Author Topic: Oscillo-confusion MHz GSa/s wfm/s Mpts  (Read 9934 times)

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Offline FriedMuleTopic starter

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #25 on: January 01, 2019, 08:52:54 pm »
Maybe I have read wrong but for me it sounds as some of you writes:

Buy high bandwidth, you will rarely use 4 channels

and other says

Buy 4 channels, you will properly not use high bandwidth anyway?
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Online tautech

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #26 on: January 01, 2019, 11:09:23 pm »
In the some years I had a 4ch 300MHz DSO there were very few times I needed the BW and when I did a scope wasn’t really the right tool for the job and when Siglent SSA’s and SVA’s became available I grabbed one of each.
I won’t be selling my beta tester SDS1104X-E SN#0012 anytime soon.
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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #27 on: January 01, 2019, 11:18:48 pm »
The OP must ensure they use the correct type of probe for the job. In particular, for some jobs such as SMPSs connected directly to the mains it is necessary to have an isolating differential probe; these cost as much as a cheap scope :)

https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
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Offline rstofer

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #28 on: January 02, 2019, 12:37:05 am »
Maybe I have read wrong but for me it sounds as some of you writes:

Buy high bandwidth, you will rarely use 4 channels

and other says

Buy 4 channels, you will properly not use high bandwidth anyway?

It goes either way!

Using 4 channels may never happen for some users or it may happen all the time.  It was my most important purchase criteria.  I already had 2 channels (analog scope).  Since I had never used a DSO, I had no idea how important Single Shot would be.  It solves so many problems.

In the old days of the analog scope, I would have to butch up my logic or code in order to have a pulse train repeat often enough to stay on the screen.  With single shot I don't need to modify anything.  I just select single shot and one pass is all it takes.  This is a HUGE deal and one of the great benefits of the DSO.

High bandwidth will never be an issue for those interested in audio.  My interests are up to around 50 MHz on uC pins and usually not that high.  Maybe something in the 2-5 MHz range.

You never said exactly what you want to do with a scope and that's good because you aren't locking yourself into one mindset and overlooking others.  Sure, in a perfect world, you would have both channels and bandwidth.  Few of us live in that world!

As also mentioned above, this is just a scope.  Better than some, not as good as others.  When you need something better, you will sell this scope for nearly as much as you paid for it and use the money toward something better.  Or you will just consider it a sunk cost and keep it.

I have mentioned that I want the Siglent SDS1204X-E.  What to do with my DS1054Z?  Well, it sits on top of one of my Comdyna analog computers, it works perfect in that application, maybe I'll just leave it there and fill the hole on my bench with the Siglent.  See?  It all works out!

 

Offline vk6zgo

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #29 on: January 02, 2019, 01:04:03 am »
Bandwidth and samples/second are completely independent. Bandwidth is all that matters in a scope, except to salesmen. I have an ~4GHz scope with ~40kS/s.

Apart from that fundamental, there are many threads on "scopes for beginners" on this forum.

The key points are to define what you will use the scope for, and whether you have other necessary equipment. Plus you shouldn't forget the cost of probes :)

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!
 

Offline FriedMuleTopic starter

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #30 on: January 02, 2019, 02:29:33 am »
That helped me a lot and maybe I should define the data before the model?
Am I understanding correct in these data titles?

All with the thought on selecting a "do whatever beginner scope that I can use to most run of the mill electronic)

bandwidth (Maybe not so extremely important but 100MHz+)
sample rate (1GSa/s+)
memory depth (As large as possible)
resolution (as many bit as possible "16bit best)
update rate (faster is better)
Capture rate (higher is better)

What would you put in as value for getting a cheap, max 1.000$ but "best" scope?
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Offline james_s

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #31 on: January 02, 2019, 05:46:52 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!

Analog broadcast TV is gone but analog video is alive and well. I use it for the FPV video link on a couple of my RC airplanes and multirotors, it's widely used in retro computers and video games, amateur broadcast, many security cameras are still analog, it's not uncommon that I find myself looking at a video waveform.
 

Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #32 on: January 02, 2019, 01:42:32 pm »
Bandwidth and samples/second are completely independent. Bandwidth is all that matters in a scope, except to salesmen. I have an ~4GHz scope with ~40kS/s.

Apart from that fundamental, there are many threads on "scopes for beginners" on this forum.

The key points are to define what you will use the scope for, and whether you have other necessary equipment. Plus you shouldn't forget the cost of probes :)

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!

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

It is only relatively recently (by my standards!) that digitising scopes have been equally usable.
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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #33 on: January 02, 2019, 01:52:31 pm »
Maybe I have read wrong but for me it sounds as some of you writes:

Buy high bandwidth, you will rarely use 4 channels

and other says

Buy 4 channels, you will properly not use high bandwidth anyway?

It goes either way!

Using 4 channels may never happen for some users or it may happen all the time.  It was my most important purchase criteria.  I already had 2 channels (analog scope).  Since I had never used a DSO, I had no idea how important Single Shot would be.  It solves so many problems.

In the old days of the analog scope, I would have to butch up my logic or code in order to have a pulse train repeat often enough to stay on the screen.  With single shot I don't need to modify anything.  I just select single shot and one pass is all it takes.  This is a HUGE deal and one of the great benefits of the DSO.

As you note, it is usually possible to design and implement a circuit in such a way that the waveforms are repetitive - in which case single-shot is not necessary.

However, there are a few events that are one-offs (e.g. monitoring bomb explosions, PSU startups), and for these a storage scope is necessary. Before digitising scopes, we had analogue storage scopes - and they were as horrible to use as the early digitising scopes were for repetitive waveforms.

Quote
High bandwidth will never be an issue for those interested in audio. 

There bits/resolution and linearity are probably more important. The Analog Discovery is pretty good in that respect.

Quote
My interests are up to around 50 MHz on uC pins and usually not that high.  Maybe something in the 2-5 MHz range.

Ahem. That 2Mb/s signal could have components above 1GHz, depending on logic family. But you know that!

Personally I'd use a cheap and nasty LA to capture/decode such signals.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline rstofer

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #34 on: January 02, 2019, 03:15:46 pm »
High bandwidth will never be an issue for those interested in audio. 

There bits/resolution and linearity are probably more important. The Analog Discovery is pretty good in that respect.
In my view, the Analog Discovery is unappreciated.  The lab courses associated with Digilent's Real Analog course demonstrate many of the features.  It truly is an electronics lab in a small box.
Quote

Quote
My interests are up to around 50 MHz on uC pins and usually not that high.  Maybe something in the 2-5 MHz range.

Ahem. That 2Mb/s signal could have components above 1GHz, depending on logic family. But you know that!

Personally I'd use a cheap and nasty LA to capture/decode such signals.
Whether the LA can display setup and hold times will be a function of its sample rate.  I have a LA that will sample at 200 MSa/s which is pretty good but before I went for one of the $8 units, I would want to look carefully at sample rate.  My scope will sample at 1 GSa/s which will more accurately display the time difference between events.

ETA:  True, the DS1054Z will only sample 4 channels at 250 MSa/s but I could momentarily be more selective and display just 2 channels with a sample rate of 500 MSa/s.

At high SPI rates, a LA sampling at 100 MSa/s might be just fine.

ETA:  Logic Analyzers will have much better triggering options.  I sometimes have to add logic just to create a trigger when using a scope.  There are a number of reasons that LAs are popular.

https://sump.org/projects/analyzer/

Note how the sample rate drops when the device is used for state analysis and the device under test provides the sample clock.  State analysis is my primary use of a LA.  I don't care what the state of logic signals is relative to some arbitrary internal LA clock, I want to know what their state is relative to the internal logic clock.
« Last Edit: January 02, 2019, 03:34:17 pm by rstofer »
 

Offline hamiltont

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #35 on: January 02, 2019, 06:01:31 pm »
The problem I had in your situation was I had no idea what my base MHz requirements were. I understood that bandwidth was a core consideration, and more bandwidth lets me scope faster signals, but....how fast are the signals I am interested in?!?!?!

To solve this I looked into common bit rates for digital protocols (my main interest area) and used a trick from stackexchange to turn them into megahertz. Here is a good table - https://en.wikipedia.org/wiki/List_of_interface_bit_rates

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.

Some other examples:

USB 2.0 High speed - 480Mbit/s e.g. 240MHz
USB 3.0 Superspeed - 5Gbit/s e.g. 2.5GHz
Ethernet 10-Bx - 10Mbit/s e.g. 5Mhz
Ethernet 100-Bx - 100Mbit/s e.g. 50MHz
I2C - 3.4Mbit/s e.g. 1.7MHz
HDMI 1.0 video - ~5Gbit/s e.g. 2.5GHz 


« Last Edit: January 02, 2019, 06:04:00 pm by hamiltont »
 
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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #36 on: January 02, 2019, 06:23:32 pm »
High bandwidth will never be an issue for those interested in audio. 

There bits/resolution and linearity are probably more important. The Analog Discovery is pretty good in that respect.
In my view, the Analog Discovery is unappreciated.  The lab courses associated with Digilent's Real Analog course demonstrate many of the features.  It truly is an electronics lab in a small box.
Quote

Quote
My interests are up to around 50 MHz on uC pins and usually not that high.  Maybe something in the 2-5 MHz range.

Ahem. That 2Mb/s signal could have components above 1GHz, depending on logic family. But you know that!

Personally I'd use a cheap and nasty LA to capture/decode such signals.
Whether the LA can display setup and hold times will be a function of its sample rate.  I have a LA that will sample at 200 MSa/s which is pretty good but before I went for one of the $8 units, I would want to look carefully at sample rate.  My scope will sample at 1 GSa/s which will more accurately display the time difference between events.

A two channel scope is better for measuring setup and hold times than an LA, and will also do the other necessary signal integrity measurements.

Only after that is it worth flipping to the digital domain with an LA.

Quote
ETA:  Logic Analyzers will have much better triggering options.  I sometimes have to add logic just to create a trigger when using a scope. 

Exactly. LAs are better at allowing you to ignore irrelevant stuff, and concentrate on the rarer important information.

Quote
Note how the sample rate drops when the device is used for state analysis and the device under test provides the sample clock.  State analysis is my primary use of a LA.  I don't care what the state of logic signals is relative to some arbitrary internal LA clock, I want to know what their state is relative to the internal logic clock.

Agreed on all counts (pun intended).

At high data rates in general purpose LAs, usually the synchronous state capture rate is only 25% of the asynchronous capture rate, since 4 samples are used to detect two edges. The memory length may or may not be reduced.

Some LAs, especially those operating at low speed or special purpose LAs, have separate clock inputs which clock the LAs logic directly. They can operate at full speed.

Often a high speed LA has "strange" speed/size limitations. These can often be understood if you know the input structures in modern FPGAs, especially the SERDES blocks.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #37 on: January 02, 2019, 06:45:20 pm »
The problem I had in your situation was I had no idea what my base MHz requirements were. I understood that bandwidth was a core consideration, and more bandwidth lets me scope faster signals, but....how fast are the signals I am interested in?!?!?!

To solve this I looked into common bit rates for digital protocols (my main interest area) and used a trick from stackexchange to turn them into megahertz.

Stackexchange is worth what you pay for it. I'm afraid your understanding is simply and completely wrong.

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.

If you don't believe standard theory and practice and my statement, perhaps the measurements shown at https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/ will convince you. It shows the very different frequency content of three different 1kHz signals:
  • a 1kHz square wave with 120μs rise and fall times – the baseline for comparison
  • a 1kHz square wave with 120ns rise and fall times – representative of a 1kHz digital clock signal
  • a 1kHz signal with a width of 10μs (1% duty cycle), 120ns rise and fall times –  representative of a general digital signal
In reality the digital signals won't have tr=120ns. Instead 12ns, 1.2ns and 0.12ns can all be found in modern jellybean logic. Even a 74LVC1G04 gate is 250ps with frequency content above 1GHz: see https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

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.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline james_s

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #38 on: January 02, 2019, 07:08:32 pm »
I don't think that's necessarily true. Yes you need a lot of bandwidth to display a really square looking square wave accurately, but the thing is most of the time you don't really need to display it accurately, you just need to understand the limitations of your test gear. Thousands of hobbyists have gotten by with scopes of 20MHz or less debugging digital circuitry. Occasionally seeing the rise time of a signal matters but most of the time you just need to see the pattern of 1s and 0s or see when a signal is changing states relative to another signal. To say someone *needs* x amount of bandwidth is somewhat subjective, more bandwidth is always nice, but you don't need a 400MHz scope to debug an Arduino, a much less exotic instrument will do the job, even if it rounds off the edges of your square waves. Even by the time you get to a 100MHz scope probes and probing technique starts to become an art and IMO it's simply not something a beginner needs to worry too much about.
 
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Offline mvs

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #39 on: January 02, 2019, 07:41:10 pm »
I am now looking at the
Siglent SDS1204X-E 200Mhz 1 GSa/s

but may be willing to buy the
Siglent SDS2352X-E 2Ch 350MHz 2 GSa/s
This specs are not quite correct.
SDS1204X-E has actually two 1GSa/s ADCs, each serving a pair of inputs (1+2, 3+4). It can sample at 1GSa/s in dual channel mode, if inputs are chosen from different ADCs.
SDS2352X-E has only one 2GSa/s ADC and it will reduce sampling speed to 1GSa/s in dual channel mode.
 

Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #40 on: January 02, 2019, 07:45:37 pm »
I don't think that's necessarily true. Yes you need a lot of bandwidth to display a really square looking square wave accurately, but the thing is most of the time you don't really need to display it accurately, you just need to understand the limitations of your test gear. Thousands of hobbyists have gotten by with scopes of 20MHz or less debugging digital circuitry. Occasionally seeing the rise time of a signal matters but most of the time you just need to see the pattern of 1s and 0s or see when a signal is changing states relative to another signal. To say someone *needs* x amount of bandwidth is somewhat subjective, more bandwidth is always nice, but you don't need a 400MHz scope to debug an Arduino, a much less exotic instrument will do the job, even if it rounds off the edges of your square waves. Even by the time you get to a 100MHz scope probes and probing technique starts to become an art and IMO it's simply not something a beginner needs to worry too much about.

I first learned the strengths and weaknesses of that approach in 1976, as I noted above in https://www.eevblog.com/forum/beginners/oscillo-confusion-mhz-gsas-wfms-mpts/msg2082310/#msg2082310 In that you will note I designed, built and debugged a computer using test equipment with a bandwidth <1Hz (let alone MHz or kHz!).

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!

Once you can be sure signal integrity problems do not exist, then you can concentrate on the "higher level" debugging - and low bandwidth equipment is often completely sufficient.

BTW all probing is an art! Even at 50Hz you need to use the right probes for the job, lest you kill your equipment or yourself or someone else.
« Last Edit: January 02, 2019, 07:47:39 pm by tggzzz »
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Offline hamiltont

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #41 on: January 02, 2019, 07:51:19 pm »
Stackexchange is worth what you pay for it. I'm afraid your understanding is simply and completely wrong.

Thanks for clarifying! It'll take me some time to grok the replies completely, but I appreciate the help - even though I didn't pay for it  ;D

« Last Edit: January 02, 2019, 07:53:39 pm by hamiltont »
 

Online ebastler

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #42 on: January 02, 2019, 08:00:41 pm »
How important would you mean that 4CH is compared to 2CH for everyday use?
I think that a later 4CH analog scope could be a good addition?

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.
 

Offline james_s

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #43 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.

Until very recently, a scope with more than 100MHz bandwidth was an exotic and very expensive instrument. In most cases there is simply no need for more bandwidth, and other details will be the limiting factor.
 
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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #44 on: January 02, 2019, 08:05:07 pm »
Stackexchange is worth what you pay for it. I'm afraid your understanding is simply and completely wrong.

Thanks for clarifying! It'll take me some time to grok the replies completely, but I appreciate the help - even though I didn't pay for it  ;D

I'm trying to reduce the cost (of your time and enthusiasm) due to misunderstandings :) You have the right attitude, so you will need less good luck.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline tggzzz

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #45 on: January 02, 2019, 08:06:42 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.

Those are strawman arguments in this context. I frequently explicitly make the same points, and implicity in my .sig.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline james_s

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #46 on: January 02, 2019, 08:07:10 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.

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.
 

Offline rstofer

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #47 on: January 02, 2019, 08:07:54 pm »
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!



 

Offline rstofer

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #48 on: January 02, 2019, 08:19:44 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.

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.
 

Offline james_s

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Re: Oscillo-confusion MHz GSa/s wfm/s Mpts
« Reply #49 on: January 02, 2019, 08:22:30 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.
 


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