Low-Cost USB Oscilloscope

[Luciano]

Hi,

You need a microcontroller with an external memory bus.
Even with an external memory bus, the oscilloscope will be
slow or better said just a toy. My assumption here is
that you need an oscilloscope for projects where
microcontrollers are involved. If you need an oscilloscope
for audio signals then go ahead, it will be fast enough.

Try to calculate how fast your oscilloscope could be.

Best regards,

Luciano

[T.Jackson]

Hi,

You need a microcontroller with an external memory bus.
Even with an external memory bus, the oscilloscope will be
slow or better said just a toy. My assumption here is
that you need an oscilloscope for projects where
microcontrollers are involved. If you need an oscilloscope
for audio signals then go ahead, it will be fast enough.

Try to calculate how fast your oscilloscope could be.

Best regards,

Luciano

For the hobby end user, most test gear is a toy. In my opinion even Bitscope falls into this category. It's not uncommon nowadays for service techs to be using 500MHz + DSOs. Of course an extreme overkill for any enthusiast and not something that you would ever really consider unless there was some considerable financial gain to be made from it from either generating new work or speeding up the process of your current service. If you don’t work with 500MHz gear then chances are you don’t need it.

SRAM: 20nS or thereabout
ADC: 40MSP/s

The upper limitation is the RAM. (1 / 20nS) = 50MSP/s

*Anything greater than this is like trying to generate free power. Or, more commonly an amplifier that supposedly puts out 200WRMS when the DC rails are only good for 100W. Like duh, did I just get ripped off or what…

*The million dollar question I guess is this. Without using too much black magic in the software for interlacing and periodical guess work, how many samples of a waveform is considered to be sufficient analysis? Sure with 40MSP/s you could take one sample of a 40MHz sine wave every second. Won’t look like much of a sine wave on the screen though. If you allowed say 20 samples of every cycle then this might suffice. (40MSP/s / 20) = 2 MHz. Undoubtedly there must be the presence of some host software magic some where a long the line. Hmm, is my DSO really 20MHz or is there just a whole heap of black magic in the software making a lot of assumptions?

Best Regards,
Trent Jackson

[Luciano]

Hi,

Bandwidth vs Sample Rate:
http://www.bitscope.com/design/hardw...r/?p=bandwidth

* * *

High speed S-RAMs: 12 ns
http://www.cypress.com

Best regards,

Luciano

[PJALM]

Here is a very simple one I found that uses a 18F4550.

http://ostry.w.szu.pl/miniscope.php?lang=en

They also have some more complex models as well but this one includes everything and its free.

[malc-c]

Here is a very simple one I found that uses a 18F4550.

http://ostry.w.szu.pl/miniscope.php?lang=en

They also have some more complex models as well but this one includes everything and its free.

OK downloaded the exe, browsed the files etc... but there is no scematic. It states on the website

Project of digital storage oscilloscope, hardware is just PIC18F4550 and few passive components.

Browsing the site there is an updated miniscope2 which shows a PCB which has a bit more than a few descrete components but the chips look too small to be a 18F4550 (assuming its a 40 pin device)



Do you still have the schematic and can you post it here or provide a link to where it can be downloaded from ?

EDIT - just re-read the bit about the ver2 -Based on AT91SAM7S64 microcontroller. - Doh !

[T.Jackson]

Here is a very simple one I found that uses a 18F4550.

http://ostry.w.szu.pl/miniscope.php?lang=en

They also have some more complex models as well but this one includes everything and its free.

Hi

The sample rate of 25kSPS won't allow for much. Won't even cover the audio spectrum. (25kSPS / 10) = 2.5KHz. For some half-decent performance, at least half a dozen or so chips are required. Analog front-end using an OPAMP, discrete ADC (at least capable of 2MSP/s), SRAM - 32KB, address counter IC, micro controller to shuffle the sampled data to the host - (PC in this case), calibrated time base’s - micro controller won't be fast enough to set sample rate or address the SRAM. Big job to come up with some decent goods. Doubt I’ll bother if I can’t get at least 1MHz of solid bandwidth.

Best Regards,

Trent Jackson

[T.Jackson]

Using a PIC18f4550 @40MHz and an analog front-end - here's my calculated guess at what to expect if things are done right. Internal ADC conversion time: (2 - 3) uS, assuming a source impedance < 200 ohms. 1 / say 2.5uS = 250kSP/S Allow most of the internal RAM for buffering. 2KB? Time taken to fill RAM? At least a few cycles. Knock off say another (2 - 3) uS from the sample rate?. At best, very best - my guess is something like 100kSP/S. Useable bandwidth of about 10KHz without any software magic in the Windows UI.

However, I reckon the author of Bitscope would manage to squeeze about (200 - 300) Khz out of this arrangement...

Here's how I think he does it: (waveform is all there but scattered)

1. Fill RAM with samples (using fastest time base.)
2. Retrieve samples from RAM & send to PC.
3. Buffer samples.
4. Analyze / sort samples (not too sure of the algorithm.)
5. Render and apply anti-aliasing to waveform.

With step 4 it might be as simple as looking for consistencies of samples and ignoring them. But what I do know for sure is that it works because the waveform is actually all there, but it’s scattered. Parts of it might be in the first few hundred samples while the rest of it might be residing in the last few thousand.

Am I kind of on the right path?, anyone care to add?

Best regards,

Trent Jackson

[T.Jackson]

This is real good. This guy has used a PIC 16f675 to make a fully self-contained CRO. For about $5 you can build this thing that simply just plugs into any spare composite video in on your TV. Bandwidth is only a few KHz, but for $5 could you really complain?

Even more notable is that he has also reinvented the game Pong using nothing more than a 16f675!
(*Very scary)

http://members.chello.nl/r.dekker49/.../uscope_e.html

Best Regards,

Trent Jackson