scope selection 1

[richerdick]

anyone care to give me a quick explanation of the relevance of the follwing to the selection of a good digital osscilloscoe.
bandwidth
a/d conversion rate
mhz(i know what it means but what is a recomended reasonable frequency)
i am looking to use the scope on analysis of simple drive problems, currently i use a low end single channel hand held scope, but have found this lacking and would like to get somethng that i can connect to my laptop so i can trend long periods of data. Something that is robust enough to sit on a dc bus but quick enough to look at encoder pulses.
can anyone make any recomendations? bearing in mind that i have used scpes pleanty of times but never had to buy one so i am not sure of the relevance of terms a/d conversion rate. i imagined it might be the resolution of the measured anaogue signals but with many of theusb type scopes that i have looked at stating an 8 bit resolution this seems a bit low
thanks for any advice

 

[ScottyUK]

I am giving very serious thought to acquiring one of the Tektronix TPS2000 series scopes. We do a lot of work on high power circuits and the built-in isolation is a major plus for me. I've always had a preference for Tek's scopes - very reliable and easy to use.

You really need to state your budget. My idea of a budget scope is probably different to yours, which is different to the other guy's opinion.

http://www.tek.com/products/oscilloscopes/tps2000/index.html

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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

There should be an FAQ on this. I don't think there is one.

First of all. Isolated channels are good. But not necessary. You do not look at motor voltages or DC bus voltages all the time. Use an isolation amplifier in those cases. Then you can widen your choice to include units with common "Common" aka scope ground.

Use fused probes whenever you work with high-powered circuits.

Get a decent DC current clamp. That will make observation of motor and mains wave-shapes a snap.

Now, the scope selection.

Eight bits are considered sufficient. That will give you a better than .5 % resolution. Twelve bits are coming and I am using one such unit since a few weeks. More about that later.

Sampling speed. You need to sample five to ten times faster than your highest frequency component. For a frequency inverter with, say, 16 kHz switching frequency, that would imply 100 to 200 kSa/s. And that is a very low value. You usually don't find hand-held scopes or USB scopes with less than 10 MSa/s.

Encoder measurements are probably the most demanding. A 4096 PPR encoder running at 12 000 RPm (unusual, but not unheard of) will output a little bit more than 800 000 Hz on each channel. A quadrature signal has four times that number, around 3.2 MHz. And, to be able to resolve that and check phase relationship, you need at least five times that BW. Ten times is better. So, you end up with a scope having 15 - 50 MHz analoge band-width, eight bit resolution and two channels. Relaxed encoder and speed specs makes a 10 MHz scope a possible choice.

I am using a four-channel 200 MHz BW LeCroy WaveSurfer. It is quite good, but somewhat pricey. Also takes some space and not so good when going by air.

I have also been using Pico Technology, many different, over the years. Dual channel USB scopes that have quite good specifications, but I am not happy with their input protection. Very short transients just above 20 V kill the inputs. And no guarantee covering that. So, I have started using other scopes.

One little device - and very good price - is the Shark USB scope. It doesn't cover high-speed applications. I think that BW is in the 1 -2 MHz range. Dual channel.

The one I am currently using is a TiePie HS4. It has 50 MSa/s, four channels and a very good SW that lets you plug several units into a powered hub. That turns a computer into a very versatile, high-speed, multichannel (16, 20 or more channels). It has scope, DMM, recorder and FFT built-in and has quite good triggering flexibility.

Sorry - breakfast is waiting.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[nbucska]

Just yesterday I have seen a TEkTRONIX 2430 for a fraction
of the new price at TIGER-TECH in Longmont, CO.
(303) 651-7919

Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[richerdick]

Thanks for the advice would anyone who has comented take alook at the following and give me their thoughts

http://www.vellemanusa.com/us/enu/product/view/?id=500243

i know its a bit of a low end version but do you feel it will meet my requirements bearing in mind that i am aware of the safety considerations when measuring a dc bus

 

[Skogsgurra]

It has all you need for drives. But, if you go to customers doing troubleshooting, they may expect you to bring something more "authorised". It is mostly psychology. The 230 does everything you need. Also fast encoders.

You may need FFT sometimes - think that over. And, the RS232 is slow and is also getting rare in new laptops. A USB connection is better.

Do not forget isolation amplifier and current clamp.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

30Mhz repetative bandwidth is low for a DSO. It will miss or corrupt short fast transients and the single shot capabilities will be limited. If you are working on fast switching power supplies operating in the high kHz range then you may struggle to pick out detail of the switching such as reverse recovery with such a low bandwidth, but to see it you'd also need fast isolation probes and / or current probes. Fast current probes in particular are expensive. If you're limited to slower higher power stuff then it looks interesting. Any idea on pricing?

Velleman used to do a range of hobby kits which looked to be at the upper end of that market - pricey but arguably worth it to those who either haven't the time or resources to make everything.

Skogs - on the subject of isolation amps we agree that they are a very good thing. There are very few excuses today for cutting the ground to a scope chassis to allow the instrument to float. The reasons I like the idea of them being built in to the Tek scope I mentioned earlier are: I need a new scope anyway; it is battery powered when I need it to be which is frequently; my isolation amps devour batteries at a scarey rate. Oh, and work is paying for it!


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Sometimes I only open my mouth to swap feet...

 

[danw2]

sounds like what I'm looking for:

>One little device - and very good price - is the Shark USB scope. It doesn't cover high-speed applications. I think that BW is in the 1 -2 MHz range. Dual channel

Skogsgurra,
Can you provide a link to this? ? I couldn't find it in 4-5 pages Googling

Dan

 

[Skogsgurra]

No wonder you couldn't find it. It is "Stingray" not Shark.

Also, it does not have 1 MHz single shot bandwidth. But it has 12 bit resolurion and two channels. So it is OK for low frequency signals. Probably perfect for sound. SW is OK, has scope, recorder, FFT modes. I bought one to test in a job where I needed to leave puter and A/D for a long time (customer provided PC). And it did its work. Can't remember price. Think it was around USD 200.

http://www.usb-instruments.com/documents/small_stingray.pdf

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...