Oscilloscope Reading

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alband

Senior Member
This is a print screen of a PC oscilloscope. The source of this little worm is (of course) a PICAXE (pin 1 you may have guessed). The commands it should be going through at this point should be…
Code: 
pulsout 1,1000
pause 2
serout 1,N2400,("1")
I have no idea if this is what it is showing and whether it is even distinguishable. If someone could confirm that it is indeed pulsoutin’ and seroutin’ that would be great.

I could probably produce a better image (bigger scale) if needed but it would take time as it was a good bit of luck I pressed Prt Scrn when I did and a bigger scale would only make it harder (quite a fun game though). :p
 

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Dippy

Moderator
Looks like a 'scope when it is AC coupled.
Dunno whether little scopes like that can be adjusted.
Certainly looks like some capacitance flying around.

If the scale is meant to be mS then your pulsout will be off to the left somewhere.
But it does look a bit odd.

Suggest you save up a £1000 and get a Tektronix..;)
 
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hippy

hippy

Ex-Staff (retired)
There's no reason it shouldn't be what you are sending and it looks like a pulse, a gap and some faster bit sequence so probably as expected.

Presumably you are looking and asking because you have some kind of problem ? Trying to interrupt on a serial line perhaps ?
 
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alband

Senior Member
It can be adjusted. Considering it's free it great. That is ms's. I'm trying to get one with a time base of 40, but the prt scrn is VERY tricky. It a shame I can't record it.
Thanks.
 
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alband

Senior Member
Did you remember hippy? Yes that is the problem. I had 2 40X's talking on a breadboard and now that one of them is "en PCB" they have gone all shy and seem to be sitting in a stubborn silence.
 
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Dippy

Moderator
Well you've got a 10mS pulse = pulsout 1000

And you've got some wobbly things for your Serout.

But those curvy things suggest capacitance. I'm sure you know about exp curves already.

Hence my comment that it looks like it is AC coupled.

If that were on a scope with DC coupling and there were no reactive bits then the pulses would be nice and square.

I'm sure you'll get it sorted and there's no need to post every Print Screen...
 
hippy

hippy

Ex-Staff (retired)
Looks about right. Without the AC-coupling effect you should be seeing something like below ( not to scale ) ...
Code: 
          __________________           _   _____     ___
_________|                  |_________| |_|     |___|   |____
 
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alband

Senior Member
It's using the sound-card input, so that's where the muck is from. Glad to know it looks good other than that.
I'm sure you'll get it sorted and there's no need to post every Print Screen...
Click to expand...
Yeah, I was just thrilled that I got the button at the exact time (how sad). As for the "I'm sure you'll get it sorted"; ooo err. I'm not sure, but since it was working and since there has been a change it has stopped, there can't be anything wrong with the code or the theoretical schematic. I suspect it will be a long process of check's that draw no conclusions then one day I'll sit down and see I have the battery the wrong way round. :rolleyes:
Thanks, though anyway.
 
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inglewoodpete

Senior Member
What you're looking at are typical digital signals on a soundcard oscilloscope. The poor low frequency response (slopes on pulses) is due to the input capacitor of the soundcard. The better the soundcard input, the better the 'picture'.

I used a soundcard oscilloscope for years. I even wrote my own oscilloscope programme in VB2005 after getting fed up with the freebies on the net.

I finally bought a 60MHz USB 'scope a few months ago after getting clearance from the finance committee (aka wife).
 
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Dippy

Moderator
Capacitance confirmed then?
Are ALL souncard 'scopes like that?

To me a scope like that is almost useless. But gives a pretty picture.
Sorry, no offence intended, but I need proper measurements that I can trust.
 
hippy

hippy

Ex-Staff (retired)
When I used a soundcard scope ( cannot remember which software ) I got far better results than that so it could be a particular issue with the soundcard or configuration.

Soundcard scopes are limited in capability and bandwidth and will never match a full featured oscilloscope but IMO are useful in some circumstances, and often better than nothing. Even here, with 'very severe AC coupling', it is possible to get a good idea of what the original signal probably was and compare with what should have been sent, though it takes additional effort and time and isn't perfect.

If you need accurate measurements then the right tool is needed, if it's a case of 'does it look right' a soundcard can do the job in many cases but not all. How "useless" depends on how well it does what one wants it to do.
 
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manie

Senior Member
"I finally bought a 60MHz USB 'scope a few months ago after getting clearance from the finance committee (aka wife)."

WOW ! 60MHZ ! And here I thought I was doing OK with my new 20MHz Analogue jobbie... but then, I don't do RF and such and the freq's in digital fits nicely in the range, but then also it is "old worldly" on the little CRT.

How fortunate we are to actually have a scope to "see" inside the goings-on.... dunno what I would do without it now...
 
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BeanieBots

Moderator
Actually, even 60Mhz is not very fast even for looking at signals which are only around 100kHz. You need what initially seems like excessive bandwidth to look closely at rising and falling edges which might be in the order of nano seconds. This is particularly important when driving things like MOSFETs.
 
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Dippy

Moderator
I think it's very easy to get muddled with these specifications.
I've even seen people get bandwidths confused with sampling rates.

I have 2 Tektronix scopes, the cheapest one being a 60MHz.
It can easily resolve a 40MHz sine wave and will easily show square wave responses of <50nS. It samples at 1GSamples/s.
The other one is a 200MHz bandwidth and samples at 2GS/s.

And my advice is to save up and buy a colour version. When those waves overlap its easy to get muddled. But not as easy as it is to get muddled between bandwidth definitions and sampling rate.

Some 'scopes have some special filling in techniques to improve their display quality when the signal input is at the limit. I believe PICO use that technique. I'm sure the £20 ones off Ebay do too ;)
 
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inglewoodpete

Senior Member
I see Dippy is fired up again. For the record, its one of these 60 MHz / 150 MS/s. It's a Hantek, all the way from China (another of Dippy's pet subjects), although the exchange rate was A$100 better when I bought it.

So far, it's worked well. I'm very happy with it.
 
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Dippy

Moderator
I don't mean to get excitable, besides guess where the cheaper Tektronixs are made now? Whereas my old Tektronix (1999) was made in USA. Still works 100%, still bang on at 20MHz sine.
(Just think, if we could all buy locally-made stuff then the credit crunch would be more of a credit crackle).

No, I was just making a point that som people think that because their 5dollar Scope-Bay device says 60Mhz that it isn't the same as a Tektronix 60Mhz @1Gs/S.
I know you will know that IP, but most schoolboys don't.
Chalk and cheese.
And I won't even ask: "If your Ebay scope shows a signal to be 150Hz, but your Father's Tektronix says 160Hz for the same signal, which one would you trust?"
The answer is that Father is obviously sensible and has saved up his money rather than blowing it on a Wii ;)
Cheapest Tek, is under £600. Bargain.
 
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papaof2

Senior Member
And sometimes you get lucky ;-)

I found a 4 channel Tektronix 2247a on Craig's List http://www.craigslist.org for $100US. The seller bought it when his company cleared out some equipment in their R&D department "because he always wanted one" - hardly a good reason and one that leads to an expensive doorstop ;-)

Now to find an equally good DSO for a similar price...

John
 
fernando_g

fernando_g

Senior Member
Dippy said:
I don't mean to get excitable, besides guess where the cheaper Tektronixs are made now? Whereas my old Tektronix (1999) was made in USA. Still works 100%, still bang on at 20MHz sine.
.
Click to expand...
Tektronix has already paid dearly for moving production to China, as knockoffs are starting to appear on Ebay, which are almost identical replicas to Tektronix, but for 60% less. The Owon comes to mind.

You are absolutely correct about the old Tek scopes. Those were the Rolls-Royces of its era. An acquaintance bought a Tek 565. This is one of the most awesome pieces of test equipment ever made! The thing must be at least 40 years old, and it still works, although some switches or knobs are a little flaky. It must weight at least around 25 kilograms.
 
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papaof2

Senior Member
I have a 545 that was still working the last time I turned it on Typically it's only used in the winter, as that 600 watts of heat isn't needed the rest of the year ;-)

John
 
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