Seeedstudio Forum

it seems we have misleading you about the bandwidth of DSO Quad. In production introduction, we used "72Mhz analog bandwidth" and "Two 72Mhz analog channels and two digital channels" ,and in the manual the same mistake was made. actully ,the 72M means sample rate, not the analog bandwidth .Sorry for the mistake we made .
as in viewtopic.php?f=22&t=1938,the Quad bandwidth is affected by many factors ,include parameters of elements, Distributed capacitance of PCB and so on .we are now checking the analog bandwidth and hope to increase it to about 15M. Any comments and suggestions about this issue are grateful here.
thanks ..

My response in viewtopic.php?f=22&t=1938, mistakenly said that the data capture was probably 14-bit, the specs state that it is only 8-bits. So now it appears that the display and the capture buffer have the same low resolution.

Because of this, the user should use V/Div to keep the signal large on the screen. This may reduce the observable steps associated with 8-bit resolution.

Here are my findings while using the engineering model Quad:

Channels A & B can simultaneously display 3.546Mhz square waves (which is 10% of the Max sample rate of 36MS/s) with reasonable shape. This indicates that the 36MS/s is not divided while using channels A & B simultaneously. Hiding channel B has no effect on the displayed sample rate resolution of channel A prior to SYS_1.31.

Signal generator square wave output is firmware unlimited but only reasonable up to 200Khz on the Quad display. The sine, saw & triangle waves are firmware limited to 20Khz or less and reasonable with 1.4us steps observed (due to 8-bit resolution). The output square wave frequency up to 8Mhz appears to be accurate in frequency using the Tektronix scope as reference, but the square wave appearance is long gone on the Quad display.

So does this mean that you have mis-sold me a device by wrongly advertising its bandwidth as 72M where as in fact I will be lucky to achieve even 15M with some upgrade ? Are you going to be offering to swap my unit to its 15m version or am I stuck with a mis-sold product that does not do what it was sold to do or even achieve 30% of its stated capability ?

Pete.

I have to say that I never expected 72MHz bandwidth, as it wouldn't be of much use with 72Msps. That being said, I had assumed that it would be somewhere north of 7.2MHz, say 10MHz? I'll wait and see what it ends up being before getting judgmental.

Neat little package, starting to find my way through the menus, but not at the point where I trust it yet... the first signal I tried it on happened to be 24 MHz, so not a good place to start. But with judicious choices (one input active, scale adjusted favorably) I could see that there was a signal...

I have tested that just yesterday using an HP signal generator, a LeCroy scope an the DSO Quad.
I must confirm that a 5Vpp square wave is altered as low as 1 MHz. At 2MHz is much shaped as a sine than a square.
I don't know whether my own DSO sample was particularly unlucky, but I can't believe you see a 3.5M square wave with "reasonable" shape...Also depends on what you mean for "reasonable".

Glad to hear from Seedstudio that there was some mistaken.
I also didn't expect having a pro-scope, because it would meant that in our labs, by purchasing 4-zeroes-Euro scopes, we would have thrown money.
Being the sample freq to 72M (or 36M), it is also true that the cut-off frequency must no higher than the half. Practically speaking no more than a third or even a quarter.
I have a peek at the schematic, and I think something could be done, but not so far. Probably 15MHz will be a dream using this hardware.
Some observation about:

the OPAs have a very large bandwidth only by keeping the gain close to the unity (look at the GBW). Can't expect having any substantial amplification over 10MHz. That's the way I have tried with a strong signal yesterday (5Vpp). That is a complex task to solve with a small circuitry...
To mutate the analog input circuitry, you are using HCMOS switches. Even having a low parasitic capacitance, it cuts off a lot the final BW. It might be expensive, but the opto-switches could perform better, I guess.

I don't think this scope is a fault, neither I feel to be betrayed. Maybe Seedstudio wanted to build an amazing scope, but I'd expect some additional consideration before initiating the production, mechanical also.
I want also wait for some concrete new about this instrument, then I will see what to do.

A thought: instead of retouching the existent hardware, I'd consider a almost-total bypass of the analog section of the DSO (almost straight to the ADC) and using an external amplifier/adapter. As for me would be a decent solution, if that will meet some concrete goal.
Thanks

I do not take measurements and then fudged the results. I am very precise in what I post on forums. There is no magic or luck here. If it looks like a square wave, walks like a square wave and quacks like a square wave, then it must be a square wave at 3.546Mhz. Reasonable means that is has a small rise-time of less than 10% of the signal's period and a slight sag (top and bottom of square wave slopes a little (which indicates a small DC gain loss in the front-end circuits)). You apparently haven't learned how to maximize the DSO Quad which has a small capture buffer (4-Kbytes per channel) and sample rate limitations. You absolutely must optimized the T/Div for the highest sample rate as described in a previous post. When you have a more costly scope with a larger capture buffer and much faster sample rate (1GS/s) then you can be sloppy with your T/Div selections.

The analog section is not the problem, the problem is the low sample rate. The front end op-amps and the DAC are not bottle-necks at 15Mhz with DAC overclocking for 144Ms/s. That stray trace capacitance would most likely not affect frequencies below 15Mhz. That only leaves the sample rate as the true bottle-neck. Your solution would have no affect on the sample rate whatsoever, therefore it would be ineffectual at best and the added stray capacitance of your additional circuitry would most likely have adverse effect.

That's probably true: I'd love if you explain me how to display a 3.5MHz well-shaped square wave.
Let me know what and how to connect the Quad and I will try.


Absolutely NOT agree at all.
The sampling rate is enough to shape even a 10MHz square wave: just consider that there would be about 7 samples per period.
Anyway, let's take a 3.5M square: that is 20 samples per period. It is a super-sampled signal!...Keep in mind that CD audio waves are sampled at 2.2xBW!!!
The real problem is just the analog section: take a look at the schematic and the OPAs datasheet!
The proof is what I see when connect as low as 1MHz square: an exponential rise and fall, instead of an edge.


Not sure to understand: do you mean a pro-scope is worser than a 150$ Quad???

Ciao!

CD audio captured data is then integrated to return back to a usable analog signal and this has nothing to do with viewing captured data.

The attached drawing demonstrates how 10, 8.5, and 4.25 samples per waveform distorts the results. As the samples per waveform decrease, then the distortion increases. Prior to SYS v1.31 you could only get 36MS/s for two channels, my 3.5xxMhz signal sampled at 36MS/s is more than 10X sample rate and displays properly on the DSO Quad as shown in the attachment below. Do your own sample rate drawing and demonstrate your point, instead of just talking about it.

I am aware of the hardware op-amp specs and they are 100Mhz band-width and 250Mhz upper limit. The front-end circuits appear to have gains of 7 or less for all V/Div settings. 100/7 = 14.285Mhz that the op-amps can pass and that is probably why Seeed Studio expects about 15Mhz with over clocking of 144MS/s. The analog mux switches that do the V/Div scaling have a rise time of 1.67ns which should easily pass 500Mhz, so that is not an analog limitation. The DAC is fed by unity gain op-amps for 100Mhz signal capability (at unity gain) and the DAC itself has a 470Mhz analog bandwidth but a limited sample rate of 100MS/s using the designed clock rate. So that is not an analog limitation. The only analog limitations would be the distributed trace capacitance and the capacitance of the frequency compensation caps in the front-end circuits, and most would agree that this band-width limiting would be minimal if any at all, below 15Mhz.

The only appreciable analog band-width limiting factor at 15Mhz is the sample rate and the resulting display distortion.

It also appears that you may be unaware of the concept of a single sweep capture at the fastest T/Div setting required to get the required sample rate, and then scroll through that capture buffer such that you can view the entire captured waveform. I have DSO Nano video lessons on YouTube that include this topic which is also applicable to the DSO Quad. Just search this Forum or YouTube for DSO Nano videos.

I have no object of insulting you, but your ignorance of this topic is no excuse for you to accuse me of falsely reporting 3.5xxMhz square waves on the DSO Quad that are of reasonable shape. The attached waveforms clearly show that you are wrong. In all this discussion (at least on my part), valid knowledge has been imparted to other Forum Users, and that is why I have continued this discussion with you. I am not sloppy in my testing and I report the scientific and repeatable facts as they are observed. You can have the final word if you need to, because I will no longer respond to your unsubstantiated remarks. I stand on the facts that I have already presented and I remain devoted towards expending my energy in helping others who want to understand the DSO Nano and the DSO Quad.

This post was edited on 4/23/2011 to clarify and to add more sample rate demonstrations in the attachment.

@lygra:
I never would have hurt you!...I really apologize if you feel that!
I was only asking you in which way did you measured/show a good shaped 3.5M square wave, nothing all.
Could you post any DSO snapshot of that wave?
I have explained what in the lab I have tried, which instrumentation and how much was the amplitude of the signals. I guess anyone can test it with a quite common scope with 100-200MHz of BW.
The wave I have seen (under several context) is far from being square.

The picture you have attached is partially correct.
It is true that having a perfect signal in the ADC input (better: the track-and-hold input) the equivalent wave will be distorted.
It is also true that never we could sample a signal having a BW over fs/2, because it generates aliasing. That is a fundamental math rule of discrete signals (Shannon). That mean we cannot feed any signal higher than 36MHz (theoretically speaking), let's say 10-20MHz practically.
Otherwise the equivalent sampled signal would be a mess, without any chance to reconstruct correctly.
This is clear represented by the frequency spectrum that would see the baseband and the lower-modulated band overlapped.

When I write "signal" here, I mean SINE waves, not square or else.
You know that the only "pure" signal is just a sine wave. Any other periodic signal can be thought as composition of harmonics.
In the case, a (perfect) square wave is one of the most "tedious", because it has only odd harmonics, decaying hyperbolically (so really slowly).

For example, let's consider a 1MHz perfect square wave.
The main sine wave is at 1MHz, of course, and it is the greater in amplitude.
It has a 3rd harmonic, whose amplitude is about 1/3 the main. Then there is the 5th, having 1/5 of amplitude, etc.

If the scope would had a 15MHz of BW (as HugeMan is going to do), then I would have attenuations over the 13-15th harmonic...my wave would be a good square anyway.
Since I do NOT see that and I see a oddly shaped wave, I'd suppose the BW is much slower.
How to prove it?
Well, simple...that was a typical high-school task.
Just feed the DSO with a 1MHz (or less) sine: no matter what's the amplitude. On the DSO display I see a good-shaped sine having about 500mVpp.
How is defined the BW? The cut-off frequency is defined as the point where the amplitude falls to 1/sqrt(2)=0.707 the nominal.
That's good...so step up the input frequency to 2MHz: how is the sine on the DSO display?...It is still a good-shaped sine, but it's amplitude is about 75% as before.
That is: the BW of the analog section of the DSO is 2-3MHz.

Hope that was clear enough.

I survived Vietnam so I am sure that you can not hurt me emotionally or physically, I just don't want valid facts to be discredited. Although against better judgment, I will do one or two more responses on this matter if you will limit yourself to provable facts and the 3.5xxMhz display as the point of discussion. I no longer have a DSO Quad because my engineering hardware v2.2 was returned to China for exchange for a v2.6 model and that is the only reason you have not seen a picture here already. When I made those measurements for the benefit of those needing more info (before the betas were available), pictures were not included because the engineering model firmware has trigger, capture, and measurement issues that need to be addressed, and those fixes would most likely change the screen displays. I was just providing the bare facts at that time.

Which part of my previous attachment is incorrect? Be specific and don't generalize because that serves no purpose. All square waves have some rise-time (both the generator and the display) due to the lack of infinite odd harmonics, and that is not shown for simplicity.

These parameters are not relative to a 3.5xxMhz square wave being viewed on the DSO Quad with SYS v1.30 or earlier.

These parameters are not relative to a 3.5xxMhz square wave being viewed on the DSO Quad with SYS v1.30 or earlier.

When you replace your word "slower" with the word "narrower", then we are approaching the crux of this misunderstanding. What is your T/Div setting when you conducted this measurement? If you don't remember, then repeat your measurements and tell me what you used for T/Div for the same results.

Unfortunately you are now talking about analog display roll-off and not band-pass roll-off. This is probably where your misunderstanding takes place. You have also switched off topic to a sine wave at a different frequency. BenF has already cited the artifact of display amplitude roll-off due to sampling rate choices. This display roll-off may be negatively influencing your viewed results. I did not compare the amplitude of the square wave at different frequencies such as 1.88Mhz like you are doing so we are now mixing apples and oranges again. Refer to page-5, my second post and BenF's following response at viewtopic.php?f=12&t=1793&start=40 for more details about display roll-off.

It wont be clear enough until you place a 3.5xxMhz square wave signal into the DSO Quad and use the faster T/Div settings to view the signal with both analog channels active, and then tell me what you see. Then we will both be on the same page. Sorry about being short and direct with you, but we have to discuss the same issues in order to communicate successfully.