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roblegion

Oscilloscope question

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Hi, don't want to stray too far from the astro but, many of you DIY'ers out there must use an oscilloscope at some point.

I decided to build an interface for PC  oscilloscope software (basically like using an Arduino etc, but with lots of protection and variability)

I decided on Rod Elliot's excellent interface, (Elliot Sound Projects) but then started to think, if this is limited to the sound card capabilities (i.e. 20kHz), why not forget the card and incorporate a higher bandwidth (but cheap) ADC to 10 - 20 +Mhz that I can then give direct USB output to the lappy.

I am 55 yrs old and only started electronics last year, the conceptuals are there, but not the practicalities!

Anyone got any ideas?.:iamwithstupid:

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I could be wrong but I don't think you'll have the bandwidth to get that data rate into the computer.  Sorry :(

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You can buy ready made USB oscilliscopes (2 channel, 20Mhz) for about £40-£50 on ebay, amazon, etc. No idea if they are any good, but I would suspect that they are good enough for most purposes. Never quite needed one enough to buy one, but have been tempted several times :)

On the data rate side, USB2 runs up to 480 megabits per second (in theory), so 8 bit samples at 10 or 20Mhz - no problem. 16 bit at 20Mhz would be starting to get near the practical limit.

cheers,

Robin

 

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Thanks Gina, just a thought.

I didn't think that  the computer was limited as low as audio frequency level and that I might be able to squeeze a bit more. back to the drawing board.

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Thanks Robin, so if 8 bit 20Mhz is possible, if the software I use (Zelscope et.al) can handle it, I should be able to bypass the soundcard with a simple ADC to USB set up, (probably not)!!!!

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Well, the questions would be

1) Can you find a USB2 speed USB ADC? (remember that USB1 is ~40 times slower than USB2, so that wouldn't get you much more than 1MHz)

2) Will the software be able to read the data from it? I guess the software might have some list of hardware it is compatible with, which you ought to check.

[Edit : It looks like Zelscope is sound card input only, so you'd have to look for alternative software ]

Robin

Edited by rwg
further information

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I am probably way off key here, but I thought the soundcard was only used for its ADC (limited) capabilities as a cheap and cheerful option, and that you are feeding the PC with an analog signal that it easily needs to "read", hence the  soundcard. The Pc doesn't actually know that it is a soundcard, merely an ADC device.

 

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There's no need for a really high speed interface if you cache the data in external RAM and only load the parts you're interested in.  A real time display can't be updated at 20MHz and the human eye couldn't follow it even if it could.  Oscilloscopes are typically used to look at repeating patterns, so subsequent scans overwrite the first ones.  For a 20Mhz resolution you could take 2000 samples at 40Mhz sample rate every 0.1 seconds and only have to input data at 20kHz.

Noel

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It's not necessarily the sampling rate that I'm trying to improve but the bandwidth, with a soundcard I would be limited to checking circuits, components up to 20Khz at whatever the hardware / software sampling rate would allow.

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1 hour ago, MarkyD said:

How about this  http://www.technobotsonline.com/windows-usb-oscilloscope-module.html

Not sure how accurate it is but I was looking at it earlier as a project.  If it doesn't work it's only £18 down the drain 

 

It looks very much like a PicAxe (£11.99), that I was looking at earlier as well !

http://www.picaxe.com/Hardware/Add-on-Modules/PCB-scope/

On that page under the "specification" Tab it gives the Analogue bandwidth (scope channels) >300 kHz, for 2 analogue channels + trigger

 

 

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22 minutes ago, SilverAstro said:

It looks very much like a PicAxe (£11.99), that I was looking at earlier as well !

http://www.picaxe.com/Hardware/Add-on-Modules/PCB-scope/

On that page under the "specification" Tab it gives the Analogue bandwidth (scope channels) >300 kHz, for 2 analogue channels + trigger

 

 

Does look exactly the same but £6 cheaper.  Even the software is the same.  I just ordered one - I 'll report back on how it is :)

--

Mark

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Hi all, thanks for the replies. I did look at 2nd hand but most of the ones I saw at sensible prices were "spares or repairs". The Picaxe one looks good, but not very much more in the bandwidth department.

I'm already halfway through my build, I'll just have to go with it for now and if I need more bandwidth in the future will have to get the wallet out again.:confused2:

 

Rob.

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If you want to use it rather than make it - how about a picoscope?

https://www.picotech.com/oscilloscope/2200/picoscope-2200-portable-oscilloscopes?gclid=CjwKEAiA9c-2BRC_vaaJ0Ybps30SJABlqxDeVrYq1Q8Xof6xj4dh5qc2KSKdKsu6EuQQFGiSnwaaLRoC6_Hw_wcB

 

I understand the appeal of making though - I made an 8-bit scope interface using a fast (for its time) DAC and a BBC Master, I think it sampled at up to about 120KHz, not bad for a 2MHz processor :-)

I've got an unpopulated board for a version using an AVR and a QVGA display, which should be able to run  a few times faster.

I use an old Hameg analogue scope for real (I used to have my dad's old Heathkit scope, but between that dying and the Hameg the Beeb scope actually got me out of a hole once or twice!

 

Beeb_scope_2.thumb.jpg.ee325f50d2aa37431

 

Beeb_scope.thumb.jpg.90365eab0ac2c451f07

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Oh Neil did you have to do that, I got Quad Core, I got terrabytes, I got VR GUI's, but god I loved my BEEB.:icon_biggrin:

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CUB monitor, twin Cuman 5 1/.4 drives, Plotmate A3, Model A with extra RAM and DDFS, together with a Master with loads of ROMs and all working - and mostly in the loft :-(

But 6502 assembly language will always be my favourite way to talk to a computer!

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In the early 80's my Brother in law wrote all his own software and ran his whole business (car radiators import, export, custom) off a BBC B,  I also had a friend who could almost write BEEB machine code in his head (weirdo). My claim to fame is I MADE ELITE, not writing it just managed to zap and make more credits than anyone else out  there. LOL.

The weird thing is ELITE didn't get any more fun on higher platforms, or perhaps I'm just getting old and cynical.

I also feel we have gone sort of retrograde, everyone says how wonderful Pi and Arduino's (and they are for a simple platform) , but for gods sake look at the I/O capabilities of the BBC Micro in its day. People in the early 80's were controlling their central heating, house lighting, burglar alarms, etc etc etc  without shields, motor boards and others.

 

OK rant over, calm pills taken.

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> I MADE ELITE

Kudos!

With games like Elite, Sentinel and Defender the basic playability was there. Everything that's come along since adds immersive effects and detail, but some of the simplest games can still be brilliant just because they work. Once the computer reacts realistically and intelligently to what you do faster than you can do it, the basic playability is there.

Lemmings on the Amiga comes to mind as well.

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It's some time since I looked at this but from memory certain sound cards will sample at a pretty high rate but tend to be expensive.

In the end I bought one of these, better value

http://www.syscompdesign.com/CGM-101_ep_56-1.html

I think that the UK sourced picoscope now offer a cheaper model as well.

John

-

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Thanks John, again it looks like limited to 200K, I think I'll finish the build and take it from there,

I still don't see that if I'm not using a "soundcard" why I should be limited to 200K

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I wrote a word processor in assembler for the BBC B including a special character set to get 80 chars to a line.

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From playing with a couple of high end sound cards I don't think they will do a real 200k. The syscomp one samples at 2mb/sec so can actually show what is there meaningfully at 200k in. Lot's of claims are made on the basis of Fourier, 2 samples can reconstruct a waveform. Only true for a sine wave. It's 11 bit too. 2048 measuring steps not 256 like the picoscopes. I have a bit of an interest in audio at times which is why I bought it. It's  also ok for other things within it's bandwidth. Picoscope do one with higher resolution but only at a price. For me faster would be better served by a logic analyser eg Picoscope 2205 = toooooo expensive for the amount if use it would see. The price of their 8bit ones is more attractive now.

There is no reason why you should be limited to 200k. Just gets harder as the frequency goes up. All the PC has to do is receive it and make the data look pretty. Over sampling the max input frequency by a factor of 10 is about right. Sampling scopes get round that but can only show steady repetitive waveforms.

John

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Ajohn,  thanks, I think now I'll just have to go with what I've got and see, this is a new venture for me.

I love " I have a bit of an interest in audio", just spent the last two years on a pre/pre supply/ dual monoblock build. but I think it was worth it. I had this thing in my late teens/early 20's, rightly or wrongly I lusted after Naim gear after hearing at my neighbours.

So when I built my own it had to look like  real clone.

IMAG0037[2].gif

Edited by roblegion
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