29 pm »
Maybe a new question can make my choice easier to make:-)

If you forget about RF and high speed digital logic. What scope would you choose for less then 1,000$

micsig to1104 since you have no clue what you really want. Neither of my digital scopes was under 1k, and my analog isn't something I'd recommend to you(tektronix 475A)
 

Offline David Hess

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #58 on: January 04, 2019, 12:04:46 am »
There are times when you want to look at signals with quite high frequency components, whilst using a very long time/cm setting.

The classic case is looking at analog video at field rate.
DSOs with  very small memories commonly reduce the sampling rate savagely at such settings, to the point where the required display is lost in a forest of aliasing.

The early DSOs ( & some currently available really cheap ones) cannot even display analog video accurately at line rate.
OK, analog video is no more, but try looking for 50/60Hz "hum" on any pulse train with HF components!

It took a while for hardware integration and performance to reach the point where real time histograms as in DPOs (Digital Phosphor Oscilloscopes) or large sample memories were feasible.  Even today the display quality of a DSO capable of displaying an analog video signal is poor.

Personally I prefer the short record length and high acquisition rate of a DPO.  See below about waveform acquisition rate.

The early digitising scopes were awful to use, except in some circumstances. Hence the correct statement that analogue scopes were usually better.

I would say the boundary between good and useless shifted with the Tektronix 2230/2232 which were the first DSOs to support peak detection.  They lacked index graded displays but at least they could capture the envelope of a single on every acquisition.

For digital signals, the only thing that matters is the transition speed, tr; the period is completely and utterly irrelevant. The usual rule-of-thumb is that the signal bandwidth is 0.35/tr. There are a few nuances, but at this level they can be ignored.

...

Hence if you have, say, an Arduino which changes the output once per hour, and it takes 5ns to go from high to low, then the maximum bandwith in the signal is 70MHz (i.e. 0.35/5e-9) - and you scope needs to be faster than that.

This cannot be stressed enough; instead of thinking in terms of bandwidth, think in terms of transition time and the 0.35 rule:

20MHz   17.5   High Voltage CMOS
50MHz   7.0ns   Standard and LS TTL and HCMOS
100MHz   3.5ns   Fast TTL
200MHz   1.75ns   ECL

Probing above 100MHz becomes increasingly difficult.  200MHz is feasible for general troubleshooting with the shortest ground lead but higher will require a expensive active probes, low-z probes, or probe tip adapters.  If you are not prepared to deal with these, then avoid instruments above 200MHz unless you plan on only using coaxial cables.

Hobbyists frequently unwittingly create the conditions in which there are infrequent random failures or failures that take a long time to become apparent. Classic examples: failing to observe data hold times (setup times are comparatively easy), incorrect termination leading to voltage spikes which slowly damage receiver inputs, non-monotonic transitions on clock lines, ground bounce, and several others.

If present, those signal integrity problems probably won't be seen on a 20MHz scope. So when their design doesn't work as expected, people will start looking in the wrong place. Seen that far too many times!

This is the place where waveform acquisition rate matters.  A real time DSO display only requires 30fps but might take days to catch an elusive metastable event.  In practice hobbyists will have to live with whatever they can find which is pretty good today compared to just a couple decades ago even on basic instruments.



As far as the number of channels, almost all two channel instruments include a trigger input somewhat relaxing the requirement for a 4 channel oscilloscope.  But if you want to decode 3 and 4 wire serial buses like SPI, 4 channels is a great improvement.  Some low cost two channel instruments (HP?  R&S?) can now use their trigger input as a third channel for SPI decoding.
 

Online tautech

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #59 on: January 04, 2019, 12:12:46 am »
Maybe a new question can make my choice easier to make:-)

If you forget about RF and high speed digital logic. What scope would you choose for less then 1,000$
For $1k you’re not gunna cover all bases and while your budget is more than double what the average novice is prepared invest in a new scope IMO getting a 2 GSa/s DSO is superfluous to the bulk of requirements.
rstofer and ebastler have given sound advice and I’ll add the cheaper of you two initial choices is a lot of scope for the money and I confidently state you won’t have buyers remorse.
Is something better just around the corner.....well probably but we don’t know exactly what is is just yet.
Im suspecting SDS2004X-E models but way 2ch models are priced they could be $1k+.
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Offline FriedMuleTopic starter

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #60 on: January 04, 2019, 01:03:54 am »
I have asked Siglent about a 4ch version of the 2000X-E but no, they don't expect to make one.

I think that all have been here where I am now, I just have a lot more scope then most could afford then.
I "just" want a scope that I do not regret as you all know and for me the Siglent looks as most smack for the diam, but there is also no reason for buying a 2MHz 20GS.

I'll try to read all your comment again for maybe the 6th time to absorb your great advices better. :-)
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Online tautech

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #61 on: January 04, 2019, 01:22:10 am »
I have asked Siglent about a 4ch version of the 2000X-E but no, they don't expect to make one.
So have I and got the same reply but if you take a step back and look at their offerings knowing what they do have coming there is a gap in their lineup.
The SDS2kX (not X-E) range has been out for a while and needs freshening up with a faster processor and while the new SDS5kX series will offer this and higher BW’s they will come at a much higher price so a mid range 2 GSa/s 4ch DSO like a SDS2354X-E will fill the gap in the lower price range of their lineup.....yet Siglent at this time maintains they’re not building one.....at least their sales dept says so.  :popcorn:
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Offline FriedMuleTopic starter

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #62 on: January 04, 2019, 01:53:50 am »
Yes you are right! :-) And they have a DSDXXX4 in every one of there scopes, so there are something missing here. Maybe wait some time while investing in other bench gear I also need? :-)
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Online tautech

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #63 on: January 04, 2019, 02:09:57 am »
Yes you are right! :-) And they have a DSDXXX4 in every one of there scopes, so there are something missing here. Maybe wait some time while investing in other bench gear I also need? :-)
The real question will be how quickly Siglent feel they need to address having to add SDS2004X-E models to their lineup while SDS2004X models are available and any possible impact 4ch X-E models will have on X model sales.
Time will tell.
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Offline rstofer

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #64 on: January 04, 2019, 04:25:15 am »
Maybe a new question can make my choice easier to make:-)

If you forget about RF and high speed digital logic. What scope would you choose for less then 1,000$

I'm seriously thinking about the Siglent 1204X-E  200 MHz, 4 Channels for around $800.
 


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