Scope Wars

[Sighound36]

Tv84

Well when I see one I will let you know 

[nctnico]

Sorry, but that is the wrong conclusion. Nyquist says that all the information from a signal is there up the fs/2. It is just that your brain can't make a signal from the dots. But that is a problem in your brain and not in the number of samples. In order to help your brain DSOs have sin x/x reconstruction which connects the dots in a mathematically correct way to show you the signal.

As the audiophools like to say, Nyquist is just a theory.  It isn't just your brain that struggles to make a signal from the dots.  The few dots there are, the harder it is to reconstruct the signal accurately.  Just look at the example he posted.  Small amplitude errors can result in huge reconstruction errors at low sample counts while at fs/10 one slightly misplaced dot won't hurt too much.  Yes, in theory you can reconstruct a sine wave at fs/2.1 with a 48-pole unity-

Sorry about the terrible picture, I had to use my phone to get the picture off the camera since I can't find my cable at the moment...
This is a 300MHz signal (the scope attenuated it quite a bit), it shows averaging in use because I forgot to take a picture without it, but it still had similar issues in the SAMPLE mode.

Well... from my own testing with oscilloscopes I've found that sin x / x reconstruction isn't always implemented correctly. I had to report this as bug to 2 different manufacturers (after which it got fixed). So it doesn't surprise me that there are oscilloscopes out there on which the sin x /x reconstruction doesn't work as goed as it can.

[SilverSolder]

So the high end is about 100x better?  -  why not the entry level.

You can get a 4-channel, 100MHz DSO with lots of memory and features for under $350. How is that not an improvement? What would that have cost 20 years ago?

1989:   HP54501A, 100MHz, 4 channels, GPIB, FFT  -  list price:  $3,465 all inclusive (which is ~$7,200 in modern play money)

2020:   DSOX3024A, 200MHz, 4 channels - list price: $5,174  (but software features are optional extras, at extra cost)


There are definitely improvements,  but hardly mind blowing considering it is 30 years -  and the price reduction is hardly mind blowing either.

Yes, I would rather have the 2020 model because it is a better scope in so many ways.  But fundamentally, you could probably do most probing jobs with either model, if you had to...  it's just more work with the old guy.

[rhb]

OK here is the DS1202Z-E sampling at 1 GSa/s. 

First a 700 MHz input:



Next an 850 MHz input:



and finally a 900 MHz input:



I'm not sure what to think about this yet.  Rather clearly if you are working at UHF you can use a 200 MHz DSO quite successfully.  Just be sure you understand what it is actually showing you as the labeling will be completely wrong.

NB: I have yet to see a DSO that did a minimum phase sinc(t) interpolation.  So technically every scope I've seen does it wrong.  Pretty sad in the $20K class.

Have Fun!
Reg

[0culus]

So the high end is about 100x better?  -  why not the entry level.

You can get a 4-channel, 100MHz DSO with lots of memory and features for under $350. How is that not an improvement? What would that have cost 20 years ago?

1989:   HP54501A, 100MHz, 4 channels, GPIB, FFT  -  list price:  $3,465 all inclusive (which is ~$7,200 in modern play money)

2020:   DSOX3024A, 200MHz, 4 channels - list price: $5,174  (but software features are optional extras, at extra cost)


There are definitely improvements,  but hardly mind blowing considering it is 30 years -  and the price reduction is hardly mind blowing either.

Yes, I would rather have the 2020 model because it is a better scope in so many ways.  But fundamentally, you could probably do most probing jobs with either model, if you had to...  it's just more work with the old guy.

If push came to shove, you could probably still make do with an analog 'scope and a camera. Many of the great technological innovations of the 20th century were done with analog scopes. Heck, even the legend himself Jim Williams often used a loaded-with-vacuum-tubes Tek Type 556 (often with the 1S1 sampling plugin) till the end of his life. You can see scope photography from it in many of his Linear app notes.

The real advantages of modern DSOs are primarily in (1) the advanced triggering and protocol decoding stuff (2) single shot acquisitions (because the storage crts sucked when they were new, and they didn't get any better with age). I would wager a skilled operator with a good analog 'scope could do everything else.

[SilverSolder]

Sorry, but that is the wrong conclusion. Nyquist says that all the information from a signal is there up the fs/2. It is just that your brain can't make a signal from the dots. But that is a problem in your brain and not in the number of samples. In order to help your brain DSOs have sin x/x reconstruction which connects the dots in a mathematically correct way to show you the signal.

As the audiophools like to say, Nyquist is just a theory.  It isn't just your brain that struggles to make a signal from the dots.  The few dots there are, the harder it is to reconstruct the signal accurately.  Just look at the example he posted.  Small amplitude errors can result in huge reconstruction errors at low sample counts while at fs/10 one slightly misplaced dot won't hurt too much.  Yes, in theory you can reconstruct a sine wave at fs/2.1 with a 48-pole unity-

Sorry about the terrible picture, I had to use my phone to get the picture off the camera since I can't find my cable at the moment...
This is a 300MHz signal (the scope attenuated it quite a bit), it shows averaging in use because I forgot to take a picture without it, but it still had similar issues in the SAMPLE mode.

Well... from my own testing with oscilloscopes I've found that sin x / x reconstruction isn't always implemented correctly. I had to report this as bug to 2 different manufacturers (after which it got fixed). So it doesn't surprise me that there are oscilloscopes out there on which the sin x /x reconstruction doesn't work as goed as it can.

The big "problem" with Nyquist is that it assumes you are sampling the sine wave at the tops...

Try offsetting the sampler by Pi/2 and the entire signal disappears....

Then you increase the sample frequency slightly to make up for it...  now you get aliasing instead, even though theoretically you are doing the right thing being over the Nyquist frequency...

Basically to be able to see fine phase changes in the signal, you need a lot more than the Nyquist sample rate, and/or dithering of the samples, and the rest of it...  or so it seems to me! 

[gf]

I am unable to interpret the Rigol behavior.  I'm using a 6k memory buffer for the FFT but unable to determine whether the spurious responses are internal noise or numerical artifacts.

The datasheet of the HMCAD1511 specifies SFDR as follows:

Spurious Free Dynamic range including interleaving spurs:

Single Ch Mode, Fs = 1000 MsPs: 49dBc
Dual Ch Mode, Fs = 500 MsPs: 44dBc
Quad Ch Mode, Fs = 250 MsPs: 57dB

As long as spurs don't exceed this amount significantly, I'd consider them within the specs.

If the raw data can be saved, they can be split into the samples from each interlaved ADC core (-> 8 cores @1GSPS, 4 cores @500MSPS and 2 cores @250MSPS), gain/offset adjustment for each core can be estimated (relative to the the first core), and then the adjusted samples can be re-combined. I tried this some time ago for the data from my scope, and it did improve spurs a little bit. Still not too much, IIRC.

EDIT: Just recognized that it was the 2CH model. Not sure if it is equipped with a HMCAD1511 too?

[bdunham7]

Well... from my own testing with oscilloscopes I've found that sin x / x reconstruction isn't always implemented correctly. I had to report this as bug to 2 different manufacturers (after which it got fixed). So it doesn't surprise me that there are oscilloscopes out there on which the sin x /x reconstruction doesn't work as goed as it can.

I don't actually know the cause of the errors in this case.  Even if the interpolation implementation is perfect, the result is still more susceptible to amplitude and jitter errors if it has less samples. It would be not possible for me, with my equipment to verify complete accuracy of the reconstruction filter on even a 200MHz scope--group delay, step response and relative amplitude and so on.  However, if I have 10X samples and my scope doesn't lie to me in the 'dots' mode, I can turn off the interpolation filter to verify what I'm seeing.  So, at the low end of the scope market, I'll take more samples over clever interpolation.

[tomato]

1989:   HP54501A, 100MHz, 4 channels, GPIB, FFT  -  list price:  $3,465 all inclusive (which is ~$7,200 in modern play money)

2020:   DSOX3024A, 200MHz, 4 channels - list price: $5,174  (but software features are optional extras, at extra cost)

There are definitely improvements,  but hardly mind blowing considering it is 30 years -  and the price reduction is hardly mind blowing either.

Not much difference? The newer scope gives you 200x improvement in bandwidth, 400x improvement in sampling rate, and a 4,000x improvement in memory:

single shot bandwidth: 1 MHz vs. 200 MHz 
sampling rate: 10 MSa/s vs. 4 GSa/s
memory: 501 pts. vs. 2Mpts

[bdunham7]

The big "problem" with Nyquist is that it assumes you are sampling the sine wave at the tops...

Try offsetting the sampler by Pi/2 and the entire signal disappears....

No, Nyquist doesn't assume that.  What you describe is what happens right AT Nyquist, when you actually need to stay below it.

Then you increase the sample frequency slightly to make up for it...  now you get aliasing instead, even though theoretically you are doing the right thing being over the Nyquist frequency...

Basically to be able to see fine phase changes in the signal, you need a lot more than the Nyquist sample rate, and/or dithering of the samples, and the rest of it...  or so it seems to me! 

Frequencies over the Nyquist limit simply fold back.  Frequencies just under the limit can theoretically be reconstructed perfectly, but this is  difficult.  'Fine changes' would imply a higher bandwidth--and the sample rate required to properly capture and reconstruct them would be over twice the BW of the 'fine changes', which is not in any way the same as the periodic frequency of the signal.

[nctnico]

Well... from my own testing with oscilloscopes I've found that sin x / x reconstruction isn't always implemented correctly. I had to report this as bug to 2 different manufacturers (after which it got fixed). So it doesn't surprise me that there are oscilloscopes out there on which the sin x /x reconstruction doesn't work as goed as it can.

I don't actually know the cause of the errors in this case.  Even if the interpolation implementation is perfect, the result is still more susceptible to amplitude and jitter errors if it has less samples.

But those errors come from the signal so in the end you get what you feed into the oscilloscope. Jitter isn't a problem because the clock will need to have a low enough jitter to satisfy the ADC's requirements.

[SilverSolder]

Hi,

I am going to speculate that more fundamental issue is the ratio between bandwidth and the sampling rate.

The Nyquist Sampling theorem tell us the bandwidth is half the sampling frequency. Then you get folding and aliasing.

There is some modulation caused by the sampling frequency.
If you look at V(sample) waveform, You have to guess at the input waveform from the sampled data.
There isn't enough data to create the waveform with any degree of confidence.

A 100Mhz scope with 500Msps isn't enough data points.

Sorry, but that is the wrong conclusion. Nyquist says that all the information from a signal is there up the fs/2. It is just that your brain can't make a signal from the dots. But that is a problem in your brain and not in the number of samples. In order to help your brain DSOs have sin x/x reconstruction which connects the dots in a mathematically correct way to show you the signal. Because there needs to be some headroom for an anti aliasing filter most DSOs have a maximum bandwidth of fs/2.5 .

Sure, there are ways to reconstruct a sinewave from 2.5 samples per period.
But I believe the signal has to be continuous and have no frequency content beyond fs/2

If you reconstruct a sinewave from a small number of data points, you are making the assumption that the signal was sinewave in the first place.

Regards,
Jay_Diddy_B

You are absolutely correct. If you are looking at 100 MHz squarewave signal on 100 MHz scope you should get 100 MHz sinewave on screen.
Because your scope must not show 300 MHz and 500 MHZ and 700 MHz at any amplitude to be visible on screen. Built in filtering must take care of that that neither of those frequencies and sharp changes in signal ever reach A/D converter.

If you need to look at 100 MHz squarewave, you need scope with 1GHz bandwith, to see at least first 9 harmonics..

Now, let's say we phase shifted the 100MHz square wave by 1 degree.   What sample rate would it take to be able to accurately portray the difference between the original square wave and the one that we just phase shifted by one degree?

[rhb]

LoL!

Here's the DS1202Z-E with a completely absurd 1.25 GHz 0 dBm input from the 8648C.  The Rigol timebase is not very stable, so I took a single shot.  The 8648C has the high stability ovenized clock option.




So can *you* figure out what the design error in the AFE of the Rigol and the Instek is?

Have Fun!
Reg

[SilverSolder]

1989:   HP54501A, 100MHz, 4 channels, GPIB, FFT  -  list price:  $3,465 all inclusive (which is ~$7,200 in modern play money)

2020:   DSOX3024A, 200MHz, 4 channels - list price: $5,174  (but software features are optional extras, at extra cost)

There are definitely improvements,  but hardly mind blowing considering it is 30 years -  and the price reduction is hardly mind blowing either.

Not much difference? The newer scope gives you 200x improvement in bandwidth, 400x improvement in sampling rate, and a 4,000x improvement in memory:

single shot bandwidth: 1 MHz vs. 200 MHz 
sampling rate: 10 MSa/s vs. 4 GSa/s
memory: 501 pts. vs. 2Mpts

You are selectively looking at specific items that have improved the most,  while ignoring the base spec of 4 channel/ 100MHz?

As @2N3055 perceptively posted earlier:  "There are no 40 ps pulses in Raspberry Pi, Arduino, switching and analog PSUs, and pretty much 90% of all electronics. Unless you are working on multigigabit data links, picosecond lasers, or designing MRI machines or such, no worry. "

Could that be the real reason basic scope performance hasn't moved on so much...    most people don't need it?

A bit like the reason a car from 1989 probably had about the same top speed as a 2020 car...

[tomato]

1989:   HP54501A, 100MHz, 4 channels, GPIB, FFT  -  list price:  $3,465 all inclusive (which is ~$7,200 in modern play money)

2020:   DSOX3024A, 200MHz, 4 channels - list price: $5,174  (but software features are optional extras, at extra cost)

There are definitely improvements,  but hardly mind blowing considering it is 30 years -  and the price reduction is hardly mind blowing either.

Not much difference? The newer scope gives you 200x improvement in bandwidth, 400x improvement in sampling rate, and a 4,000x improvement in memory:

single shot bandwidth: 1 MHz vs. 200 MHz 
sampling rate: 10 MSa/s vs. 4 GSa/s
memory: 501 pts. vs. 2Mpts

You are selectively looking at specific items that have improved the most,  while ignoring the base spec of 4 channel/ 100MHz?

Single shot bandwidth, sampling rate, and memory depth are probably the most basic defining specifications of a general purpose oscilloscope. 

The 1989 scope isn't even a true 4 channel scope, it's a "2+2" channel scope. So, you're really comparing a 2 channel, 1 MHz, 10 MSa/s scope to a 4 channel, 200 MHz, 4 GSa/s scope.  Apples vs. oranges watermelons.

[tom66]

1989:   HP54501A, 100MHz, 4 channels, GPIB, FFT  -  list price:  $3,465 all inclusive (which is ~$7,200 in modern play money)

I had a HP 54501A as my first scope.  10MSa/s ADC (so 1MHz single shot B/W), 50 wfm/sec, 500 points memory.  Also like a DOS terminal to use with single rotary encoder.

I can tell you it is a bitch to use... It was my main oscilloscope for about 4 years.  It is hardly comparable to modern scopes.

Edit: others beat me to it...

[rhb]

The relevant questions are: 

"What is the rise time of that GPIO on the Pi?"

 "What do I need to measure it accurately?".

"Do I have problems with the impedance of the trace to the device at the other end?"

Embedded systems probably constitute the vast majority of electronic engineering tasks by a wide margin.  An STM32Fxxx running at 120 MHz clock has some rather fast edges.  They are not picosecond steps, but they are quick.  Accurately probing and displaying those is not trivial.  A Pi or Beagle is even harder.

Have Fun!
Reg

[nctnico]

Now, let's say we phase shifted the 100MHz square wave by 1 degree.   What sample rate would it take to be able to accurately portray the difference between the original square wave and the one that we just phase shifted by one degree?

The samplerate doesn't matter. But you'd need to resolve tens of ps in this scenario.

[rhb]

Now, let's say we phase shifted the 100MHz square wave by 1 degree.   What sample rate would it take to be able to accurately portray the difference between the original square wave and the one that we just phase shifted by one degree?

The samplerate doesn't matter. But you'd need to resolve tens of ps in this scenario.

There are two factors: sample rate and bit depth. 

You have to have enough resolution to resolve the phase difference.  At 200 Msa/s you only have 180 degrees of phase sampling no matter what your bit depth.  And you have no accurate amplitude information.  However, you can resolve a 1 degree phase difference if you have 8 bit data, but not if you have 7 bit data.

At 400 MSa/s you have phase sampling of 90 degrees.  That guarantees that you can accurately reconstruct the amplitude and phase to within the amplitude resolution of the data.  With 8 bit data you have 90/256 degrees of phase resolution.

In between those extremes pick a sample rate and bit depth such that phase_angle/bit_depth = 1.

Have Fun!
Reg

[2N3055]

Hi,

I am going to speculate that more fundamental issue is the ratio between bandwidth and the sampling rate.

The Nyquist Sampling theorem tell us the bandwidth is half the sampling frequency. Then you get folding and aliasing.

There is some modulation caused by the sampling frequency.
If you look at V(sample) waveform, You have to guess at the input waveform from the sampled data.
There isn't enough data to create the waveform with any degree of confidence.

A 100Mhz scope with 500Msps isn't enough data points.

Sorry, but that is the wrong conclusion. Nyquist says that all the information from a signal is there up the fs/2. It is just that your brain can't make a signal from the dots. But that is a problem in your brain and not in the number of samples. In order to help your brain DSOs have sin x/x reconstruction which connects the dots in a mathematically correct way to show you the signal. Because there needs to be some headroom for an anti aliasing filter most DSOs have a maximum bandwidth of fs/2.5 .

Sure, there are ways to reconstruct a sinewave from 2.5 samples per period.
But I believe the signal has to be continuous and have no frequency content beyond fs/2

If you reconstruct a sinewave from a small number of data points, you are making the assumption that the signal was sinewave in the first place.

Regards,
Jay_Diddy_B

You are absolutely correct. If you are looking at 100 MHz squarewave signal on 100 MHz scope you should get 100 MHz sinewave on screen.
Because your scope must not show 300 MHz and 500 MHZ and 700 MHz at any amplitude to be visible on screen. Built in filtering must take care of that that neither of those frequencies and sharp changes in signal ever reach A/D converter.

If you need to look at 100 MHz squarewave, you need scope with 1GHz bandwith, to see at least first 9 harmonics..

Now, let's say we phase shifted the 100MHz square wave by 1 degree.   What sample rate would it take to be able to accurately portray the difference between the original square wave and the one that we just phase shifted by one degree?

You need 1GHz scope to see 9th harmonics of 100MHz squarewave. That would make decent approximation of squarewave, but still bumpy and not perfect. Sampling at 5 GS/s, at 200 ps intervals.

1% of phase shift according to what ? Scope have triggering system to synchronise to signal. Trigger point is synch point.
And that would mean shifting sample point 100ps. What do you expect to see? It won't make a squat of the difference on your 3.5 ns edges on your 100MHz squarewave. Because that is what edge will look like on 100 MHz scope even if you put 40ps edge in it.
On a 1GHZ scope it will have 300-400ps edge.

Every time this kind of topic pops up, I'm sad that in schools they don't plot math functions and solve graphical solutions by hand on millimeter paper and pencil anymore. That was great way to get good feeling for sense of scale.

Get piece of paper (or CAD program that works in scale if you wish) and plot 100MHz squarewave from horizontal points of 200 ps. And then from 400 ps points. That is respectively 5 and 2.5 GS/s.  Than step away from plot and squint at it so it is roughly the size of scope screen form your perspective...

Writing this I think I start to understand where confusion is coming from.  People who say that you need  at least 10-20 points or more per period to reconstruct arbitrary shape signal are correct. But that is because to look at 100MHz sinewave you need 100 MHz scope (with more than 250 MHz sampling). In order to look at some arbitrary shaped signal repeating at 100 MHz, if you were to look that signal on SA, you would see, say, harmonics going to 2 GHz or more. So to look at that signal you need 2 GHz scope, although signal is periodic  with repetition rate of 100 MHz, because that is signal with 2 GHz bandwidth.

Square wave consists of negative harmonics on quite simple formula, 3rd harmonic with 1/3 of amplitude, 5th harmonic with 1/5 of amplitude etc.. To infinity. Of course, very soon you don't need to go further, so in practice on scopes for visual representation, after you pass 11th or 13th harmonic you don't need to go further.. So that is your formula,. Repetition rate of your squarewave multiplied by 10-15. That is your scope bandwidth needed. And that at least 2.5 more will give sampling frequency.

So 100MHz squarewave, 1GHz scope, 2.5 GS/s sampling.
If you could afford 2GHz scope with 10 GS/s that would be better.

[Sighound36]

I will happily plot that 100Mhz SWF for you with my everyday lab scope 2.2Ghz and 10G/s sampling rate tomorrow I will produce the plot up to the 15th harmonic for you chaps

[rhb]

What does it take to measure a 1 degree phase shift of a 100 MHz sine wave?

That is *all* you need if you want to *measure* a 1 degree phase shift of a 100 MHz square wave.

Representing it correctly is an entirely other matter.  And for that a DSO with over 1 GHz of BW and a clean step response are needed.  So it needs to sample at 4 GSa/s or faster.

Have Fun!
Reg

BTW Could we please not have quotes nested 8-10 deep?  Trim off the stuff not relevant to your point.  My measurements have been buried under people arguing.

[gf]

NB: I have yet to see a DSO that did a minimum phase sinc(t) interpolation. So technically every scope I've seen does it wrong.

Just a few thought - but I may be wrong:

Agreed that an anti-alias filter is basically supposed to be minimum phase (particularly if it is an analog one, of if it is supposed to mimic an analog one).

But why do you think that the interpolator (i.e. the reconstruction filter) needs to be minimum phase?
The aim of the reconstruction filter is to reconstruct exactly that (band-limited) signal waveform, which came out from the AA filter and entered the ADC.
Doesn't a zero phase sinc reconstruction filter do exactly that anyway? (w/o introducing additional delays, phase shifts or artifacts)
And would a minimum phase reconstruction filter with the same boxcar magnitude response actually reconstruct exactly the same waveform?
(If not, then it would not lead to an exact reconstruction of the (band-limited) signal which entered the ADC, and the reconstruction filter would miss its goal.)

I do not see why I should blame the interpolator for additional (pre)ringing which was not present in the output of the AA filter, if the actual root cause is the AA filter, which has done a bad job in the first place, not limiting the BW properly.

Please correct me if I missed anything.

[tomato]

BTW Could we please not have quotes nested 8-10 deep?  Trim off the stuff not relevant to your point.  My measurements have been buried under people arguing.

Funny.

[StillTrying]

So can *you* figure out what the design error in the AFE of the Rigol and the Instek is?

Unless they've left a big peak in the response, no.

In this one:  https://www.eevblog.com/forum/testgear/scope-wars/msg3119138/#msg3119138
Peak detect needs at last 2 ADC samples to work, to me it looks like the GW MSO-2204EA might be halving the displayed 500MSa/s rate to give the 2 peak samples in each X sample position.