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Building my own scope

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pcprogrammer:

--- Quote from: py-bb on October 22, 2022, 03:03:02 am ---Analogue ones are much much easier.

--- End quote ---

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


--- Quote from: py-bb on October 22, 2022, 03:03:02 am ---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.

--- End quote ---

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.


--- Quote from: py-bb on October 22, 2022, 03:03:02 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.

--- End quote ---

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.

py-bb:

--- Quote from: pcprogrammer on October 22, 2022, 06:13:25 am ---
--- Quote from: py-bb on October 22, 2022, 03:03:02 am ---Analogue ones are much much easier.

--- End quote ---

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


--- Quote from: py-bb on October 22, 2022, 03:03:02 am ---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.

--- End quote ---

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.


--- Quote from: py-bb on October 22, 2022, 03:03:02 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.

--- End quote ---

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.

--- End quote ---

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.

pcprogrammer:
Not going to start a discussion about nyquist. There are plenty around here on the forum.


--- Quote from: py-bb on October 22, 2022, 07:49:00 am ---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.

--- End quote ---

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.

py-bb:

--- Quote from: pcprogrammer on October 22, 2022, 08:21:47 am ---Not going to start a discussion about nyquist. There are plenty around here on the forum.


--- Quote from: py-bb on October 22, 2022, 07:49:00 am ---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.

--- End quote ---

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.

--- End quote ---

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

tggzzz:

--- Quote from: py-bb on October 22, 2022, 03:03:02 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.

--- End quote ---

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.

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