rookie scope question

[chancecasey]

Can analog scopes "record"? Seems the drawback with DSOs is the low resolution and sampling rate - but, if analog scopes are capturing all that "analog" data - seems it would require lots of storage like a tape or something. If they can record, is there a way to get it to your PC?

 

[RadioRon]

Nope, regular analog scopes can't record. However, before there was digital storage, there was a thing called an analog storage scope. These had the ability to store one or more traces on a screen using a special screen phospor. But they were not as useful as the DSO and usually more expensive than the analog scope.

There was also an add-on box for some units called a storage normalizer. This would transform an analog display into a crude digital storage one.

Actually, the most common storage method back in the old days was where the human operator would observe the analog scope, then sketch the waveform into his notebook using that old word processing tool, the pencil. Keeping a log book was a good practice in those days and good engineers nowadays still keep such logs, but often they are kept on a pc.

 

[ericgibbs]

hi,
In the 'old' days, many of the higher quality CRO's came with a camera attachment located over the screen. The camera was triggered by a sync pulse from the CRO. The pictures were excellent.

Like radioron I have sketched CRO traces, one method is to use a a china graph pencil and trace over the displayed trace, a little messy.

You can also buy a PC prn port module, [12 bit adc] with software, it works
but its not high resolution.

You can still buy second hand storage scopes.

Regards
EricG

 

[Hero999]

hi,
In the 'old' days, many of the higher quality CRO's came with a camera attachment located over the screen.

That's a good idea.

How about just taking a picture with a digital camera?

 

[ericgibbs]

hi hero,
If you planned to use 'film' camera.
You would require a fast film, with a suitable colour response.
I am sure the local camera shop would advise.

Why don't you give the digcam a go, if it works let us know.

A home made module to detect the trace start sync, to trigger the camera
should be possible.

EricG

 

[ericgibbs]

hi hero,
Just tried a quick test using digicam and CRO.
Attached a couple of pix, camera hand held, manual sync.

Used Paint Shop to reduce to monochrome and cropped.

Using PSPro the quality/contrast could be improved, some interesting effects.

Encouraging for anyone else who would like to try it out.

Regards
EricG

 

[Nigel Goodwin]

No disrespect, but those are REALLY boring scope pictures!. They are also much bigger than they need to be, because you kept them as JPG.

 

[Revolvr]

I photograph the scope images ocasionally using a digitalcam. Turn on the macro feature, set shutter priority to force 1/25 sec or faster - about the slowest useful handheld shutter time. Don't use flash. Consider taking another shot of the entire scope to capture all of the settings.

These don't usually need much post-procesing. The most I will do is correct barrel distortion common at close-up macro shots, perspective correction if I didn't hold the camera straight. Sometimes brightness and contrast. For posting I resize and increase the jpg compresion.

Extra credit - what might be causing this waveform and what is wrong with the circuit?

-- Dan

 

[audioguru]

Extra credit?? For marks in school?? Which school?? Who's school??

The positive-going part of the waveform has ringing then has a nice flat DC-coupled top. The negative-going sags back like it is capacitor-coupled. Weird.

The real photo looks a lot better than those black and white blobs.

 

[Nigel Goodwin]

These don't usually need much post-procesing. The most I will do is correct barrel distortion common at close-up macro shots, perspective correction if I didn't hold the camera straight. Sometimes brightness and contrast. For posting I resize and increase the jpg compresion.

Now that's the sort of picture we're talking about!.

Extra credit - what might be causing this waveform and what is wrong with the circuit?

It's called ringing, I wouldn't suggest a cause without knowing the circuit it came from!.

 

[ericgibbs]

Hi nigel,
I agree the pix are boring, I didn't post them as a work of art, just to see
what results I could get, as a first off.

The pix posted by revolvr are first class, I'm sure the OP will now have something to encourage him to try for himself.

Chill out, dont be so negative at least we are trying to answer the OP with
the resources we have available.

Best Regards
EricG

 

[Revolvr]

Extra Credit was just to see what people could come up with.

For those interested, I have been fiddling with boost converters recently. The photo is the voltage across an inductor being charged and discharged by a square waveform, into a Schottky diode and a 100uF cap - the circuit acting as a DC-DC boost converter. I am not sure what causes the ringing, and the output is not efficient - cannot get much current out of it. Probably poor selection of choke value, too much choke internal resistance.

I was using a PIC to generate a PWM waveform, connected to two Pots so I could dynamically vary the duty cycle and period. Also connected a variable voltage supply to the input to the choke.

I took other photos too, some showing no ringing - good square waves, others ringing like a bell.

Cheers,

-- Dan

 

[Dean Huster]

The standard scope phospher was P31. A P7 or other "blue" to "near UV" phosphor was an option on high bandwidth scopes for companies needing single-shot photos at very high sweep rates using fast film. Even though the highest speed analog storage scopes such as the Tektronix 7834 had high writing rates, it still wasn't as fast as the special phosphers and fast black and white film. Companies doing nuclear and laser research used the latter methods a lot.

The analog storage version of an analog scope was ALWAYS more expensive and less reliable. It's only the $30,000+ digital scopes that have storage speeds and resolution equal to the 1980 analog high-speed storage scopes. Affordable DSOs in the $2000 range are nice, but limited. They'll be perfect for what most of us work on.

Dean

 

[chancecasey]

Wow - thanks for all the input! Seems the other disadvantage of digital, besides resolution, is memory. A few thousand data samples is the norm for older, affordable used ones. Should still be sufficient for my immediate need, which is to record a very short amount of a 3 or 5 MHz signal, then see if I can decode the data contained in it. I'll post up some pics if I can ever get my hands on one. Darn things on eBay always seem to rocket up in price right before the auction ends...

 

[Sceadwian]

They're always hot buys, it does take a while, but all it takes is one decent bid on an item that 'falls through the cracks' and you save huge bucks, depends on what you want how bad you want it, and how little you have to spend =) DSO's can be useful but are also over rated. You can do a LOT more with a decent analog scope. If you need a logic analyzer you can build those with an AVR or a PIC. AVR's make excelent logic analyzers as their instruction clock runs at the same rate as their system clock so you can sample a LOT of I/O lines VERY fast and then send them serially to some other device for processing or logging. A good ADC will even turn one into a low voltage scope.

 

[chancecasey]

Wow - that makes total sense - not a job for the beginner, but I see exactly what you're saying. What's the clock speed of the AVR?

 

[Sceadwian]

For analyizing logic signals the code is really pretty straight forward, basically you just read the input pins at a specific clock rate using a timer or a calibrated delay loop and send the data to a PC or store it locally. The biggest limit is the data transfer to the PC or storage. The fastests AVR's I've heard of are clocked at 20mhz. They can be overclocked but that's sort of forbidden teritory =) You could probably sample around 1mhz easily,with a 20mhz clock faster if you wrote really good code. A single I/O port is 8 bits wide so more channels is just a matter of adding a few more instructions. As I said though, storage or transfer to an external device is the trickiest part.

 

[chancecasey]

Never saw atmel and pic stuff before - just checked out the atmel web site, pretty amazing. Looks like they have USB comms built in now.

1Mhz makes the "old" DSOs capable of 100 MHz sample rates seem pretty impressive by comparison, but I guess if you have the luxury of fabbing your own chip design, the sky (or silicon) is the limit.

Might be fast enough for info on a 5Mhz carrier - depending on how quickly the info is modulated. I'm assuming 1Mhz is the sampling rate. As long as the modulation happens no faster than the sampling rate, you should be golden for attempting demodulation right?

 

[Sceadwian]

How much does a 100MHZ DSO cost? AVR's are usually under 10 dollars per chip. And no, your maximum frequency still follows Nyquist, half the sampling rate, any transistion shorter than half the sampleing rate could be missed, all logic analyzers are is a 1 bit DAC.

 

[chancecasey]

100Mhz sampling rate in a DSO could be had in the $200's on eBay - but probably without a lot of extra features. There was a pretty nice HP which had 500/1000 Mhz sampling rate with 4 channels and a few other goodies just went for $366 - which i just missed out on. item 260077490508