Products > Test Equipment
Building my own scope
tggzzz:
--- Quote from: pcprogrammer on October 22, 2022, 05:57:32 am ---
--- Quote from: David Hess on October 21, 2022, 11:14:55 pm ---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.
--- End quote ---
Correct. Take equivalent time sampling, but for that to work you need a reliable fine scaled delay...
--- End quote ---
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 :)
Fungus:
--- Quote from: pcprogrammer on October 22, 2022, 06:13:25 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.
--- End quote ---
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
tggzzz:
--- 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)
py-bb:
--- 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.
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.
balnazzar:
I'm following (or trying to follow) your discussion with the utmost interest.
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