Show us your square wave

[tautech]

I used My DS2000 to show a Perfect 62.5MHZ square wave Sampled at 2 GSa/s

Source?

[tautech]

Source?

NO source!,
I was leading you on,
I saved a waveform data ,
I then Patched the Data. to values to show a step change.
I then loaded it back into the DSO. 

I thought everyone could see it was too prefect.

I figured as much  and know now to watch out for any waveforms you might post. 

[joeqsmith]

Square wave is series of sinewaves

Indeed.......with a fundamental, and upper and lower order harmonics....here are two plots with a higher interpolated sample rate.....one showing the dynamic fundamentals of the center frequencies and the other showing the total roll-off of those fundamentals....

I am a bit puzzled by the topic of "show us your square wave".....too many variables between the oscillator/gen and the scope reading.....I don't think it says a hell of a lot about the quality of a sig gen, without analyzing the entire signal path

I did the plots posted using coax (properly terminated), but the results will be completely different with a more stable probing solution....and different i/o & measurement settings......to have any kind of real meaning in reality, there has to be a standard/definition of measurement parameters....

maybe I am missing the point of the question?

A saw your post some time ago.  You brought up some good points.   You stated the DSO you used for this data was a WR64MXI with 600MHz BW with a 15MHz input using coax.    I did not understand the THz FFT but thought I would try and replicate your results.

I am using my Sony AWG2041 to generate a 15MHz squareware with roughly the same amplitude as you show.  This generator has a little sharper edges.  I am using Thermax RGU-400 with SMAs torqued to 8"/lbs.   ARB uses crappy BNCs, so I have a BNC to SMA for this test.   My DSO is an older Wavemaster 8500A, 5GHz.     

As I would expect, the FFT drops like a rock well before I reach 10G.    Amazing how different the two setups behave. 





 

     

[joeqsmith]

Same setup but with an crappy RF generator attached with 3.8GHz signal.   



20Gs/s with 50ohm MWM terminator attached.  -92dBm. 

[Electro Fan]

How should these colors be interpreted - what do they signify or represent?  Thanks

[Jay_Diddy_B]

Hi,

I had a look at this picture:



I can explain it. The square wave is 15 MHz, so the spectral lines in the FFT should be 15MHz apart. From the shape of the waveform I would expect to see the higher order harmonics attenuated.

The FFT shows spectral lines that are 5 GHz apart. This is sampling frequency of the scope 5G samples per second. The Lecroy scope is using RIS acquisition.

Regards,

Jay_Diddy_B

[Wuerstchenhund]

How should these colors be interpreted - what do they signify or represent?  Thanks

The picture is a 3D color persistence screen, which is essentially a more advanced variant of the common 2D persistence function many scopes have today.

Most scopes have a monochrome persistence function (i.e. intensity grading) where individual waveforms overlap each other on a single "depth" layer (the screen plane) and where the brightness represents the amount of times a waveform has passed through a particular spot on the screen. This can be used (in limits) to see jitter or other signal anomalies. More advanced scopes use color grading, i.e. the color shows how often a waveform has passed through a particular spot, where red ('hot') shows places of high occurrence, yellow for medium occurrence and blue for low occurrence.

In a 3D persistence screen each waveform is 'stacked' on top of the other, which shows much better how the waveforms change over time. Again, the colors indicate the occurrence of a waveform passing through a certain spot in the horizontal (x/y) plane.

It's a great tool but for this specific exercise I don't think it adds any value, and since the scope was running in RIS (ETS) mode I'm not sure the waveform that is displayed has much to do with the input signal. The picture shows noticeable variations in the waveform, which means RIS is unsuitable here.

[T3sl4co1l]

Or to put it another way, it's a histogram of the waveform hitting those x,y (time/voltage, frequency/amplitude) points.  If you set up an external clock trigger and play random data, you automatically get your eye diagram that way.

Tim

[Kibi]

I have the results from two generators. One is a GW Instek SFG-2110 (10MHz bandwidth). The second will the from the built in wavegen function of my Agilent DSO-X 2002A (10MHz bandwidth).

First up: GW Instek SFG-2110

1MHz


5MHz, jitter is starting to creep in.


5MHz, with averaging switched on.


10MHz, at full chat now and jitter is quite bad.


10MHz, with averaging.




Next up is the DSO-X 2002A built in wavegen option. This seems to have better jitter performance, but not as good with the high frequency harmonics.

1MHz


1MHz with averaging


5MHz


5MHz with averaging


10MHz, full chat. I checked and it is still supposed to be outputting a square wave.


10MHz with averaging

[TunerSandwich]

It's a great tool but for this specific exercise I don't think it adds any value, and since the scope was running in RIS (ETS) mode I'm not sure the waveform that is displayed has much to do with the input signal. The picture shows noticeable variations in the waveform, which means RIS is unsuitable here.

It shows just how much the measurement device itself can influence the result....RIS + sinX interpolation + 3d persistence.....really shows just how many artifacts can be attributed to the WAY in which a device interpolates what it's being fed.....I wanted to show just how far away we can get from something "trustworthy", especially when sampling and interpolating a signal....I wish the scope had an AES/EBU input and I could lock it to an external sample clock (via word clock sync).....I would love to see how a pure digital interpolation of a synthesized signal is skewed by the scopes DSP/processing engine.....

I have a cool V.A.S.T synthesis engine here, that can build block chains of various types of modulations and sources (thanks Ray Kurzweil).  Sadly I think the d/a a/d and "analog" cable run would skew the results beyond the ability to make any solid predictions about HOW the scopes own interpolation transforms the original source.....(the synth has a digital out, but the scope doesn't have the corresponding input....I don't have the proper accessory)

Producing a model of a square wave, should be a very straight forward endeavor.....but everything in between that model and the way it's interpolated to the screen of a discreet device makes the whole concept quite a bit more "complicated".....in the "analog world" we live in....square waves just don't seem to want to exist.....and nature seems to like to avoid straight lines....

The real world is quite a complex place.....the virtual world is so much more simple.....shows just how far off we are of the vastness and complexity of the natural world....sometimes I step back and think we are still "living in caves"

somehow through all the muck we still manage to send 0 and 1 through global interconnection systems....most of which use far "shoddier" cabling solutions than we are displaying here (with lots of band-aids on them)....

kind of trying to re-enforce my earlier point, that showing square waves, even between "premium" gear is a rather pointless.....as it's very difficult to attribute exactly where the non linearities are coming from....are they induced (EM), are they processing "errors" etc etc etc....I could see one spending vast resources to produce a "perfect" square wave....only to realize it has no real practical application

 


it really is turtles all the way down

[TunerSandwich]

Square wave is series of sinewaves

Indeed.......with a fundamental, and upper and lower order harmonics....here are two plots with a higher interpolated sample rate.....one showing the dynamic fundamentals of the center frequencies and the other showing the total roll-off of those fundamentals....

I am a bit puzzled by the topic of "show us your square wave".....too many variables between the oscillator/gen and the scope reading.....I don't think it says a hell of a lot about the quality of a sig gen, without analyzing the entire signal path

I did the plots posted using coax (properly terminated), but the results will be completely different with a more stable probing solution....and different i/o & measurement settings......to have any kind of real meaning in reality, there has to be a standard/definition of measurement parameters....

maybe I am missing the point of the question?

A saw your post some time ago.  You brought up some good points.   You stated the DSO you used for this data was a WR64MXI with 600MHz BW with a 15MHz input using coax.    I did not understand the THz FFT but thought I would try and replicate your results.

I am using my Sony AWG2041 to generate a 15MHz squareware with roughly the same amplitude as you show.  This generator has a little sharper edges.  I am using Thermax RGU-400 with SMAs torqued to 8"/lbs.   ARB uses crappy BNCs, so I have a BNC to SMA for this test.   My DSO is an older Wavemaster 8500A, 5GHz.     

As I would expect, the FFT drops like a rock well before I reach 10G.    Amazing how different the two setups behave. 





 

   

your cabling solution is vastly superior to the one I used....(I used the coax that came with the rigol 2102a/s)

In the original post the statement was put forth....use the maximum "horsepower" of your scope to display a square wave.....just trying to show how misleading that can really be.....

It is amazing how different the results are, between the two devices.....no doubt about that....I would argue that you have the superior scope for this kind of test.....?

[Pjotr]

kind of trying to re-enforce my earlier point, that showing square waves, even between "premium" gear is a rather pointless.....as it's very difficult to attribute exactly where the non linearities are coming from....are they induced (EM), are they processing "errors" etc etc etc....I could see one spending vast resources to produce a "perfect" square wave....only to realize it has no real practical application

 


it really is turtles all the way down

Well, there is at least one turtle that benefits from a near perfect square wave source: Adjusting your high impedance HF probes as good as it gets

[TunerSandwich]

I personally thought this was the most interesting signal i had posted....the square wave is still in there, amongst the garbage of the antenna

[TunerSandwich]

also this.....look at the timebase    where exactly is that "squarewave" coming from.....it sure as hell ain't the sigGen....I got a good laugh out of it anyhow

[TunerSandwich]

kind of trying to re-enforce my earlier point, that showing square waves, even between "premium" gear is a rather pointless.....as it's very difficult to attribute exactly where the non linearities are coming from....are they induced (EM), are they processing "errors" etc etc etc....I could see one spending vast resources to produce a "perfect" square wave....only to realize it has no real practical application

 


it really is turtles all the way down

Well, there is at least one turtle that benefits from a near perfect square wave source: Adjusting your high impedance HF probes as good as it gets

yes, no doubt about that....which I suppose has some VERY useful application.....when looking at impulse responses and the potential phase issues....touche'

[EV]

I see Rigol DSO displays are being posted on this thread and comments about overshoot.

Here are pictures you asked:

Generator: Rigol DG4162, sync out, 40 MHz square wave, connected straight to scope with about 1 m RG58 cable.

Scope: Tektronix R7103
1. pic: Timebase 7B15, Vertical amp 7A29, BW 1 GHz
2. pic: Timebase 7T11, Vertical amp 7S11 with sampling head S-2, BW ?
3. pic: Timebase 7T11, Vertical amp 7S11 with sampling head S-4, BW ?
           This picture does not look good.
4. pic: Timebase 7T11, Vertical amp 7S11 with sampling head S-4, BW ?
          RG58 cable is connected with 20 dB attenuator to the scope

Edit:  Pic 4 added. Pic 3 removed.

[Wuerstchenhund]

I did a quick check of the Rigol DG1062z's square wave on a spectrum analyzer. There wasn't much time so I only tested 1MHz (where the square wave looks pretty ok) and 25MHz (where it's essentially shit). Cable is a new Huber & Suhner Sucoflex 100 which goes up to 18GHz. The BNC to N adapter is also Huber & Suhner, again rated to 18GHz.

The spectrum analyzer is an Agilent E7495B (actually it's a Base Station Test Set but pretty universal, I'll probably do a review at some point). The screenshots are a bit crap (light green on white background isn't nice) because I didn't check the capture settings where I guess the background can be changed to black as on the screen.

The two screenshots show nicely the various odd multiples of f₀ which make the 1MHz square wave while the 25MHz "square" wave only shows two peaks.

[Wuerstchenhund]

It's a great tool but for this specific exercise I don't think it adds any value, and since the scope was running in RIS (ETS) mode I'm not sure the waveform that is displayed has much to do with the input signal. The picture shows noticeable variations in the waveform, which means RIS is unsuitable here.

It shows just how much the measurement device itself can influence the result....RIS + sinX interpolation + 3d persistence.....really shows just how many artifacts can be attributed to the WAY in which a device interpolates what it's being fed.....I wanted to show just how far away we can get from something "trustworthy", especially when sampling and interpolating a signal..

I know, but I thought I should highlight the RIS issue anyways to avoid that someone takes the screenshot too literally.

Producing a model of a square wave, should be a very straight forward endeavor.....but everything in between that model and the way it's interpolated to the screen of a discreet device makes the whole concept quite a bit more "complicated".....in the "analog world" we live in....square waves just don't seem to want to exist.....and nature seems to like to avoid straight lines....

A perfect square wave can't exist, it's physically impossible as it assumes a indefinitely fast transition between low and high (which in this analog world is not possble). All we can get are more or less close approximations.

[TunerSandwich]

YIKES!!!

That Rigol "25MHz square wave" is pretty useless.....I wonder just how bad the ds2000A-S gen is @ 15MHz.  My plots showed some serious issues, but I assumed they were more related to cabling and the fact that the "specAN" in the MXi isn't a "real" discreet spectral analysis tool.....you don't happen to have a ds2000A-S in house do you? 

My guess is running the built-in gen on the a-s at it's "limit" is producing useless data (in terms of analyzing the cabling and acquisition tool.

You have far nicer spectrum analyzer package than anything i have kicking around....

[Pjotr]

YIKES!!!

That Rigol "25MHz square wave" is pretty useless.....I wonder just how bad the ds2000A-S gen is @ 15MHz.  My plots showed some serious issues, but I assumed they were more related to cabling and the fact that the "specAN" in the MXi isn't a "real" discreet spectral analysis tool.....you don't happen to have a ds2000A-S in house do you? 

There are many more issues here, like PCB layout and PSU pulse response/decoupling at the output drivers. Capacitors and inductors are as always, everywhere and ring as they like.

[Wuerstchenhund]

YIKES!!!

That Rigol "25MHz square wave" is pretty useless.....I wonder just how bad the ds2000A-S gen is @ 15MHz.  My plots showed some serious issues, but I assumed they were more related to cabling and the fact that the "specAN" in the MXi isn't a "real" discreet spectral analysis tool.....

Don't underestimate your scope  The WRXi-SPECTRUM option should turn your scope into a "real" spectrum analyzer, although a vector one and not an old-style swept one (which isn't necessarily a disadvantage), and of course with only 600MHz bandwidth.

you don't happen to have a ds2000A-S in house do you?

No, and what I've seen from Rigol so far nowadays I probably wouldn't touch another of their products with a bargepole. Admittedly some Rigol devices aren't as bad as say, Siglent, but after the DG1062z I've decided to stay with the big names.

You have far nicer spectrum analyzer package than anything i have kicking around....

You haven't seen my R&S CMU200 yet 

But seriously, the E7495A/Bs are great devices, where else can you get a portable Spectrum Analyzer (500kHz to 2.7GHz) that also comes with an RF generator (IIRC 200MHz to 2.7GHz), a power meter and a cable tester (1-port and 2-port measurements), all in a rugged and weatherproof case, for less than $1500?

It run Linux so there might even be a chance to extend the functionality by "hacking".

[TunerSandwich]

I know, but I thought I should highlight the RIS issue anyways to avoid that someone takes the screenshot too literally.

A perfect square wave can't exist, it's physically impossible as it assumes a indefinitely fast transition between low and high (which in this analog world is not possble). All we can get are more or less close approximations.

Yes, I had brought that up when one gentleman said the "1THz bandwidth" of the FFT function was impressive.....considering it can only be achieved with RIS and sin x/x I think it's a somewhat "useless" measure.

My main point in this whole discussion, is really regarding the original post.  In which it was sort of implied "use all of the horsepower you've got".  At least in my situation all that "horsepower" doesn't lead to anything of real value.  It's all interpolated and based on signal processing, and has very little bearing on the actual analog front end characteristics of the 64MXi.

I think that is a very important point to drive home....that the more bells and whistles and interpolations of the actual signal, the farther we get away from anything of merit or value.  In some ways the more impressive results are those showing as little interpolation and augmentation of the "true signal" as possible.  Which obviously can get VERY resource intensive ($$$$).  Cabling, true quality of source, post ADC "artifacts" etc....at some point I wonder just how useful a DSO is in regards to truly "characterizing" the "quality" of the sigGen. 

Seems like they are great for looking at the impulse responses (phase issues/ringing).....but for truly analyzing the "rise time" of the sigGen....hmmmm, seems like it's not so cut and dry....

[TunerSandwich]

YIKES!!!

That Rigol "25MHz square wave" is pretty useless.....I wonder just how bad the ds2000A-S gen is @ 15MHz.  My plots showed some serious issues, but I assumed they were more related to cabling and the fact that the "specAN" in the MXi isn't a "real" discreet spectral analysis tool.....you don't happen to have a ds2000A-S in house do you? 

There are many more issues here, like PCB layout and PSU pulse response/decoupling at the output drivers. Capacitors and inductors are as always, everywhere and ring as they like.

absolutely.....I have to assume that is a given....I can't imagine any scope based FFT/specAn is going to have the clarity/fidelity of a dedicated device....which focuses it's investment resources on doing one thing "as good as it can"

[tautech]

My guess is running the built-in gen on the a-s at it's "limit" is producing useless data (in terms of analyzing the cabling and acquisition tool.

Yep. This is normally the case, dedicated FG's are superior.

Working on my main PC ATM, I'll post some waveforms later of what the inbuilt AWG can do in the SDS2304.
And as the have been a temp graded ones I'll do some of those too.

[TunerSandwich]

No, and what I've seen from Rigol so far nowadays I probably wouldn't touch another of their products with a bargepole. Admittedly some Rigol devices aren't as bad as say, Siglent, but after the DG1062z I've decided to stay with the big names.

As a "general purpose" "hobby" type scope the ds2000A is not too bad.  I actually really like it for some very specific things, but the math operations (especially FFT) is/are utterly useless.....I don't see any purpose in their "higher end" products though.....they just don't stack up, and you can get used examples of "big name" kit for roughly the same prices or less.....I imagine they don't sell many $5000 scopes or analyzers, and the company floats on the "hobby grade" stuff....