DSO Quad bandwidth

--DSO203

Re: DSO Quad bandwidth

by HugeMan » Mon May 09, 2011 5:51 pm

vernarim wrote:HugeMan, I don't know whether your Quad has the same hardware as the mine, but *of sure* the mine has a -3dB cutoff around 2-3MHz.
Please, specify whether the probe could be so relevant in the test, because I have considered it always in my tests.

I don't understand what you stated also: at the very beginning of your post you are saying that the analog bandwidth is 10MHz, then at the end you say is 5MHz. What is the correct value?
I'd love to detect a 10-15MHz of *real* bandwidth. It means inputting a 10MHz @1Vpp sine on the probe and reading around 700mVpp on the display. Being step-shaped, no matter if I read 700mVpp or 750mVpp. On my test I read a *supposed* 400mVpp!
If you have a Quad granting 10MHz of overall bandwidth, I'll swap with the mine as soon. I will pay the shipping charge also!

About the sampling rate, I have also detected 72Ms/s and it could be fully acceptable from my viewpoint.
Thanks.

hi, i promise my hardware is the same as the one in your hand.
1. i say the "10M"bandwidth i means the input(connector) to the ADC input(U5 and U16 pin13), i say this just want to clarify confusion on the problem(is the input channel or the sample rate the limitation of the system bandwidth ).so ,we can say: the input analog channel bandwidth is 10M.
2. i say the "5M"bandwidth i means the system bandwidth. i input a signal of 5M (sinewave),and found the wave on the Quad screen is less than -3db. and if a signal of 10M(sine) , the wave is more the -6db.that is : a 10M signal can pass the input channle(gain less than -3db),while it came into the ADC ,because of the sample rate,the whole gain became more the -6db, so,we can not say the bandwidth is 10M.instead,just about 5m.
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Re: DSO Quad bandwidth

by Slimfish » Mon May 09, 2011 7:00 pm

Hi Lygra,

lygra wrote:I finally found time to research this topic some more. There appears to be a mistake in your calculations here. The first and subsequent odd harmonics for a 1Mhz square wave would be 3,5,7,9,11,13,15,17,19,21,23,25 and 27Mhz to pass the 13th odd order harmonic.

sorry, but you're mistaken here. The n-th armonic of a wave is defined as: f_n-th = f * n-th, where f is the fundamental frequency of the wave and n-th is the harmonic. So for a 1kHz square wave, its 5th armonic will be 5 kHz. And zero harmonic is the DC component. It's true that perfectly square waves doesn't have even harmonics (and you counted to the n-th position of a odd harmonic), but that's another tale.

lygra wrote:Another assertion claimed by you and others is that the Nyquist theory only requires 2x of the signal for a sampling rate.

I would say not only, but at least. Difference is significant.
lygra wrote:You failed to include the rest of that definition which says "a signal must be sampled at least twice as fast as it's highest frequency component." With the square wave this would require many odd order harmonics, so in your example above, to include the 13th odd harmonic, then you would require a sample rate of 54MSa/s to reproduce that 1Mhz square wave waveform accurately.

A perfectly square wave has infinite odd harmonics, and hence can't be perfectly sampled (also produced). With the corrected calculations you will require 30 Msps (15 Mhz x 2). With the arguments you gave in the previous messages... why not sampling 13th harmonic with 20x? So now its enough with 2x?. You are mixing theory and practise, and that's quite difficult without a solid ground. That was the point to stress.

lygra wrote:So even if the analog bandwidth could pass the 27Mhz harmonic (and it apparently can't), then the sample rate would still be destroyed according to Nyquist while using a 36MSa/s sample rate.

Although wikipedia says exactly what you posted before, in reality the signal has to be sampled with at least twice its bandwidth to be perfectly reconstructed.

lygra wrote:It also might be pointed out here in the bandwidth discussion, that if the DSO Quad design had a hardware trigger circuit (and it does not) then for repeating waveforms, the firmware could be designed for equivalent time sampling with a magnitude order increase in the maximum observable waveform frequency.

I don't know how you know this since FPGA is closed source ;-). But i can bet my mother (only speaking, don't take me too serious :-)) that trigger is hardware (obviously is not an analog one, but digital). So QUAD can use equivalent time sampling for sure.

I'd like this thread not to diverge into sampling theory, because its large and complex. So for me, the theory discussion is over.
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Re: DSO Quad bandwidth

by vernarim » Mon May 09, 2011 8:09 pm

@HugeMan: I will check the analog bandwidth by open my Quad, as soon I have spare time to do it.

@lygra: I appreciate your effort to demonstrate (better: trying to do it) with tons of theories, but I guess that you are mixing many concepts together.
A square wave is yes composed by odd harmonics (I agree with slimfish about the numbering), but the shapes you drawn could never be what the analog section outputs. You missed a *VERY* important parameter: the phase!
Remembers that here, for simplicity, we are talking about the modulus of a signal (i.e. a square), but we *MUST* take in account the phase-shift caused by the analog section. We "should" take in account the phase-shift introduced by the sampling also, but...never mind.

Just take a simple square-wave generator and a R-C lowpass filter. Assuming the time-constant similar to the period, what do you expect to see on a scope?
The input is a square-wave, composed of an infinite number of odd-harmonics. The output cuts them at -6db/oct.
That would be having an output shaped as you drawn?...Yes, if only the filter won't shift the phase, smoothly from 0 to -90 degrees.
Of course, by scoping the output signal you will see an exponential fragment, repeated on each period. Exactly the same as having a switch charging then discharging the capacitor, via the resistor.
All that only for a simple R-C filter, being just-one-pole complex function. Imagine what would be having a 10+ poles network!

Cheers
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Re: DSO Quad bandwidth

by lygra » Wed May 11, 2011 12:04 am

Slimfish wrote:sorry, but you're mistaken here. The n-th armonic of a wave is defined as: f_n-th = f * n-th, where f is the fundamental frequency of the wave and n-th is the harmonic. So for a 1kHz square wave, its 5th armonic will be 5 kHz. And zero harmonic is the DC component.
I never talked about the fifth harmonic, I only referred to the 5th odd harmonic, not the same. If the first harmonic is even (2Mhz) then the pattern after the fundamental would be 2nd=even, 3rd=odd,4th=even,5th=odd; so the fifth harmonic would be odd and is 5Mhz, but I was referring to the second odd harmonic (which is the actually the 4th harmonic if you count the none existent even harmonics too) when I said 5x, so once again you are mistaken. As you have already said, the even harmonics don't exist, so they can not be counted. :)

Slimfish wrote:A perfectly square wave has infinite odd harmonics, and hence can't be perfectly sampled (also produced). With the corrected calculations you will require 30 Msps (15 Mhz x 2). With the arguments you gave in the previous messages... why not sampling 13th harmonic with 20x? So now its enough with 2x?. You are mixing theory and practise, and that's quite difficult without a solid ground. That was the point to stress.
See previous response.

Slimfish wrote:Although wikipedia says exactly what you posted before, in reality the signal has to be sampled with at least twice its bandwidth to be perfectly reconstructed.
My reference was not Wikipedia, my reference was a professional application note produced by Tektronix engineers. If you would bother to read this reference then you may become more informed. As you have already said, the odd harmonics are part of the square wave, and hence the sample rate must be twice the highest odd harmonic expected, unless Nyquist is wrong and you are right.

Slimfish wrote:But i can bet my mother (only speaking, don't take me too serious :-)) that trigger is hardware (obviously is not an analog one, but digital). So QUAD can use equivalent time sampling for sure.
This is a fool's bet for someone who doesn't know the FPGA source code. But that FPGA source code may not be necessary if you look at the STM source code and find it's triggers source.

Slimfish wrote:I'd like this thread not to diverge into sampling theory, because its large and complex.
I believe that it was you that started with the equivalent time sampling theory, not myself. Sampling theory is not that complex and is explained quite well in the Tektronix reference that I provided in my earlier post. If you look at the Nano post viewtopic.php?f=12&t=1793&start=80 on page 9, BenF fully explains in his May 8th post just exactly how the Nano firmware trigger detection works without a hardware trigger. That discussion is also not very complex. You can also read my Tektronix reference, that also explain exactly how equivalent time sampling works. So I don't see anything here that is overly large and complex.

The only thing I find complex is trying to determine a hardware trigger within the FPGA, without first looking at the STM source code (which is available) to see if a software trigger is present. An even simpler approach would be to point out the Quad menu choice which provides enabling of this fictional equivalent sampling mode. If the Quad were in continuous equivalent sampling mode, then you could not look at non-repeating signals, but guess what; you can look at non-repeating signals with the Quad. Therefore if you assertion is true, then there must be a Quad menu choice to switch back and forth between equivalent sampling mode and real-time sampling mode. Please be so kind as to point out this menu choice, or stop wasting everyone's time with your non-validated theories.

Slimfish wrote:So for me, the theory discussion is over.
What a relief that is. :D
Look, my goal here is not to constantly criticize you, my only goal is to provide factual enlightenment for those looking for same. If you agree to stop spewing theory, then I will agree not to be critical in return. Do some research on your own as I have done, and present those research findings that support your claims. ;)
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Re: DSO Quad bandwidth

by lygra » Wed May 11, 2011 12:19 am

vernarim wrote:@HugeMan: I will check the analog bandwidth by open my Quad, as soon I have spare time to do it.
@lygra: I appreciate your effort to demonstrate (better: trying to do it) with tons of theories, but I guess that you are mixing many concepts together. A square wave is yes composed by odd harmonics (I agree with slimfish about the numbering), but the shapes you drawn could never be what the analog section outputs. You missed a *VERY* important parameter: the phase! Cheers
Have you measured this phase shift when a 1Mhz square wave passes through the Quad front-end circuits? If so, how much phase shift did you measure for the fundamental, 1st odd harmonic and the 2nd odd harmonic?

When I present a theory, I always use a "maybe" or "might be" during that theory. The rest is not theory but common knowledge of the trade. If you haven't measured the above phase shift, then it is yourself that is now presenting not tons but ounces of theory.

Thanks
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Re: DSO Quad bandwidth

by Bainesbunch » Wed May 11, 2011 3:21 am

My input into all of this is really quite simple.

It requires no degree in mathematics or foundation courses in laplacian and fourier calculations and transforms for calculating the nth harmonic of a fundamental :? , buffer sizes, sampling rates, time divs, ADCs, unity gain op-amps, low impedance signal path, parasitics and finally disembowelling :shock: my quad to fiddle with its innards.

My question is simple.

Have we been mis sold a device that, from what I can see and extrapolate from all the theory, is only good for about 5Meg bandwidth when is said quite clearly 27Meg when it was sold to me.

This is not half, not a quarter, not even one fifth of what I paid good money for.

My question is how are Seeedstudio going to address this issue of mis selling and then saying nothing to their customers apart from ....

HugeMan wrote:it seems we have misleading you about the bandwidth of DSO Quad. ......... Any comments and suggestions about this issue are grateful here.


Well this is my comment and I leave it to you to decide on the action ;)

Ohhh and BTW before the flaming starts .... this is not a serous post I just wanted to lighten the mood a little :P

To be honest firmware issue aside i quite like it and once the niggles are ironed out and my confidence in its abilities is a little higher it will have pride of place on my hobby bench.


Cheers Pete.
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Re: DSO Quad bandwidth

by Slimfish » Wed May 11, 2011 5:03 pm

lygra wrote:I never talked about the fifth harmonic, I only referred to the 5th odd harmonic, not the same. If the first harmonic is even (2Mhz) then the pattern after the fundamental would be 2nd=even, 3rd=odd,4th=even,5th=odd; so the fifth harmonic would be odd and is 5Mhz, but I was referring to the second odd harmonic (which is the actually the 4th harmonic if you count the none existent even harmonics too) when I said 5x, so once again you are mistaken. As you have already said, the even harmonics don't exist, so they can not be counted. :)

If you want to count only odd armonics to fade your mistake is up to you. But fifth armonic of a 1 kHz wave is 5 kHz. And venarim was refering to that.

lygra wrote:My reference was not Wikipedia, my reference was a professional application note produced by Tektronix engineers. If you would bother to read this reference then you may become more informed. As you have already said, the odd harmonics are part of the square wave, and hence the sample rate must be twice the highest odd harmonic expected, unless Nyquist is wrong and you are right.

Your reference is a manual (a.k.a. tutorial - simplify things) which by the way i've read. Also i've read more than "a couple" of books that support what i'm saying. Even i built a radio (which works by the way :-) which samples @ 1.5 times the carrier frequency. I suppose at this time our world as we know it is collapsing :-). And by the way, Nyquist is STILL correct (because the signal bandwidth is 3 kHz).

lygra wrote:This is a fool's bet for someone who doesn't know the FPGA source code. But that FPGA source code may not be necessary if you look at the STM source code and find it's triggers source.

A fool's bet. Yeah. I did look into STM the code, did you? Of course not, because you are wrong again. Look @ BIOS.h, process.c (__set function) and you will understand (i hope so). And again, what would be the purpose of sampling at 72 Msps when you can not trigger with a precision of more than 1us (in case internal ADC would be used)?.

lygra wrote:I believe that it was you that started with the equivalent time sampling theory, not myself. Sampling theory is not that complex and is explained quite well in the Tektronix reference that I provided in my earlier post.

Of course, the sampling theory is very simple. Read a manual and that's it. Shannon and Nyquist would commited suicide if they could read you.

lygra wrote:The only thing I find complex is trying to determine a hardware trigger within the FPGA, without first looking at the STM source code (which is available) to see if a software trigger is present. An even simpler approach would be to point out the Quad menu choice which provides enabling of this fictional equivalent sampling mode. If the Quad were in continuous equivalent sampling mode, then you could not look at non-repeating signals, but guess what; you can look at non-repeating signals with the Quad. Therefore if you assertion is true, then there must be a Quad menu choice to switch back and forth between equivalent sampling mode and real-time sampling mode. Please be so kind as to point out this menu choice, or stop wasting everyone's time with your non-validated theories.

I've never said that equivalent time sampling is implemented in QUAD. I said it is POSSIBLE for sure. Excuse me but you know we have to wait until Benf took the source code disaster of DSO Nano and made a pretty decent firmware... not expected for seeed programmers to evolve so fast.

lygra wrote:
Slimfish wrote:So for me, the theory discussion is over.
What a relief that is. :D
Look, my goal here is not to constantly criticize you, my only goal is to provide factual enlightenment for those looking for same. If you agree to stop spewing theory, then I will agree not to be critical in return. Do some research on your own as I have done, and present those research findings that support your claims. ;)

Huh, that's funny. I'm a professional researcher/university teacher (+20 years in real electronic design). I'm proud of being always open to criticism. As Bainesbunch have said, this have gone too far. I'm glad you finally have some relief.

The Last word is all yours. Enjoy it.
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Re: DSO Quad bandwidth

by vernarim » Wed May 11, 2011 8:52 pm

The story is always funnier than expected.
I have done some measurements right now.
As HugeMan/lygra suggested, I feed the ch-A input (without any probe) directly with the signal generator. BTW: the instrumentation is still exactly the same.
I checked the signal either on the Quad display and with the LeCroy scope on U5 pin13.
The amplitude of the input was constant at 2Vpp.

Sine wave test.
A manual sweep from 1 to 15MHz (step by 1MHz) clearly shown that the band is *NOT* constant (ref U5 pin 13).
Assuming 100% at 1MHz, the amp will decrease briefly to 50% (-6dB) at about 3MHz. Still remain at 50% until you reach about 7-8MHz, then raises to about 75% around 10MHz. Hereinafter will raise even more: at 15MHz (the max I can test) is about 200%.
So, at 10MHz the attenuation is about 75% (i.e. -3dB)...that's the way HugeMan says the bandwidth is 10MHz...but *FOR ME* the bandwidth has to be taken as the VERY FIRST POINT where the attenuation falls to -3dB...once again the point is around 2MHz.

Square wave test.
Just a simpler test than before: always scoping the pin 13 U5.
At 1MHz the wave has the rising edge clearly rounded. The displayed wave on Quad looks pretty the same as the LeCroy's one.
Going higher with the freq, 3MHz, the square is almost a triangle/sine, with a well-visible spike just before the falling edge.
Above 4-5MHz the wave on the LeCroy scope looks as sine.
The spike is clearly due to the unexpected amplitude above 10MHz.

My conclusion:
    the analog section has a bandwidth of 2-3MHz;
    the "supposed" bandpass is *very* far to be flat (as should be);
    the analog switching strategy is faulty: if the signal cannot be shown as it is, the Quad should constrain the user's selection;
    as the current analog section hardware, this Quad is good for signals under 100-200KHz.
I will leave my Quad in the lab, if someone of you is asking for any additional test.
Cheers

PS: suggestion to HugeMan.
I would think about an external box, embedding a good analog section, that can feed a good bandpass and a reliable way to display signals.
I am not so happy about this toy.
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Re: DSO Quad bandwidth

by lygra » Thu May 12, 2011 2:19 am

Slimfish wrote:the last word is all yours ...
My last word is in line with Bainsbunch, in that I apologize to you for my being an ass at times :oops:
Last edited by lygra on Thu May 12, 2011 2:46 am, edited 1 time in total.
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Re: DSO Quad bandwidth

by lygra » Thu May 12, 2011 2:41 am

vernarim wrote:The story is always funnier than expected. I have done some measurements right now.
As HugeMan/lygra suggested, I feed the ch-A input (without any probe) directly with the signal generator. BTW: the instrumentation is still exactly the same.e briefly to 50% (-6dB) at about 3MHz. Still remain at 50% until you reach about 7-8MHz, then raises to about 75% around 10MHz. Hereinafter will raise even more: at 15MHz (the max I can test) is about 200%. So, at 10MHz the attenuation is about 75% (i.e. -3dB)...that's the way HugeMan says the bandwidth is 10MHz...but *FOR ME* the bandwidth has to be taken as the VERY FIRST POINT where the attenuation falls to -3dB...once again the point is around 2MHz. I checked the signal either on the Quad display and with the LeCroy scope on U5 pin13. The amplitude of the input was constant at 2Vpp.
Thanks for providing these very necessary measurements.

Neither you nor HugeMan have specified which range scale you are using for these measurements. As long as the range scale is not changed, then I would suspect that calibration of that range scale would not be a factor. On the other hand, the probe compensation adjustments for that channel could have very dramatic affect.

U5 pin "Y", is that what you are measuring? The pin numbering seems to be inconsistent on the schematic. It appears that U5 "Y" is the input to the op-amp U-7, and U5 "X" is the output of the op-amp U-7. If so, then U5 "X" will be the low impedence signal into the ADC, and that is where the measurement should take place.

I am still awaiting my replacement Quad. Maybe you could upgrade to the latest firmware, conduct the new probe compensation procedures provided by HugeMan, and then repeat your test to see if the results are different. Hopefully the probe compensation procedure will help to flatten this bandwidth. It may be robbing Peter (2-3Mhz) to pay Paul (15Mhz). If this is true then the bandwidth will probably end up being more than 15Mhz.

Thanks for sharing this info.
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