home-built DSO (digital oscilloscope) ideas

[petrv]

Hi all,

I have always wanted to have an oscilloscope for my hobby but the professional ones are quite expensive ($$$$) and while I have found a few construction most of them are toys - like "connect and ADC to the parallel port of the PC & so on ...)

A few days ago I have received samples from National (ADC08060) which is 60 MSPS 8-bit ADC - should give frequency response 20-30 MHz. But of course, a common MCU (PIC... etc) cannot read the data from the ADC fast enough. I am thinking about using a FPGA - luckily I have 2 FPGA boards (XSA3S1000 from XESS and Spartan-3E starter kit from Digilent, Inc.) Then I think I can interface the FPGA to a MCU and then PC (serial or better USB).

I am wondering if someone already did something similar (or is planning to do)

I was thinking about using even faster ADCs - they are available from National or other companies but the challenge is to read the data fast enough from those chips and store them in RAM...

Petr

 

[Nigel Goodwin]

There are lot's of such projects on the net, it's a fairly common final project for University students.

 

[3v0]

This is a common topic but I too would like to revisit it. I do not want to take the time to design one from scratch.

With DSO's better is often taken to be faster, wider, deeper. Faster is just that. Wider is more traces, and deeper is more trace memory.

This is one DIY that may be what you want.

http://www.embedds.com/diy-40msps-avr-oscilloscope-with-graphical-lcd/

Maximum sample frequency: 40MSPS
Maximum input frequency: 5MHz
Maximum displayed frequency without aliasing: 10MHz
Input circuit bandwidth: 20MHz
Display resolution: 240×128 total, trace resolution 200×125
Sensitivity: 40mV/div
Coupling: DC
Input impedance: 10K
Power supply: single DC source 8V..10V, 1A
No incremental mode
Time base: 1s/div, 500ms/div, 200ms/div, 100ms/div, 50ms/div/, 20ms/div, 10ms/div, 5ms/div, 2ms/div, 1ms/div, 500us/div, 200us/div, 100us/div, 50us/div, 20us/div, 10us/div, 5us/div, 2us/div, 1us/div, 500ns/div
Trigger: digitally adjustable
Trace offset: digitally adjustable

I have not priced the parts.

 

[petrv]

I have found 2 projects (excluding "toy" projects using the A/D converter in a PIC micro), both using FIFO memories. (This one and another, older project).
I'd like to avoid FIFO memories if possible as they are not common parts (try to search DigiKey or Mouser for FIFO memory -> 0 matches...).
When using a FPGA I can use its internal memory.

 

[Papabravo]

Modern automated manufacturing methods and component purchasing in large quantities mean that for all practical purposes it is IMPOSSIBLE to design and build, one of anything, at a lower cost than it can be purchased for. If you want a DSO, then save your nickels and dimes until you can buy one. Don't even waste your time thinking about this one.

 

[petrv]

I am sorry papabravo but I do not agree with you. Electronic test equipment like scopes is not produced in a big quantities as the market is not so big and it is very expensive - even low-end USB osciloscope can cost well over $100. I believe it is possible to build a cheaper and better one (e.g. the construction mentioned by 3v0 and you can remove the GLCD display if using PC, just replace the MCU with a different MCU with USB ....) The same goes for other test equipment, like RF spectrum analyzers etc

 

[3v0]

Modern automated manufacturing methods and component purchasing in large quantities mean that for all practical purposes it is IMPOSSIBLE to design and build, one of anything, at a lower cost than it can be purchased for. If you want a DSO, then save your nickels and dimes until you can buy one. Don't even wast your time thinking about this one.

I agree about design and build. But if the design already exists?

If I can find a similar assembled scope for the price of the parts I would buy a few. Suggestions ?

 

[blueroomelectronics]

It's a single channel design, that GLCD display probably won't be cheap. What is the purpose of the FPGA? What's the part # of the A/D?

The USB PoScope comes to mind $100, or Owon brand scopes (color for about $300)

 

[petrv]

The design for a hobbyist is a fun thing to do so I don't count it as an expense. Also no need to design it from scratch, it is possible to take parts from other designs - for example the analog front-end, as I am much better in digital design than analog. Maybe it could be produced as a kit - PCB + components.

 

[petrv]

Hi Bill,
there are a few ADCs available, I have samples of ADC08060 from National but there are others and also from other companies (AD,TI,Intersil ...)

The purpose of FPGA is to write the 8 output bits from the ADC to memory (inside the FPGA) - and at the speed of 60 MHz (or more if using faster ADC)
Microcontrollers like PIC are not fast enough to do the job. After the signal is sampled, a MCU will read it from the FPGA (at much slower speed) and send the data over USB to PC. You can do the same thing without FPGA by using a special kind of memory (FIFO memory) but these are hard to get. You can buy Xilinx FPGAs in DigiKey (and of course the design is more flexible as you can reprogram -recofigure the logic inside the FPGA).
Petr

 

[3v0]

It's a single channel design, that GLCD display probably won't be cheap. What is the purpose of the FPGA? What's the part # of the A/D?

The USB PoScope comes to mind $100, or Owon brand scopes (color for about $300)

The HDS1022M which is 2 channels 100MS/s could work for me. I still have not found it for $300. More like 5.

The proscope sample rate only goes up to 200 kHz or .2MS/s but it could work for school.

Would have to look at the specs more. On cheap scopes they fake 2 channels by using a single channel to sample 2 inputs. You see the results from 2 channels but at 1/2 the speed and depth.

The DIY could be made into a 2 channel scope. The software could be major work.

 

[ericgibbs]

Hi Bill,
there are a few ADCs available, I have samples of ADC08060 from National but there are others and also from other companies (AD,TI,Intersil ...)

The purpose of FPGA is to write the 8 output bits from the ADC to memory (inside the FPGA) - and at the speed of 60 MHz (or more if using faster ADC)
Microcontrollers like PIC are not fast enough to do the job. After the signal is sampled, a MCU will read it from the FPGA (at much slower speed) and send the data over USB to PC. You can do the same thing without FPGA by using a special kind of memory (FIFO memory) but these are hard to get. You can buy Xilinx FPGAs in DigiKey (and of course the design is more flexible as you can reprogram -recofigure the logic inside the FPGA).
Petr

hi,
How many samples to expect to collect at 60MHz from the ADC before you start outputting to the display.?

 

[petrv]

The designs I saw with FIFO memory had 512 or 1024 byte memory. I'd like to use Xilinx XC3S100E FPGA which has 8 kbyte block ram, in VQFP-100 package (how I hate to do PCB layout for this .... but no choice, better than BGAs).

So it can be anything (power of 2) until the available memory size, no reason why it could not be configurable (another advantage of using the FPGA)

It could be also possible to connect another ADC to have second channel ...

The design in VHDL should be quite simple, I need to try what is the maximal speed the design can run in this FPGA ($ 9.48 in DigiKey it is not such an expensive part that's why I'd like to use it).

 

[ericgibbs]

The designs I saw with FIFO memory had 512 or 1024 byte memory. I'd like to use Xilinx XC3S100E FPGA which has 8 kbyte block ram, in VQFP-100 package (how I hate to do PCB layout for this .... but no choice, better than BGAs).

So it can be anything (power of 2) until the available memory size, no reason why it could not be configurable (another advantage of using the FPGA)

It could be also possible to connect another ADC to have second channel ...

The design in VHDL should be quite simple, I need to try what is the maximal speed the design can run in this FPGA ($ 9.48 in DigiKey it is not such an expensive part that's why I'd like to use it).

hi,
Have you looked at www.analog.com for their range of FPGA, FIFO's, they also have an evaluation board. No idea of cost.

http://search.analog.com/search/default.aspx?query=FIFO&local=en

 

[petrv]

Yes they do have an FPGA board for ADC demo but .. $300, thank you very much ...
HSC-ADC-FPGA-8Z Prodn EVALUATION BOARDS - Comm $300.00
Evaluation boards are usually expensive and AD components are not cheap as well.

I'd rather buy a general purpose FPGA board for less money, like this one : (Nexys-2 : $99)

http://www.digilentinc.com/Products...ucts&Nav2=Programmable&Cat=Programmable Logic

Petr

 

[ericgibbs]

Yes they do have an FPGA board for ADC demo but .. $300, thank you very much ...
HSC-ADC-FPGA-8Z Prodn EVALUATION BOARDS - Comm $300.00
Evaluation boards are usually expensive and AD components are not cheap as well.
Petr

Analog are very helpful with 'free' samples..

 

[petrv]

Samples usually do not include eval. boards, only chips ... I did not try yet to ask for samples from AD, but I have a good experience with MAXIM, On Semiconductor and National (That is how I got the ADCs from National
Petr

 

[Papabravo]

I am sorry papabravo but I do not agree with you. Electronic test equipment like scopes is not produced in a big quantities as the market is not so big and it is very expensive - even low-end USB osciloscope can cost well over $100. I believe it is possible to build a cheaper and better one (e.g. the construction mentioned by 3v0 and you can remove the GLCD display if using PC, just replace the MCU with a different MCU with USB ....) The same goes for other test equipment, like RF spectrum analyzers etc

You are of course perfectly free to disagree. I would suggest that some of your assumptions are questionable especially given the manifest problems people have obtaining various common parts depending on their location. Buying a single display is often a project killer, I think the cost of a dedicated PC should be factored into the cost. I know they are cheap, but they are not free. What you're going to end up with is very likely to be outgrown rather quickly. Instead save your money and buy something that will be around for a while. If your credit is good you can even get a loan to buy a scope. I did that 30 years ago and I still use my Tek scope with the VF frequency counter.

 

[petrv]

Buying a display ? Dedicated PC ? Of course not, nothing of that. I think (almost) everyone today has a PC and no need to have a dedicated one and no need to have a display if you have your PC -> display cost equals 0. For example I have bought an USB logic analyzer for a fraction of the price of a standalone logic analyzer (Intronix) and I don't need a dedicated PC (which you would argue bring the price to the same level as the standalone L.A.)

Anyway I started the thread to discuss technical ideas how to build such an device and not if or why yes or no.

And even if you would be correct with the price, we do things for fun,
this is our hobby and not a business.

Petr

 

[BeeBop]

Just wonder if you are aware of the Bitscope? https://www.bitscope.com/design/

16F628 PIC and a Lattice LSI 1032 PLD to implement a high speed dual stream data capture engine. Captured data is recorded to two 32k x 8 bit SRAM buffers.

They provide schematics and information on their design.

I have one of these, but prefer my ancient Tek analog scope, any day of the week.