Scope Wars

[rhb]

The Fourier transform of a Dirac functional is a constant from DC to infinity.

The 100 ps pulse I've been using is only 10% of the 1 ns sample interval, but it still shows up.

Mathematically even a 1 ps spike should show up at slower sampling rates.  However, I find that a 5 ns impulse at 1 second intervals does not consistently show up unless I set peak detection mode.  Why I still do not understand, but clearly it is useful.

I do find that the peak detection mode in the DS1102E works just as well as on the DS1202Z-E and the MSO-2204EA.  I've not yet tried the Owon.

So my question for David Hess, is which model Rigol?

This thread is intended to be an antidote to rumor and insinuation as much as a goad to fix issues.

Have Fun!
Reg

[0culus]

With careful attention to the FFT settings to keep the span within range of the actual bandwidth of the scope, an almost credible looking FFT comes out of the 54622d. 

It is so sensitive to even minor tweaking that it is hard to know when you can trust what you are seeing...

Presumably the magic of HP's dithering methods is what allows greater than Nyquist performance.  Scope sample rate is 200Msamples/sec, so Nyquist is 100MHz, at the center vertical line in the screen - but the FFT clearly performs well beyond that.

Offset -50dB,  20dB/division
(Attachment Link)

I have a TDS5000 series DPO at work that will happily show you "signals" in FFT out to 10s of GHz. It's a 1 GHz scope.  Sorry, no pictures will be forthcoming from that.

[bdunham7]

Watching the live display, the distribution of the dots varies with time.  I suspect it is simply the result of clock jitter rather than a design feature.  However, dot mode and persistence let you get a very good representation of the pulse.

Have Fun!
Reg

This was hashed out here a while ago and I think we concluded that, at least on the DS1054Z, that the 'dots' mode was deceptive in that the dots were not actual sample values, but just dots places where the scope thought they should be.

[SilverSolder]

With careful attention to the FFT settings to keep the span within range of the actual bandwidth of the scope, an almost credible looking FFT comes out of the 54622d. 

It is so sensitive to even minor tweaking that it is hard to know when you can trust what you are seeing...

Presumably the magic of HP's dithering methods is what allows greater than Nyquist performance.  Scope sample rate is 200Msamples/sec, so Nyquist is 100MHz, at the center vertical line in the screen - but the FFT clearly performs well beyond that.

Offset -50dB,  20dB/division
(Attachment Link)

I have a TDS5000 series DPO at work that will happily show you "signals" in FFT out to 10s of GHz. It's a 1 GHz scope.  Sorry, no pictures will be forthcoming from that.

It's as if they only had one piece of code that does FFT, taken from their highest end product, that they use everywhere!

Look at this ridiculous display on a 100MHz scope:

[rf-loop]

Look then this Good Will trigged edge position. Totally out of order.  Is this GoodWill model at all with digital trigger engine..  In 5ns/din display scale position is 2.5ns wrong when sampling interval is 1ns. 2ns/div time scale just "game over"...

If you look more closely you'll see the actual trigger point is off-screen. What you see is the next pulse. I don't know what rhb did in the second image. I can't reproduce it on my unit.

Yep, my error.  When I look images I assume he did even somehow comparative tests when he show Rigol images and then GoodWill.  Also Owon was there. But yes, GW trig position is out of screen and my previous comment is bullshit. His images are not at all comparable - for what reasons they are - I do not want even guess.

[0culus]

With careful attention to the FFT settings to keep the span within range of the actual bandwidth of the scope, an almost credible looking FFT comes out of the 54622d. 

It is so sensitive to even minor tweaking that it is hard to know when you can trust what you are seeing...

Presumably the magic of HP's dithering methods is what allows greater than Nyquist performance.  Scope sample rate is 200Msamples/sec, so Nyquist is 100MHz, at the center vertical line in the screen - but the FFT clearly performs well beyond that.

Offset -50dB,  20dB/division
(Attachment Link)

I have a TDS5000 series DPO at work that will happily show you "signals" in FFT out to 10s of GHz. It's a 1 GHz scope.  Sorry, no pictures will be forthcoming from that.

It's as if they only had one piece of code that does FFT, taken from their highest end product, that they use everywhere!

Look at this ridiculous display on a 100MHz scope:

(Attachment Link)

Yeah, it does seem that the software people just couldn't be arsed to change the code for lesser models, leading to that. My scope is a decent enough scope, the DPO mode is pretty handy sometimes. We have a far better Keysight MSO-X6000 series in the lab though. But that's a $30k+ scope, which is a little outside the price ranges being considered here.

[David Hess]

So "peak detection" substitutes the peak value during the sample period for the value at the time of the sample clock?  If so, that has some very *interesting* effects.   None of which seems to me desirable.

That is not how it is implemented.

During decimation which would normally produce a fractional number of points in the acquisition record by dropping samples, instead the maximum and minimum samples during the duration of two points of the acquisition record are saved which necessarily halves the acquisition record length.  So an envelope of the input signal is captured during a single acquisition, within the limits of the maximum sample rate.

CCD based digitizers also did peak detection but in the analog domain and I am not sure how; it sure was not with analog comparators or diode peak detectors.  I suspect Asgard technology was involved.

If that is a correct interpretation I cannot understand why anyone would want to do it except for lack of comprehension of sampling theory.  Aliasing is a consequence of regular sampling, so there is *no* possibility of "peak detection" preventing aliasing.  This is dead obvious if you simply draw the time domain multiplication and the corresponding frequency domain convolution.

What is ultimately captured is the envelope of the input signal during a single acquisition record.  Under conditions where aliasing would be present because of decimation, a non-graded envelope is captured.  A DPO (digital phosphor) style DSO produces a similar display but captures a histogram instead of an envelope so it may be index graded; DPO operation requires an order of magnitude more acquisition memory.

As far as the practicality of the display, in most cases it looks normal but with all peak-to-peak noise displayed unless you are Tektronix who managed to implement a noise reduction algorithm to produce a display which looks like a sample display but also shows detected peaks.  Luckily this was Tektronix of the past and this feature was configurable.

Below is an example of what peak detection allows.  The display shows 30+ kHz switching noise on an analog control signal for a switching power supply controller during startup so it was a single shot acquisition.  A coaxial connection was required to get a clean signal.  If peak detection was not used, then the sample rate would have been 50 kHz instead of 100 MHz and the shaded area of the trace would be entertaining but questionable.

This image was how I recognized that Tektronix had implemented a noise reduction algorithm for use during peak detection.  I "knew" it in the sense that I had read about it when I read the manual cover to cover but had not made the connection to what it actually did until examining this photograph later and thinking, "That does not look right.  Where is the added noise from peak detection?  The readout says peak detection."  Since then I have never found their noise reduction algorithm lying, but I still never trust it.  Honestly though it has revealed details which would normally be missed.  I might trust it if Tektronix had described how it works.

The 100 ps pulse I've been using is only 10% of the 1 ns sample interval, but it still shows up.

Mathematically even a 1 ps spike should show up at slower sampling rates.  However, I find that a 5 ns impulse at 1 second intervals does not consistently show up unless I set peak detection mode.  Why I still do not understand, but clearly it is useful.

Narrow pulses get spread and attenuated by the limited bandwidth before sampling so you end up with a probably to capture a given pulse width to a specific accuracy.  On older DSOs, Tektronix actually listed these probabilities and accuracies but later DSOs had sample rates high enough to essential capture the results of narrow pulse which made it through the input amplifiers.

So my question for David Hess, is which model Rigol?

This thread is intended to be an antidote to rumor and insinuation as much as a goad to fix issues.

It  has been a couple years but I can check my notes; it was the DS1000D and DS1000E series.  From page 2-56 of the manual:

Peak Detect Aqusition: Peak Detect mode captures the maximum and minimum values of a signal.  Finds highest and lowest record points over many acquisitions.

That is not peak detection as it was commonly understood and it contradicts later Rigol documentation for their DSOs which do support peak detection.  I went to the manual after informally evaluating a DS1000D.

What I find a little ironic is that the ancient Tektronix 2440 series of DSOs are advertised as having peak detection, and they do, but have no such acquisition mode; the designers were a little too clever and implemented peak detection as envelope mode with the number of acquisitions set to the minimum of 1.

[SilverSolder]

[...] the designers were a little too clever and implemented peak detection as envelope mode with the number of acquisitions set to the minimum of 1.

Well, technically...   

[rf-loop]

With careful attention to the FFT settings to keep the span within range of the actual bandwidth of the scope, an almost credible looking FFT comes out of the 54622d. 

It is so sensitive to even minor tweaking that it is hard to know when you can trust what you are seeing...

Presumably the magic of HP's dithering methods is what allows greater than Nyquist performance.  Scope sample rate is 200Msamples/sec, so Nyquist is 100MHz, at the center vertical line in the screen - but the FFT clearly performs well beyond that.

Offset -50dB,  20dB/division
(Attachment Link)

I have a TDS5000 series DPO at work that will happily show you "signals" in FFT out to 10s of GHz. It's a 1 GHz scope.  Sorry, no pictures will be forthcoming from that.

It's as if they only had one piece of code that does FFT, taken from their highest end product, that they use everywhere!

Look at this ridiculous display on a 100MHz scope:

(Attachment Link)

It is not at all any kind of problem because A+ brand manufacturer did it, so B brands can follow and if someone claim it is wrong they can  jump behind A brand and say "look  how They did, so what is wrong in our products"  just like example in history I send some images to manufacturer X how they scope have some (other) things what need repair. Next day I get image from  manufacturer X when they did same test with A brand one model and they show same thing more bad and ask...how I can think this name X product is bad because this A brand show more bad... what to do after then. Only think that keep your X name shits and I keep my money in my pocket... and shut off mouth because A brand is always right.

[SilverSolder]

With careful attention to the FFT settings to keep the span within range of the actual bandwidth of the scope, an almost credible looking FFT comes out of the 54622d. 

It is so sensitive to even minor tweaking that it is hard to know when you can trust what you are seeing...

Presumably the magic of HP's dithering methods is what allows greater than Nyquist performance.  Scope sample rate is 200Msamples/sec, so Nyquist is 100MHz, at the center vertical line in the screen - but the FFT clearly performs well beyond that.

Offset -50dB,  20dB/division
(Attachment Link)

I have a TDS5000 series DPO at work that will happily show you "signals" in FFT out to 10s of GHz. It's a 1 GHz scope.  Sorry, no pictures will be forthcoming from that.

It's as if they only had one piece of code that does FFT, taken from their highest end product, that they use everywhere!

Look at this ridiculous display on a 100MHz scope:

(Attachment Link)

It is not at all any kind of problem because A+ brand manufacturer did it, so B brands can follow and if someone claim it is wrong they can  jump behind A brand and say "look  how They did, so what is wrong in our products"  just like example in history I send some images to manufacturer X how they scope have some (other) things what need repair. Next day I get image from  manufacturer X when they did same test with A brand one model and they show same thing more bad and ask...how I can think this name X product is bad because this A brand show more bad... what to do after then. Only think that keep your X name shits and I keep my money in my pocket... and shut off mouth because A brand is always right.

You are probably right, any improvements have to start in the "good" brands (unless one of the "B" brands sees an opportunity to look better than the competition).  I wonder how often people actually use the FFT functions on their scopes, though -  if it is not a super popular feature, it might always end up as the lowest priority for the r&d team...

[0culus]

I personally would never use a scope FFT if I had a spectrum analyzer available. I see the FFT functionality as more of a quick and dirty way to get frequency domain. When it gets more interesting is something like the Tek MDOs where you can time correlate the spectrum analyzer trace with the time domain. If only they weren't so bloody overpriced. 

[rf-loop]

What I find a little ironic is that the ancient Tektronix 2440 series of DSOs are advertised as having peak detection, and they do, but have no such acquisition mode; the designers were a little too clever and implemented peak detection as envelope mode with the number of acquisitions set to the minimum of 1.

I remember it.  Long time ago I have owned and used it lot.
2440 Peak detect works well.
These years Tek engineers did nice job - mostly. 



 

[David Hess]

[...] the designers were a little too clever and implemented peak detection as envelope mode with the number of acquisitions set to the minimum of 1.

Well, technically...   

Envelope mode over 1 acquisition is a contradiction in terms unless peak detection is also used, which is what they did.  For documentation purposes, it would have been nice to have a separate peak detection setting and that is what they did with later DSOs.

I remember it.  Long time ago I have owned and used it lot.
2440 Peak detect works well.
These years Tek engineers did nice job - mostly.

Considering how they work, they are amazing.

[SilverSolder]

I personally would never use a scope FFT if I had a spectrum analyzer available. I see the FFT functionality as more of a quick and dirty way to get frequency domain. When it gets more interesting is something like the Tek MDOs where you can time correlate the spectrum analyzer trace with the time domain. If only they weren't so bloody overpriced. 

I never appreciated until this thread, that you really have to keep your wits about you when looking at the scope FFT, it is way too easy to get it to display near total nonsense in a plausible looking way!   

[bdunham7]

I wonder how often people actually use the FFT functions on their scopes, though -  if it is not a super popular feature, it might always end up as the lowest priority for the r&d team...

I 'used' (as opposed to played around with) the FFT on my Rigol DS1054Z exactly one time--to verify that a garage door opener was transmitting and determine on what frequency--and it was 315MHz.  It worked.

[Fungus]

This was hashed out here a while ago and I think we concluded that, at least on the DS1054Z, that the 'dots' mode was deceptive in that the dots were not actual sample values, but just dots places where the scope thought they should be.

The dots are not raw sample values? Correct.

That doesn't mean they're random though, they have sin(x)/x applied to them until you turn on more than 2 channels at maximum zoom. At this point it seems to use a special rigol reconstruction filter that works slightly better than sin(x)/x on the outer limits.

Bottom line: Any plans along the lines of "I'll download the sample data and process it myself" don't really work out.

Edit: Well.... they do so long as you're not getting too close to Nyquist.

[2N3055]

Scopes are not acquisition digitizers, despite sometimes being used like that. That is exactly where LeCroy's business is coming from: people who sample signal to analyze it, not just as an interactive scope.
Scope optimized for digitizing will have different priorities to one optimized for interactive work (comparison LeCroy - Keysight 3000/4000/6000 series).
And it is debatable that a scope can be made (without being unnecessarily expensive) that could do both right.

As far as FFT on a scope goes, that is also always a mixed bag. FFT is never trivial. You need to mindful of bin width (resolution bandwidth in SA speak), windowing used (which one has minimum spectral leakage, which one will have best amplitude accuracy at peak, what is correlation between peak size and bin width, what is correlation of numbers of bins, sample rate and bin width, and windowing and amplitudes......... Also aliasing effects will downsample frequencies above Nyquist to lower frequencies and show them there in a wrong place...

But I keep hearing that people say they don't use FFT on scopes because that is SA job. Well, that is true if you're doing RF.  FFT on  scope is really useful at lower frequencies. Also, it is real time, simultaneous bandwidth. My Picoscope will show you 200 MHz (or another one 5 MHz) of bandwidth simultaneously from single capture. And with 1 Mbins ( 2 MPoints) it will show it with very high frequency resolution.

So yes it is very useful, but it needs to be done properly and you need to understand what are you looking at.

And also, more than once, I just needed to see if something is sending something, so scope FFT is great for this.

The more these kinds of discussions are dragging out, the more I think many people don't understand some basic things.

On some level scopes are like multimeters ( and any other instrument really) : you have 3.5 digit meters, and 4.5 digit meters, and 5.5 and 6.5 and 7.5 and 8.5.
3.5 and 4.5 digit meters are used 99.5% of time and 99% of people. Because most of the time we need to check something is there and it is roughly in spec.
But 3.5 and 4.5 digit meters look retarded and broken when compared to, say, 7.5 digit meters.
So we should stop using that "shit", right. Wrong.

Most of the people will rarely have need for a scope that has "better signal fidelity" than Rigol DS1054Z. Most people will never have need for anything better than Rigol DS5000 and Siglent SDS200X+, even in professional environments, if money is thigh and will need to make do. We buy better because we can afford it (we all like our toys) and sometimes it makes for better productivity. Sometimes we buy it because our customers expect us to have nice shiny "ping machines"

[Sighound36]

I actually find the scope FFT useful especially when performing power analysis on incoming a/c it gives a respectable indicator as to weather you need to dig further into the harmonic content with the real time SA.

Also when exploring data signal fidelity having an eye diagram/histogram and an FFT  really does give you a decent chance of chasing down the exact cause of the issues.

Not to be forgotten basic RF probing again will show whether to fire up the SA or not first, though I do use multi domain investigations more these and find it very useful.

Here is my thoughts, you can spend six figures plus on a scope or under £400 as 2N3055 has mentioned and I also advocate WHAT are you looking to achieve with it?

I could happily live with the Rigol MSO 5000 or the Siglant 2000x + and bit more model  both the scopes have many features and have performance that a few years ago on the big boys were charging five figures + for.

For myself I need to ability to perform really low noise dynamic measurements, I have a desirable volt meter and power analyzer, the Rigol 8000 is almost there in certain areas and not in others. The Lecroy Wavepro is just the ticket it does exactly what it is does, no fuss and it delivers accurate repeatable and RELIABLE results at the low noise floor I require. I can justify the cost as business capital purchase, however in the real world do you really need to have such a device 100% of the time?

Careful selection of test equipment FOR YOUR NEEDS will better serve your purpose and importantly budget

[tv84]

Reading you all I come to the conclusion that people throw in more money in their scope buy in the expectation that it covers their lack of knowledge of the fundamentals.  Some of you show such a deep knowledge that you make do with "almost" any model.

[rsjsouza]

2N3055, all very well and eloquently said.

FFT on  scope is really useful at lower frequencies.
(...)
So yes it is very useful, but it needs to be done properly and you need to understand what are you looking at.

That is absolutely true. My DS4014 has terrible FFT, but it is alright when you choose properly its frequency of interest and bandwidth.

3.5 and 4.5 digit meters are used 99.5% of time and 99% of people. Because most of the time we need to check something is there and it is roughly in spec.

Exactly. A 3.5 digit meter, a DS1202Z or a SDS1202X-E are more than enough for heaps of people. Heck, even a cheaper Owon VDS1022 USB-based could be considered the new 20MHz/2ch staple scope to see wiggly lines.

[rhb]

If downsampling is being done in a DSO by throwing away samples it would explain the need to invent a new sampling mode.  It's also brain dead stupid DSP.

As for the "dots" stuff, the server borked my last post and sent 2 photos to never never land.

However, the DS1202Z-E, DS1102E and MSO-2204EA do *not* have a sinc(t) applied to them.  That should be obvious on inspection of the leading edge.

I'll write more when the server is feeling better. There are quite a few other things that have been said that are not accurate

Reg

[rhb]

What I'm seeing in the comments above and my tests is pretty appalling.

The DSP in DSOs is a major fail even from the A list.  It's not even good enough to get a passing grade as a DSP 101 homework exercise.   The first chapter of any DSP text explains aliasing and why you cannot decimate data by throwing away samples.  You *have* to low pass filter the data.  That's not hard to do and doesn't require a lot of resources, so I'm agog that DSOs are decimating data by discarding samples.  The results of my tests with a 5 ns pulse at 1 s intervals showed that "peak detection" is necessary to offset the absolute bodge of downsampling by decimation.

I'd always wondered what "peak detection" was when I got my DS1102E.  BTW it does just as well as the 1202Z-E and MSO-2204EA.  I'll post photos when the server is feeling better. Now I know and I am quite appalled at the reason it exists.

Most users don't understand the FFT at all.  As an example, window functions for the FFT.  Keysight doesn't have a triangle  (aka Bartlett) window option,  essential to get good spectral resolution, on the MSOX-3104T,  a $20K list instrument.  And the FFT is so limited as to be almost completely useless.

The chief problem is that the FFT on most DSOs is *just* an FFT with almost no control over the parameters.  To be useful it must be configured the same way as a spectrum analyzer.  That  *is* implemented in the SA app for the Instek MDO series, although it clearly has errors in the implementation.  I'll sort those out eventually by analyzing a scope record and comparing my spectrum analysis to what they show.  The FFT is just a tool, it is not a solution.  And if you you don't have adequate control over the parameters it's not even useful.  The main limitation of spectrum analysis on a DSO is the dynamic range. The other problems can be fixed by implementing it correctly.

The notion that you cannot process a sampled series for any reason other than an inability to get it out of the DSO is preposterous.  It doesn't matter if it's aliased or what the spectrum is at all.  It is sampled data and the mathematics were fully developed by Norbert Wiener in 1940 and published in 1949 as "The Extrapolation, Interpolation and smoothing of Stationary Time Series".

A skilled person can measure microsecond level multiplexer skew in 2 ms sample rate multichannel data with a maximum signal content of 60 Hz.

Have Fun!
Reg

[SilverSolder]

Why does the spectrum look different when you change the time base?  -  I am not well versed in the math of FFT, but intuitively, it seems to me that whether you are looking at two cycles of a wave form, or 20 cycles, the spectral content should be exactly the same - after all, the wave didn't fundamentally change just because you look at more or less cycles of it.  But it does make a big difference to the scope!  To wit, looking at exactly the same 10MHz signal with two different timebase settings:

Agilent 54622D, timebase:  200ns/div,   Scale: 10dBV/div    Offset: -50.0dBV



Timebase:   50ns/div

[gf]

Why does the spectrum look different when you change the time base?

The basic shape does not look so diffferent, though. The 2nd one was obvioulsy calculated from a lower number of points (-> lower frequency resolution), and was up-sampled then, in order to obtain the same number of frequency bins as in the first one.

You can basically achieve this by zero-padding the available samples to a larger number, and calculating the FFT from the zero-padded samples then.

EDIT: I presume that you did not select different window functions, did you?

[Elasia]

Reading you all I come to the conclusion that people throw in more money in their scope buy in the expectation that it covers their lack of knowledge of the fundamentals.  Some of you show such a deep knowledge that you make do with "almost" any model.

That's always been my take on it, biggest thing i look in a scope for is time alignment of whatever two or more signals i am looking for / debugging, once you have the raw data you can parse it to however many ends as you want, weather thats inside the same scope or another computer etc