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Offline pcprogrammer

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Re: Building my own scope
« Reply #75 on: October 22, 2022, 06:13:25 am »
Analogue ones are much much easier.

With a cathode ray tube sure, but try making something for a LCD.

With digital ones the problem is that you need to have 1 amplifier (at least for each channel) and one thing sampling it (the ADC). This is because if you had 2 and switched between them (say taking it in turns) they'd be slightly different in gain and sampling. We often don't expect channels to be exactly the same so you can get away with it but still.

And yet many scope use this to get higher sampling rates. Mostly with the ADC's integrated into a single chip. Good ones will be matched by laser trimming in the factory.

Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

There are some threads on this forum with discussions about the needed sample rate. For a scope the opinions differ from 2.5 to 10 or more times for proper signal representation. Times 2 is the nyquist theorem for the ability to reconstruct a sine wave. How often is it that you are just interested in a pure sine wave when using a scope? Maybe you want to see some distortion on your signal. You need a much higher ratio to be able to see it. Or use a spectrum analyzer of course.

Offline py-bb

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Re: Building my own scope
« Reply #76 on: October 22, 2022, 07:49:00 am »
Analogue ones are much much easier.

With a cathode ray tube sure, but try making something for a LCD.

With digital ones the problem is that you need to have 1 amplifier (at least for each channel) and one thing sampling it (the ADC). This is because if you had 2 and switched between them (say taking it in turns) they'd be slightly different in gain and sampling. We often don't expect channels to be exactly the same so you can get away with it but still.

And yet many scope use this to get higher sampling rates. Mostly with the ADC's integrated into a single chip. Good ones will be matched by laser trimming in the factory.

Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

There are some threads on this forum with discussions about the needed sample rate. For a scope the opinions differ from 2.5 to 10 or more times for proper signal representation. Times 2 is the nyquist theorem for the ability to reconstruct a sine wave. How often is it that you are just interested in a pure sine wave when using a scope? Maybe you want to see some distortion on your signal. You need a much higher ratio to be able to see it. Or use a spectrum analyzer of course.

With the first thing... OK good point.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.


As for 3, yes you're right that's the nyquist limit. I think we can all agree you need to go at least that high because otherwise it's not possible to recover a signal.

That's the definition of it, whilst good you know that, you should have a look on wikipedia I think about the significance of it. If anyone tells you they need < 2x the bandwidth for a signal they want, they are simply wrong.

Putting it at 2.5, 10, sqrt(5), pi, whatever are all above 2 - and we venture into practical constraints.


In the ideal world, 2 is fine, but practice varies based on everything and you cannot give it a number of the same "mathematical rigor" as the Nyquist Limit.

Finally, I picked 1 ghz because it's 1e12 hz and this would have been nice for back of the envelope calculations, it's not special.

Hope this helps.

Addendum: You may have heard of the Nyquist Bound - this term is used when we mean that anything with a sampling rate >= 2 x frequency we care about will do. The Nyquist Limit is the lowest sampling rate you can have and have an (idealised) chance of catching the frequency you care about.

They're the same thing.
« Last Edit: October 22, 2022, 07:59:21 am by py-bb »
 

Offline pcprogrammer

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Re: Building my own scope
« Reply #77 on: October 22, 2022, 08:21:47 am »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

Offline py-bb

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Re: Building my own scope
« Reply #78 on: October 22, 2022, 08:36:03 am »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

If you're going that high you need knowledge of microwave propagation... which also doesn't help OP.

 

Online tggzzz

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Re: Building my own scope
« Reply #79 on: October 22, 2022, 10:06:49 am »
Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

Not true. The bandwidth and sampling rate are independent.

Examples:
  • SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
  • TDR. My 1970s Tek 1502 displays <140ps risetimes and its sampling rate is <100kS/s
  • Scopes. One I used many decades ago is the HP54100A: 1GHz/350ps, 40MS/s
  • X-Y plotters. In the mid 70s I saw an experimental sampling scope with >1MHz bandwidth that used an XY pen-plotter as an output device.
There are many many other examples.
« Last Edit: October 22, 2022, 10:17:03 am by tggzzz »
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Online tggzzz

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Re: Building my own scope
« Reply #80 on: October 22, 2022, 10:10:40 am »
A low sampling rate sampling oscilloscope could be made for bandwidths above 1 GHz but that entails several other design disciplines and is less generally useful.

Correct. Take equivalent time sampling, but for that to work you need a reliable fine scaled delay...

Delay isn't too much of a problem: see the very simple circuits in the Tektronix 1502 TDR.

Triggering from the signal is more challenging. The Tek 1502 avoids that by having an, ahem, external trigger that is generated internally :)
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Offline Fungus

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Re: Building my own scope
« Reply #81 on: October 22, 2022, 10:12:24 am »
There are some threads on this forum with discussions about the needed sample rate. For a scope the opinions differ from 2.5 to 10 or more times for proper signal representation. Times 2 is the nyquist theorem for the ability to reconstruct a sine wave.

Times 2 can reconstruct any bandwidth limited signal, not just sine waves.

The main problems are:
a) Finding a bandwidth limited signal in real life
and
b) Using an infinitely wide filter to reconstruct it
 
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Online tggzzz

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Re: Building my own scope
« Reply #82 on: October 22, 2022, 10:18:27 am »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

If you're going that high you need knowledge of microwave propagation... which also doesn't help OP.

Not true.

Here's an example that doesn't - and can use a pen plotter as an output device http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf http://www.redrok.com/sampscope.htm

Shame that author measures time intervals in mohs (e.g. 50nS)
« Last Edit: October 22, 2022, 10:20:39 am by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline py-bb

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Re: Building my own scope
« Reply #83 on: October 22, 2022, 11:15:00 am »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

If you're going that high you need knowledge of microwave propagation... which also doesn't help OP.

Not true.

Here's an example that doesn't - and can use a pen plotter as an output device http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf http://www.redrok.com/sampscope.htm

Shame that author measures time intervals in mohs (e.g. 50nS)

I'm not convinced it'll be any good, trust me at ~ghz speeds things are a lot less nice.

At ~4ghz digital stuff you need to zig-zag traces to avoid problems from the fibres in the circuit board. Width and bends in "traces" matter - it's really non-trivial.

Also that link is analogue.

If you don't believe me BTW, I encourage you to try it (seriously, love to know how you get on) PROVIDED you have some equipment to know if it's working well or not. I'm sure it'll do something! The question is it it accurately reflecting that something.

 

Offline balnazzarTopic starter

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Re: Building my own scope
« Reply #84 on: October 22, 2022, 11:20:13 am »
I'm following (or trying to follow) your discussion with the utmost interest.
 

Offline pcprogrammer

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Re: Building my own scope
« Reply #85 on: October 22, 2022, 11:28:20 am »
Maybe you will find this video interesting then :)


Online tggzzz

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Re: Building my own scope
« Reply #86 on: October 22, 2022, 11:40:02 am »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

If you're going that high you need knowledge of microwave propagation... which also doesn't help OP.

Not true.

Here's an example that doesn't - and can use a pen plotter as an output device http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf http://www.redrok.com/sampscope.htm

Shame that author measures time intervals in mohs (e.g. 50nS)

I'm not convinced it'll be any good, trust me at ~ghz speeds things are a lot less nice.

If you had bothered to understand the techniques in that circuit, you would see that the high frequencies are limited to a very small part of the board. They are entirely tractable; see the old scopes I referred to earlier.

Besides, zig zag traces aren't principally used to avoid "fibres in the circuit board", and there are standard ways of reducing how the weave affects the spatial variation of impedance.

Quote
At ~4ghz digital stuff you need to zig-zag traces to avoid problems from the fibres in the circuit board. Width and bends in "traces" matter - it's really non-trivial.

Here's an example of a 4GHz sampling circuit where the weave in the PCB is clearly visible (Tek 1502, from half a century ago).



The only interesting point is the notch next to the CR1732 sampling diodes, used to counteract the effect of the diode's capacitance.

Quote
Also that link is analogue.

Irrelevant. All circuits are analogue - except arguably for photon counting and femto-amp circuits (and Josephson Junction, but I don't know enough about those).

You could easily add a very slow ADC and it would work unchanged. For example, you could use an Arduino's ADC if you wanted to show the result on a computer screen.  Alternatively ( if you used skill and imagination to circumvent a lack of DC response) you  could use a PC Sound card as an ADC :)

Quote
If you don't believe me BTW, I encourage you to try it (seriously, love to know how you get on) PROVIDED you have some equipment to know if it's working well or not. I'm sure it'll do something! The question is it it accurately reflecting that something.

That's valid for each and every approach, therefore doesn't distinguish between techniques.
« Last Edit: October 22, 2022, 11:49:27 am by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline rob77

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Re: Building my own scope
« Reply #87 on: October 22, 2022, 11:47:59 am »
Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

Not true. The bandwidth and sampling rate are independent.

Examples:
  • SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
  • TDR. My 1970s Tek 1502 displays <140ps risetimes and its sampling rate is <100kS/s
  • Scopes. One I used many decades ago is the HP54100A: 1GHz/350ps, 40MS/s
  • X-Y plotters. In the mid 70s I saw an experimental sampling scope with >1MHz bandwidth that used an XY pen-plotter as an output device.
There are many many other examples.

are you trolling or are you just simply dumb ? that's a question not a statement.

Quote
Not true. The bandwidth and sampling rate are independent.
discuss this please wit mister Nyquist and mister Shannon , the best place for the discussion would be a class in a school.

Quote
SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
wtf ? do you even know how it works ? how the heck would you sample a waveform with a sample rate several orders of magnitude lower that the sampled frequency ?
the SDR fronted downmixes the signal to lower freqeucy and that low frequency is sampled. e.g. you're interested in a 10Mhz wide band @ 1Gz, then you downmix it and sample the resulting low frequency.

probably you're getting seriously confused by the fact there are sampling scopes which in fact can sample higher frequency than the sampling rate, but the signal must be repeating and it works by sampling different part of the waveform each cycle.
 

Online tggzzz

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Re: Building my own scope
« Reply #88 on: October 22, 2022, 12:01:31 pm »
Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

Not true. The bandwidth and sampling rate are independent.

Examples:
  • SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
  • TDR. My 1970s Tek 1502 displays <140ps risetimes and its sampling rate is <100kS/s
  • Scopes. One I used many decades ago is the HP54100A: 1GHz/350ps, 40MS/s
  • X-Y plotters. In the mid 70s I saw an experimental sampling scope with >1MHz bandwidth that used an XY pen-plotter as an output device.
There are many many other examples.

are you trolling or are you just simply dumb ? that's a question not a statement.

No, it is an accurate statement, as demonstrated in the commercial examples.

Quote
Quote
Not true. The bandwidth and sampling rate are independent.
discuss this please wit mister Nyquist and mister Shannon , the best place for the discussion would be a class in a school.

I don't need to.

I, and others, have used it as a question for job interviewees in the form of "you have an audio signal superimposed on a 10MHz carrier. What is the minimum sampling rate you can use to recover the audio signal?" Anybody answering >10MS/s doesn't understand the fundamentals.

Quote
Quote
SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
wtf ? do you even know how it works ? how the heck would you sample a waveform with a sample rate several orders of magnitude lower that the sampled frequency ?
the SDR fronted downmixes the signal to lower freqeucy and that low frequency is sampled. e.g. you're interested in a 10Mhz wide band @ 1Gz, then you downmix it and sample the resulting low frequency.

I was using such analogue sampling techniques in 1979, based on an 1957 Bell Labs Technical Journal paper. As I've noted elsewhere, when I had time to return to the topic, I was irked that Tayloe had "got there first".

I will admit to phrasing the point somewhat ambiguously, in order to make the point about the independence of sampling rate and bandwidth.

Quote
probably you're getting seriously confused by the fact there are sampling scopes which in fact can sample higher frequency than the sampling rate, but the signal must be repeating and it works by sampling different part of the waveform each cycle.

And there you have the answer. Your confusion is that you haven't understood the inter-relationships between Nyquyist, signal bandwidth, bandwidth and aliasing. Or maybe you do understand them, but haven't articulated the relevance to this problem.
« Last Edit: October 22, 2022, 12:12:49 pm by tggzzz »
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Offline balnazzarTopic starter

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Re: Building my own scope
« Reply #89 on: October 22, 2022, 12:20:28 pm »
Maybe you will find this video interesting then :)



You did actually read my mind. That's the scope I finally decided to buy.  8)
 
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Offline Mechatrommer

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Re: Building my own scope
« Reply #90 on: October 22, 2022, 12:39:03 pm »
Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

Not true. The bandwidth and sampling rate are independent.

Examples:
  • SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
  • TDR. My 1970s Tek 1502 displays <140ps risetimes and its sampling rate is <100kS/s
  • Scopes. One I used many decades ago is the HP54100A: 1GHz/350ps, 40MS/s
  • X-Y plotters. In the mid 70s I saw an experimental sampling scope with >1MHz bandwidth that used an XY pen-plotter as an output device.
There are many many other examples.
its funny you have a habit to get into disagreement with people and giving lecture philosophically about "in context" discussion, strawman fallacy etc (in other thread) without realizing you are the one you are lecturing about. i browse this thread quickly and nothing talk about sampling scope (which is a specialized scope), you cant bode plot with sampling scope. people talked about nyquist criteria (2.5X BW to be exact, not 2X) which is what we understand normally as what a normal scope do, and then you wandered off to suggest 1GHz BW with KSps scope, and how unimportant is trace length and propagation delay is in GHz domain. its just out of context, and even incorrect in some context... no need to further mess the already complicated subject (now i quote you fully so you have less chance to invent strawman argument on me ;D)

btw, i agree with tautech, OP is going into rabbit hole, or worse a worm or black hole. if you really want to waste your time you can dig i also created a thread like this to ask many years ago (while the spirit is high) and then i never made the first step, knowing more (or just a little bit) of what i actually dealing with... if you think you can beat the already cheap Rigol or Siglent and their in-house engineers, then imho you are delusional. lest if you want to talk about saving your pocket money. and later after that, i was thinking about the 10GSps or greater DSO segment where we dont have cheap option yet, maybe i can implement Rigol brilliant idea of interleaving 10x 1GSps ADC? but when i browsed digikey for the price, i closed the browser and never went back to the idea again. going fishing is much much better things to do for the sake of your brain and health.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline pcprogrammer

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Re: Building my own scope
« Reply #91 on: October 22, 2022, 12:40:40 pm »
There are some threads on this forum with discussions about the needed sample rate. For a scope the opinions differ from 2.5 to 10 or more times for proper signal representation. Times 2 is the nyquist theorem for the ability to reconstruct a sine wave.

Times 2 can reconstruct any bandwidth limited signal, not just sine waves.

The main problems are:
a) Finding a bandwidth limited signal in real life
and
b) Using an infinitely wide filter to reconstruct it

You are correct, and I should have be more specific. When you only have two samples for a period of the signal you can only reconstruct it to a sine wave, as long as the samples are not taken on the zero crossing of course. And it is missing information about the max amplitude unless the samples are taken on the top and bottom of the wave.

To reconstruct a square wave you need much more samples to be able to do it to some extend. Fourier comes into play here. But as I wrote in another post, there are enough threads discussing this.

Offline Messtechniker

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Re: Building my own scope
« Reply #92 on: October 22, 2022, 12:56:24 pm »
To start with, I'd keep it simple. Like an USB controlled
relay attenuator/amplifier box ahead of a standard sound card.
Once you have programmed time domain, XY, FFT, Bode and all the rest,
you might go for an extended frequency response by writing your own driver
having a 1 MHz+ sample rate for a specifically modded sound card.
In an even further step you might want a sound card generator with a similar USB controlled relay/amplifier output box.
So this would mostly be a software project.
Feeling up to it? Even this will not be a one-man-show. Or it will take a very long time
(10 yrs + ?) to implement.
Just an idea.
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Offline Fungus

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Re: Building my own scope
« Reply #93 on: October 22, 2022, 01:22:13 pm »
You are correct, and I should have be more specific. When you only have two samples for a period of the signal you can only reconstruct it to a sine wave, as long as the samples are not taken on the zero crossing of course. And it is missing information about the max amplitude unless the samples are taken on the top and bottom of the wave.

Reconstruction only works at less than Nyquist.

If it's 99.99999% of Nyquist the sampler will go in and out of phase with the signal and still reconstruct correctly if the signal+filter are infinitely wide.

(which they won't ever be in real life, obviously)

 

Offline py-bb

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Re: Building my own scope
« Reply #94 on: October 22, 2022, 01:37:50 pm »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

If you're going that high you need knowledge of microwave propagation... which also doesn't help OP.

Not true.

Here's an example that doesn't - and can use a pen plotter as an output device http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf http://www.redrok.com/sampscope.htm

Shame that author measures time intervals in mohs (e.g. 50nS)

I'm not convinced it'll be any good, trust me at ~ghz speeds things are a lot less nice.

If you had bothered to understand the techniques in that circuit, you would see that the high frequencies are limited to a very small part of the board. They are entirely tractable; see the old scopes I referred to earlier.

Besides, zig zag traces aren't principally used to avoid "fibres in the circuit board", and there are standard ways of reducing how the weave affects the spatial variation of impedance.

Quote
At ~4ghz digital stuff you need to zig-zag traces to avoid problems from the fibres in the circuit board. Width and bends in "traces" matter - it's really non-trivial.

Here's an example of a 4GHz sampling circuit where the weave in the PCB is clearly visible (Tek 1502, from half a century ago).



The only interesting point is the notch next to the CR1732 sampling diodes, used to counteract the effect of the diode's capacitance.

Quote
Also that link is analogue.

Irrelevant. All circuits are analogue - except arguably for photon counting and femto-amp circuits (and Josephson Junction, but I don't know enough about those).

You could easily add a very slow ADC and it would work unchanged. For example, you could use an Arduino's ADC if you wanted to show the result on a computer screen.  Alternatively ( if you used skill and imagination to circumvent a lack of DC response) you  could use a PC Sound card as an ADC :)

Quote
If you don't believe me BTW, I encourage you to try it (seriously, love to know how you get on) PROVIDED you have some equipment to know if it's working well or not. I'm sure it'll do something! The question is it it accurately reflecting that something.

That's valid for each and every approach, therefore doesn't distinguish between techniques.

In the picture you linked to a scope you see those fat copper traces - waveguides, because it had to be designed with great care to be useful.

That they succeeded doesn't mean it's trivial or easy.
 

Online tggzzz

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Re: Building my own scope
« Reply #95 on: October 22, 2022, 01:40:50 pm »
Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

Not true. The bandwidth and sampling rate are independent.

Examples:
  • SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
  • TDR. My 1970s Tek 1502 displays <140ps risetimes and its sampling rate is <100kS/s
  • Scopes. One I used many decades ago is the HP54100A: 1GHz/350ps, 40MS/s
  • X-Y plotters. In the mid 70s I saw an experimental sampling scope with >1MHz bandwidth that used an XY pen-plotter as an output device.
There are many many other examples.
its funny you have a habit to get into disagreement with people and giving lecture philosophically about "in context" discussion, strawman fallacy etc (in other thread) without realizing you are the one you are lecturing about. i browse this thread quickly and nothing talk about sampling scope (which is a specialized scope),

You think a Rigol DS1054 or the many equivalent low end scopes are "a specialised scope"? I don't think many people would agree.

Quote
you cant bode plot with sampling scope.

Yes you can. It is standard on the entry-level Digilent Analog Discovery, and I believe it is an option on many other low-end scopes.

With skill and imagination you can do it on any scope, snalogue or digital, with or without digital post-processing. You might find it convenient to have a numerically controlled oscillator or voltage controlled oscillator (with log scaling), plus an Arduino's ADC, but you can do it.


Quote
people talked about nyquist criteria (2.5X BW to be exact, not 2X) which is what we understand normally as what a normal scope do, and then you wandered off to suggest 1GHz BW with KSps scope,

So you discount commercial scopes such as the HP54110A 1GHz bandwidth, 40MS/s? That was a "normal" scope in its day, albeit top of the range.

Or the Tektronix 11801 with SD-22 plugin: 12.5GHz, 200kS/s. I know people actively creating advanced electronic products using those.

Your definition of "normal scope" appears to be "scopes that I think of using", which is rather limiting if you are trying to understand the minimum necessary to make a useful and usable scope.


Quote
and how unimportant is trace length and propagation delay is in GHz domain. its just out of context, and even incorrect in some context... no need to further mess the already complicated subject (now i quote you fully so you have less chance to invent strawman argument on me ;D)

I've made no such statements, of course.

I have stated that you can make the traces so short that their length is unimportant - and proved that with a photo of a commercial product.

Quote
btw, i agree with tautech, OP is going into rabbit hole, or worse a worm or black hole. if you really want to waste your time you can dig i also created a thread like this to ask many years ago (while the spirit is high) and then i never made the first step, knowing more (or just a little bit) of what i actually dealing with... if you think you can beat the already cheap Rigol or Siglent and their in-house engineers, then imho you are delusional. lest if you want to talk about saving your pocket money. and later after that, i was thinking about the 10GSps or greater DSO segment where we dont have cheap option yet, maybe i can implement Rigol brilliant idea of interleaving 10x 1GSps ADC? but when i browsed digikey for the price, i closed the browser and never went back to the idea again. going fishing is much much better things to do for the sake of your brain and health.

I'll leave the OP to decide what is relevant for him. It would be arrogant for me to speak for him.

I too think the OP is biting off more than they can chew. I suspect he does realise that, but he is enjoying the conversation, viz

I'm following (or trying to follow) your discussion with the utmost interest.

From what little I can infer about the OP, he is having fun understanding what's in a scope and the many imaginative ways you can build them. Whether he builds one is a separate issue.

Having fun while pushing the boundaries of his understanding and capabilities? I like that attitude :)
There are lies, damned lies, statistics - and ADC/DAC specs.
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Online 2N3055

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Re: Building my own scope
« Reply #96 on: October 22, 2022, 01:59:27 pm »
Say you want 1ghz bandwidth, you'd need to sample at at least 2ghz, this is where you run into problems, you can't just dump that into DRAM (you can get about ~100m/sec (100mhz) requests from DRAM, they're faster than this at sequential transfers once you've opened the page) - so you'll need to buffer there.

Not true. The bandwidth and sampling rate are independent.

Examples:
  • SDR. Even the cheap dongles capture 1.5GHz waveforms, and their sampling rate is several orders of magnitude lower
  • TDR. My 1970s Tek 1502 displays <140ps risetimes and its sampling rate is <100kS/s
  • Scopes. One I used many decades ago is the HP54100A: 1GHz/350ps, 40MS/s
  • X-Y plotters. In the mid 70s I saw an experimental sampling scope with >1MHz bandwidth that used an XY pen-plotter as an output device.
There are many many other examples.

And all of these are wrong statements.

SDR is digital downconversion device that deliberately breaks Nyquist to make downconversion. Those are narrowband receiver devices and do not belong into discussion about scopes.

Speed with which sampling scopes assemble screen representation is NOT their SAMPLE rate.
Their sample rate in relation to input signal is their equivalent sample rate (ETS, RIS or whatever), basically 1/sampling aperture of S/H circuit + a bit more. And since they are not realtime but a repetitive process, it takes a lot of time to, dot by dot, reassemble whole screen worth of waveform. And stable repetitive signal, because what you see on the screen is reassembled from thousands of separate different trigger events.
And since their apparent (equivalent) sample rate is not breaking Nyquist it is possible to reconstruct signal.

Most of the modern scopes designed in last 10 years don't have ETS mode at all (some do but most don't) because of very limited usability on modern signals.
Could you please accept the fact that when people speak about sample rate they mean realtime sample rate nowadays..?
« Last Edit: October 22, 2022, 04:19:45 pm by 2N3055 »
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Offline Mechatrommer

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Re: Building my own scope
« Reply #97 on: October 22, 2022, 02:14:55 pm »
You think a Rigol DS1054 or the many equivalent low end scopes are "a specialised scope"? I don't think many people would agree.
i'm not sure whats your problem is... Rigol DS1054 is one of the among "normal" (and cheapest) scope today. we are talking about whats "normal" today, not what is "normal" 80 years ago. or maybe you need to update your terminology... https://blogs.keysight.com/blogs/tech/bench.entry.html/2022/05/09/what_s_the_differencebetweenareal-timeandsamp-WuT8.html i mean "normal" is the abundant of cheap "real-time" dso you can find today.

Quote
you cant bode plot with sampling scope.
Yes you can. It is standard on the entry-level Digilent Analog Discovery, and I believe it is an option on many other low-end scopes.
yes we can with some not so straight forward tricks. please do a bit of homework first before posting, Analog Discovery (100MSps) is not a "sampling" scope. https://www.jensign.com/Discovery/bode/index.html

i know sometime during the time, "sampling" used to means generally anything that sample digitally at interval of time (including Rigol DS1054Z), when analog scope dominating the world, and ADC was the kind of luxury.. but not today. maybe this explained your confusion. if "sampling" that you understand is the better way, think why they dont do it today? opps yeah i know, some minority of people did it, reinventing the wheel minus the boat anchor weight... https://hackaday.io/project/167292-8-ghz-sampling-oscilloscope i know one guy did it enclosed nicely and sell it, but i cant find it right now. yes it has some specialized use today, we can invent some new devices out of it, but only within its limitation and context (repeatetive signals) when you need it cheap, affordable and if thats the only option available during the time, like when your stated "sampling" scopes was manufactured. today real-time 8GHz scope costs your soul, you tell me why companies still buy it? instead of cheaper sampling scope? are they fool or some other reason?

So you discount commercial scopes such as the HP54110A 1GHz bandwidth, 40MS/s? That was a "normal" scope in its day, albeit top of the range.
so you discount Shannon-Nyquist Theorem? circa 1915
« Last Edit: October 22, 2022, 02:17:51 pm by Mechatrommer »
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Online tggzzz

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Re: Building my own scope
« Reply #98 on: October 22, 2022, 02:15:16 pm »
Not going to start a discussion about nyquist. There are plenty around here on the forum.

As for the 2nd one, why? I am sure at some point separate ADCs becomes sensible, but when and for what? I imagine really really high frequencies because multiplexing the sample values (rather than ADCs) is easier and gives good results.

But either way you've gotta demux the samples.

Well most likely money. High speed ADC's are expensive and using two that are capable of half the speed might cost way less than the single high speed one.

Take the FNIRSI-1013D and 1014D, these use two AD9288 chips. One for each channel, and this chip has 2 ADC's in it. This way they have 200MSa/s, but do need calibration to equalize the readings between the two.

The Hantek DSO2000 series use a ADC08D500 chip. This is a 2 channel 500MSa/s ADC where the two ADC can be connected to the same signal inside the chip. So a single channel can do 1GSa/s. Have not looked into the software of this one, but assume it also needs some calibration.

These are the ones I know of, but there will be others.

Using a single ADC to sample multiple channels can of course also be done, but it lowers the sample rate instead of increasing it.

For really high frequencies (> GHz) it requires a lot more knowledge to get it right with multiple ADC's, and this is not within the realm of what I know.

If you're going that high you need knowledge of microwave propagation... which also doesn't help OP.

Not true.

Here's an example that doesn't - and can use a pen plotter as an output device http://www.redrok.com/Circuits_1GHz-samplig-Oscilloscope-Front-End.pdf http://www.redrok.com/sampscope.htm

Shame that author measures time intervals in mohs (e.g. 50nS)

I'm not convinced it'll be any good, trust me at ~ghz speeds things are a lot less nice.

If you had bothered to understand the techniques in that circuit, you would see that the high frequencies are limited to a very small part of the board. They are entirely tractable; see the old scopes I referred to earlier.

Besides, zig zag traces aren't principally used to avoid "fibres in the circuit board", and there are standard ways of reducing how the weave affects the spatial variation of impedance.

Quote
At ~4ghz digital stuff you need to zig-zag traces to avoid problems from the fibres in the circuit board. Width and bends in "traces" matter - it's really non-trivial.

Here's an example of a 4GHz sampling circuit where the weave in the PCB is clearly visible (Tek 1502, from half a century ago).



The only interesting point is the notch next to the CR1732 sampling diodes, used to counteract the effect of the diode's capacitance.

Quote
Also that link is analogue.

Irrelevant. All circuits are analogue - except arguably for photon counting and femto-amp circuits (and Josephson Junction, but I don't know enough about those).

You could easily add a very slow ADC and it would work unchanged. For example, you could use an Arduino's ADC if you wanted to show the result on a computer screen.  Alternatively ( if you used skill and imagination to circumvent a lack of DC response) you  could use a PC Sound card as an ADC :)

Quote
If you don't believe me BTW, I encourage you to try it (seriously, love to know how you get on) PROVIDED you have some equipment to know if it's working well or not. I'm sure it'll do something! The question is it it accurately reflecting that something.

That's valid for each and every approach, therefore doesn't distinguish between techniques.

In the picture you linked to a scope you see those fat copper traces - waveguides, because it had to be designed with great care to be useful.

That they succeeded doesn't mean it's trivial or easy.

Not quite. Perhaps I should have been more explicit.

The scope part of the TDR is measuring the voltage as it whizzes past CR1732 sampling diode. The strip is part of the internal signal generator creating a step with a <50ps risetime.

In the (expandable) picture below the signal source CR1703 (a tunnel diode) is on the bottom right. The signal travels along the strip to the BNC socket on the bottom left, and hence into the UUT (a cable). Impedance variations in the UUT cause reflections which return to the BNC socket and then whizz back past the sampling diode.



The BNC socket contains a special internal shorting strap, so that if no cable is connected, the "scope" captures the signal rising as it goes past the sampling diode and falling as the reflection returns.

In the picture below, the 1-div wide pulse shows the signal travelling the (72mm??) from the sampling diode to the BNC short, and back again. (It looks like the pulse could be expanded by a factor of 4 horizontally, to give better time/distance resolution).

Note the 140ps risetime can also be captured on the chart recorder. No ADC, no microprocessors (1972!), and the most complex ICs are 741 class op amps, CMOS quad gates and dual flip-flops, and a CMOS 7-stage ripple counter acting as a DAC.

There are lies, damned lies, statistics - and ADC/DAC specs.
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Online tggzzz

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Re: Building my own scope
« Reply #99 on: October 22, 2022, 02:30:43 pm »
You think a Rigol DS1054 or the many equivalent low end scopes are "a specialised scope"? I don't think many people would agree.
i'm not sure whats your problem is... Rigol DS1054 is one of the among "normal" (and cheapest) scope today. we are talking about whats "normal" today, not what is "normal" 80 years ago. or maybe you need to update your terminology... https://blogs.keysight.com/blogs/tech/bench.entry.html/2022/05/09/what_s_the_differencebetweenareal-timeandsamp-WuT8.html i mean "normal" is the abundant of cheap "real-time" dso you can find today.

Read more carefully. I think the DS1054 scope is a normal scope. The context you chose to omit indicates you think it isn't normal.

Quote
Quote
you cant bode plot with sampling scope.
Yes you can. It is standard on the entry-level Digilent Analog Discovery, and I believe it is an option on many other low-end scopes.
yes we can with some not so straight forward tricks.

We clearly have different concepts of "normal" and "tricks". In addition, in what way is the AD's bode plotting not "normal"?

Quote
please do a bit of homework first before posting, Analog Discovery (100MSps) is not a "sampling" scope. https://www.jensign.com/Discovery/bode/index.html

Yes, the AD is a sampling scope. The samples are clearly visible in the displayed waveform!

What exactly do you think an ADC is doing when it captures a sample?

Quote
i know sometime during the time, "sampling" used to means generally anything that sample digitally at interval of time (including Rigol DS1054Z), when analog scope dominating the world, and ADC was the kind of luxury.. but not today. maybe this explained your confusion. if "sampling" that you understand is the better way, think why they dont do it today? opps yeah i know, some minority of people did it, reinventing the wheel minus the boat anchor weight... https://hackaday.io/project/167292-8-ghz-sampling-oscilloscope i know one guy did it enclosed nicely and sell it, but i cant find it right now. yes it has some specialized use today, we can invent some new devices out of it, but only within its limitation and context (repeatetive signals) when you need it cheap, affordable and if thats the only option available during the time, like when your stated "sampling" scopes was manufactured. today real-time 8GHz scope costs your soul, you tell me why companies still buy it? instead of cheaper sampling scope? are they fool or some other reason?

I can't be bothered to parse and attempt and understand that rant.

The key point is that "sampling" is capturing and measuring something at an instant in time. The capture is always analogue, but the measurement and subsequent processing can be either digital or analogue. Sampling scopes, both digitising sampling scopes and analogue sampling scopes repeatedly sample the input voltage. Some have, but not all, have anti-aliasing filters in front of the sampler or in the digital processing subsystem.
There are lies, damned lies, statistics - and ADC/DAC specs.
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