Products > Test Equipment
Building my own scope
pcprogrammer:
Maybe you will find this video interesting then :)
tggzzz:
--- Quote from: py-bb on October 22, 2022, 11:15:00 am ---
--- Quote from: tggzzz on October 22, 2022, 10:18:27 am ---
--- Quote from: py-bb on October 22, 2022, 08:36:03 am ---
--- 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.
--- End quote ---
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)
--- End quote ---
I'm not convinced it'll be any good, trust me at ~ghz speeds things are a lot less nice.
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
That's valid for each and every approach, therefore doesn't distinguish between techniques.
rob77:
--- Quote from: tggzzz on October 22, 2022, 10:06:49 am ---
--- 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.
--- End quote ---
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.
--- End quote ---
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
--- End quote ---
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.
tggzzz:
--- Quote from: rob77 on October 22, 2022, 11:47:59 am ---
--- Quote from: tggzzz on October 22, 2022, 10:06:49 am ---
--- 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.
--- End quote ---
are you trolling or are you just simply dumb ? that's a question not a statement.
--- End quote ---
No, it is an accurate statement, as demonstrated in the commercial examples.
--- Quote ---
--- Quote ---Not true. The bandwidth and sampling rate are independent.
--- End quote ---
discuss this please wit mister Nyquist and mister Shannon , the best place for the discussion would be a class in a school.
--- End quote ---
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
--- End quote ---
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.
--- End quote ---
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.
--- End quote ---
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.
balnazzar:
--- Quote from: pcprogrammer on October 22, 2022, 11:28:20 am ---Maybe you will find this video interesting then :)
--- End quote ---
You did actually read my mind. That's the scope I finally decided to buy. 8)
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