Products > Test Equipment
Building my own scope
David Hess:
--- Quote from: balnazzar on October 20, 2022, 05:59:30 pm ---1. If I stick a PCIe frontend/ADC card into a desktop computer, can the computer do what the FPGA does in commercial scopes?
--- End quote ---
A PC processor has the performance to implement the decimation and triggering which the FPGA handles, but many high speed PCIe and USB digitizers cannot stream continuously at high sample rates.
--- Quote ---2. Do USB scopes have a FPGA board inside??
--- End quote ---
Yes, USB scopes have an FPGA or equivalent for handling decimation and triggering.
--- Quote from: alm on October 20, 2022, 06:07:23 pm ---Don't forget the analog front-end. You'll want a 1 MOhm in parallel with 10-25 pF or so input impedance to work with standard 10x probes, and some amplification and/or attenuation to extend the range beyond the native range of the ADC. Especially since you value low noise, which presumably means you want to look at low-level signals.
You also may want some input protection so you don't blow up your expensive ADCs.
--- End quote ---
Do not underestimate the finesse that goes into a high impedance input. For high impedance attenuators, I would consider picking up a couple of the 4-pin hybrid attenuators Tektronix used in the 1970s. Add a DPDT telecom relay and you have a switchable high impedance attenuator good to 100 or maybe 200 MHz.
Input noise is dominated by the high impedance buffer, typically an FET being used as a source follower, which is the noisiest part.
--- Quote from: balnazzar on October 20, 2022, 06:11:56 pm ---Well said. In the end, low noise amounts to a well-thought front end. I don't have the faintest about how to learn concocting good front ends.
--- End quote ---
The Tektronix Circuit Concepts books "Vertical Amplifier Circuits" and "Oscilloscope Probe Circuits" available here are a good place to start.
"Signal Conditioning in Oscilloscopes and the Spirit of Invention" by Steve Roach included in "The Art and Science of Analog Circuit Design" edited by Jim Williams discusses modern front end designs.
"Good Engineering and Fast Vertical Amplifiers" by John Addis in "Analog Circuit Design - Art, Science, Personalities" edited by Jim Williams would be useful for a discrete amplifier design.
Studying various Tektronix service manuals for their solid state oscilloscopes from the 1970s to 1980s will be helpful. They include detailed theory sections describing every circuit. The Tektronix 2230 and 2232 DSOs will be particularly helpful. The 2230 implemented a sampling rate of 20 MHz with a TTL design.
--- Quote from: pcprogrammer on October 20, 2022, 07:44:54 pm ---A scope without a FPGA is certainly possible, but would not have a high sampling rate. Take a look at these little scopes made with a STM32103 MCU. Just 1MSa/s, but good as a basis to learn about how a scope works.
--- End quote ---
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.
--- Quote ---For higher sample rates without a FPGA you would need a more powerful processor and an external ADC. Don't have examples on this.
--- End quote ---
Last year I figured there should be standard interface ADCs and processors to allow for a high sample rate design without an FPGA or custom logic, and there are, but they are incredibly expensive.
balnazzar:
--- Quote from: tggzzz on October 21, 2022, 10:24:56 pm ---or even RFC1149 (or revision RFC2549)
--- End quote ---
;D
balnazzar:
--- Quote from: David Hess on October 21, 2022, 11:14:55 pm ---
The Tektronix Circuit Concepts books "Vertical Amplifier Circuits" and "Oscilloscope Probe Circuits" available here are a good place to start.
"Signal Conditioning in Oscilloscopes and the Spirit of Invention" by Steve Roach included in "The Art and Science of Analog Circuit Design" edited by Jim Williams discusses modern front end designs.
"Good Engineering and Fast Vertical Amplifiers" by John Addis in "Analog Circuit Design - Art, Science, Personalities" edited by Jim Williams would be useful for a discrete amplifier design.
Studying various Tektronix service manuals for their solid state oscilloscopes from the 1970s to 1980s will be helpful. They include detailed theory sections describing every circuit. The Tektronix 2230 and 2232 DSOs will be particularly helpful. The 2230 implemented a sampling rate of 20 MHz with a TTL design.
--- End quote ---
Thanks. You seem to be remarkably knowledgeable. Do you work in the field of oscilloscope manufacturing?
(P.S. the link to Tek literature contains a typo, but removing the trailing 'url' makes it work)
py-bb:
--- Quote from: balnazzar on October 20, 2022, 11:34:09 am ---
--- Quote from: pcprogrammer on October 20, 2022, 11:21:19 am ---It is also the question why. If you need a scope for your hobby it is easier and probably cheaper to just buy one. If it is a hobby to build one then nothing is stopping you to just try. It can be a lot of fun.
--- End quote ---
I'm motivated by two reasons, essentially:
1. I'm dissatisfied by commercial scopes within my financial reach (say 1 grand).
2. I think I'll learn a lot of things by building my own scope.
That said, I'm having difficulties finding the ADCs, primarily because I don't know exactly where to search, and apart from the main specs (e.g. resolution in bits, etc..), I don't know what to look for... Here is where I need advice.
--- End quote ---
Analogue ones are much much easier.
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.
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.
What you need are say ~20 things able to latch the digital reading in a round-robin fashion (this might be doable on the FPGA, but getting it onto it could be tough) - then you can read each at ~100mhz which is bordering on practical.
This is a demultiplexor and something the FPGA can be good at. Then you need to get this to a circular buffer in RAM. I'd also make it so you knew via software if say a "latch" hadn't been read and had to latch a new value (meaning you missed a sample)
That'll be the hard bit, the rest is software or the same as analogue scopes.
pcprogrammer:
--- 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, or more useful for spectrum analysis a heterodyne mixer to bring your signal down in frequency. But these require a lot of additional electronics to make it work.
https://en.wikipedia.org/wiki/Heterodyne
--- Quote from: David Hess on October 21, 2022, 11:14:55 pm ---Last year I figured there should be standard interface ADCs and processors to allow for a high sample rate design without an FPGA or custom logic, and there are, but they are incredibly expensive.
--- End quote ---
You mean the FMC based systems?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version