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USB digital scope

Dear Joel, thanks for your considerations: you are right, the channel synchronization is VERY difficult...I approached this problem in this way:

1=2E I distribute the clock signal (20MHz) from the first scope board (channel 1 and 2) to my second board (channel 3 and 4). The clock goes only to fpga (then the fpga distribute them to the A/Dcs), and for this the capacitive loading on this wire is quite low. 2=2E I have a bidirectional wire, the trigger, that comes from first board to the second if the trigger is on channel 1 or 2 or from the second board to the first if the trigger is on channel 3 or 4 (this is set by the applicative program). The trigger is a "normal" signal synchronous with the 20MHz clock signal: this will generate the sweeps with exactly the same start point and the same end point (referred to the 20MHz clock cycle). 3=2E The equivalent time sampling will be rebuilt after this phase, calculating with interpolation the trigger instant on a sub-cycle basis (interpolation) and shifting the overall waveform of this fractional value. For this the waveforms from the different boards needs to be synchronized at the clock cycle and not at higher frequency.

Finally, this scope has not very high features but from my point of view is quite cheap (about 100=80 for components) and easy to use (powered by USB only). Another very important point (always from my point of view) is the "teaching" peculiarity of this instrument: you can change the DSO firmware (FPGA) directly from the USB with no programmers or other things, only Quartus and a USB cable.

hi,

Davide

Reply to
Davide
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XMLHttpRequest() is a JavaScript subroutine (so it runs on the web browser) that monitors the web server for new data. It does this without polling so it is very fast. This routine is the core of what is called AJAX and is what makes sites like Google maps work.

A web interface is just one possibility. We could also to a TCL interface if you wish. We probably can't do native X-windows or MS-windows interfaces since you'll almost certainly want a portable interface.

Bob

Reply to
Bob Smith

I disagree Jörg,

12 bits are very good for some things. I hate these lazy 8 bit scopes. 20 ns would be fine too, but I don't need these high frequencies very often.

Marte

who is very interesting too...

Reply to
Marte Schwarz

Hi Dave,

The PCB is intended to mount also two 12 bit 50MSPS A/D converter (I do not yet tested this A/D converter but I hope this works fine...) in place of the two 12 bit 20MSPS; The PCB has an "expansion port" for a daughter board for:

  1. An arbitrary waveform generator (the scope lose one channel)
  2. A 16 bit digital port for analog/digital mixed oscilloscope function (the scope lose one channel)
  3. To stack multiple board in order to have 4,6 or 8 channel scope.

this sounds very very cool...

Marte

Reply to
Marte Schwarz

"Helmut Sennewald" schrieb im Newsbeitrag news:dn2g58$t1m$00$ snipped-for-privacy@news.t-online.com...

----- Original Message ----- From: "Helmut Sennewald" Newsgroups: sci.electronics.design Sent: Monday, December 05, 2005 11:49 PM Subject: Re: USB digital scope

Hello Davide,

Thanks for sending your schematic to me. I have taken a look to it.

I have read in another message from you that this circuit can save 1k-samples per channel.

Is it possible to have a higher number of samples stored with a pin-compatible FPGA?

Best regards, Helmut

Reply to
Helmut Sennewald

I think it do need 12 bits, it give better range and resolution to do some post calculation.

I done similar project but using ethernet and C# code. I used 18F4820 PIC because it has large RAM memory to enable single capture (1024 byte) and then transfer data later. I was using AD7708 which is low frequnecy type ADC. The C# code interface tektronix TDS3014 scope and aligent 33120 function generator which give me spectrum analyser function (low frequnecy) and the code includes automated qualification test setup. It worked very well. I learn a lot from OOP, .Net framework and C# coding technique.

The point is that what I did find that 9 bit ADC from TDS3014 is insufficient for spectrum analysis where amplitude varies with frequnecy. I perfers 16 bits which is by I'm using AD7708 setup.

20MSPS versa 50MSPS is useful if you're working high frequnecy where rise time measurement is important on certain application.

Trigger is important feature of the USB DOS.

Reply to
Riscy

Hi I'm very interested on these DLL, how to try out these module, can they be intergrated into C# code from VS 2003 pro?.

What kind of AD are you using?

Regards

Davide wrote:

Reply to
Riscy

Marte

What is the part number of the ADC you just mention below

Why not one 16 bit and one 12 bits, it give best of two(!)

Riscy

Marte Schwarz wrote:

Reply to
Riscy

Nice design, but the small memory is the killer. Memory depth is by far the #1 requirement for a DSO. 512 samples is incredibly limiting. Check out the cleverscope for a nice USB scope that has 4MB of sample memory (SDRAM). It trys to compete with the Agilent mixed signal scopes and actually doesn't do too bad a job:

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The software (Labview) lets you do lots of clever custom programmable maths directly on the waveforms too.

Dave :)

Reply to
David L. Jones

You're perfectly right. My goal was a 100$ instrument and not 1000$ instrument!!! The comparison is not from instruments in the same "segment".

Reply to
Davide

I'm sorry, I'm not sure that the DLL works in this envronment. I checked the DLL only with delphi and Visual Basic. I think this can work also with Visual C++.

The A/D is the ADS805 or ADS807 from TI. (the price is the same and the second has 50 MSPS smaple rate).

hi,

Davide

Reply to
Davide

Yup!

I love it how supposed "specialists" can't even explain how their Labview "code" works, nor can they fix the damn thing 12 months later because they can't understand what they did. Love it how you can't readily "peer review" it either. And NI push it like it's the greatest thing since sliced bread and what wonderful benefits it has for our organisation, all we have to do is send every one of our programmers on the Labview BASICs course and we'll all be instant experts :->

Dave :)

Reply to
David L. Jones

Yes, you are right. It is a great design for the price, although the lack of memory really cripples it, regardless of how cheap it is. I suspect more people would happily pay double or triple the cost for say a 1MB scope, or even 10KB worth of sample memory.

How much does a few MB of SDRAM cost in order to get a vast improvement in functionality? I've seen inside the CleverScope, and it is not much more than 2 SDRAM chips, an FPGA, and the input circuitry. Pretty pricey, but volumes are low and it fills a niche.

Dave :)

Reply to
David L. Jones

I suspect not. Those after that sort of performance would most likely not want a USB based scope, they want a "real" scope they can play with on the bench.

500MHz analog bandwidth is incredibly difficult to do, and at 1GS/s+ you need to roll your own ADCs which is what the big scope manufacturers do. That takes massive R&D time and $$$$$$$ You can't just slap together a bunch of parts from Digikey and get anywhere near that performance.

They aren't as fast as the real scopes because you can't go much above that with off-the-shelf parts.

Dave :)

Reply to
David L. Jones

Labview is the killer. Did you ever spend a month trying to bring the wires through some silly walls while knowing with some decent compiler it would have been a matter of minutes ?

Rene

Reply to
Rene Tschaggelar

Hello Riscy,

Yes, if you want to do FFT and stuff like that. But I thought this thread was more about the scope nature of a digital scope. When you want to use it to track down runt pulses, phase jitter, setup and hold violations and the other usual stuff the ADC needs to be as fast as can be. Even if that comes at the expense of bits. I have done quite some work with 6-bit digitizers because that's all we had in them days when you needed more than 100MSPS. We actually needed 400MSPS. The ENOB up there had probably been 4 bits or less but it did the job.

Regards, Joerg

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Reply to
Joerg

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