# Shopping for oscilloscope



## Tesseract (Sep 27, 2008)

You can get by with 2 channels but they need to be fully isolated if you are going to be looking at the motor waveforms. That sorta knocks out most of the inexpensive USB oscilloscopes right there, IIRC.

Probably a portable/DMM-like scope is going to be the least-expensive option that is - by default - isolated, but make sure there is channel-to-channel isolation as well.

You need at least twice as much bandwidth as the fastest sine wave signal you want to look at. If the shape of the waveform is important then you need 10x.


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## etischer (Jun 16, 2008)

I bought a fluke 97 for around $450 used on ebay. Scopes with knobs are easier to use but for as often as I use it, I don't mind it having only buttons. It seems to be much more reliable than analog scopes. It also can display cursor position data for displaying dT, dV and calculate measurements like Pk-Pk, frequency, rise time... 

It is also very portable and can run off batteries. It's nice to know it is isolated, especially when I know my pack/charger is not. For the price, it's not bad, I'm happy with mine. 

http://etischer.com/awdev/encoder/IMG_8631.jpg


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## steven4601 (Nov 11, 2010)

Only if, you can afford get a newer model with the white background. Those green ones are (ill put it mildly) not as nice compared to a 99B.

The menu's are improved quite a bit. Make sure the firmware is one of the latests, I used to work where they made these scopes. 

Small world eh.


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## david85 (Nov 12, 2007)

Isolated - and that would by why I asked here first!

Having seen what Eric was able to do with his fluke 97, I was already looking at that.

So how do I get a rough idea as to what sort of frequency I need based on my motor and controller specs? I'm assuming its related to the highest RPM, right?


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## etischer (Jun 16, 2008)

The switching frequency on my inverter is only ~3.5khz. The scope is 50hmz, so speed isn't an issue for me. 

Here's a shot of the signal that fires off my IGBT. You can setup the cursors to get a real accurate measure of time and difference in voltage. You can also have it calculate. If I had the choice between an older analog scope, and the fluke, I'd pick the fluke. Seems the analog scopes I've used don't like to be moved around much. 












david85 said:


> Isolated - and that would by why I asked here first!
> 
> Having seen what Eric was able to do with his fluke 97, I was already looking at that.
> 
> So how do I get a rough idea as to what sort of frequency I need based on my motor and controller specs? I'm assuming its related to the highest RPM, right?


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## helluvaengineer96 (Oct 8, 2010)

Man I love o-scopes... Just was on ebay the other day reminiscing. I'm looking for one too.. A little surprised by the high price for used scopes.
Anyway here is what I would recommend.


Digital. You don't want to mess with an analog.. But you need to worry about alaising (sp?).
No PC junk, fine for logic analyzers, but would not use for a scope.
2 or 4 channel.... can make due with two.. but with less probes a trigger becomes more important.
Trigger options are important.. usually you'll be looking for a certain event. LeCroy's easily have the best triggers.
Sample rate... Nothing happening in the an EV that is all that fast, but you might need to watch for impulses... but 40 Mhz sample is plenty.
Probes probes... quality is important and of course they need to match the scope.
Relatively compact... Want to be able to see it while you drive right?

An OK in-exepensive scope that is small and can be used while driving is an Tektronix TDS210.. These were the poor man's scope when they were new. But they are just about ideal for an EV I would think... Note I've not used one for an EV...but will be soon.

Good luck.



david85 said:


> I'm starting to order parts, components and tools needed to put together my own BLDC controller and would like some imput on what to get for oscilloscopes.
> 
> I'm thinking something that can plug into a laptop since I have one but I'm wondering what sort of features I need to look for. Do I need something with 4 channels or are 2 enough?


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## david85 (Nov 12, 2007)

Alright, so PC based is loosing ground....

So far we have the fluke 97 shown here:









Is there any reason not to buy one used?
http://cgi.ebay.com/FLUKE-97-50MHz-...570?pt=LH_DefaultDomain_0&hash=item4aa6e8b192

And then we have the Tektronix TDS210.








http://cgi.ebay.com/Tektronix-TDS21...10842?pt=BI_Oscilloscopes&hash=item19c14e367a

I have no idea if these prices are typical but one thing I like about the Fluke 97 is that it's battery powered and can be taken even on the road.
Can the Tectronix do that too?


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## peggus (Feb 18, 2008)

It is not the switching frequency but the rise time and the frequency of the transients you are interested in. 500nS rise time is 300kHz, parasitic transients and ringing can be much higher. A 50MHz scope is sufficient for most power electronics though.


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## steven4601 (Nov 11, 2010)

peggus said:


> It is not the switching frequency but the rise time and the frequency of the transients you are interested in. 500nS rise time is 300kHz, parasitic transients and ringing can be much higher


I agree with the first part. The second part needs more explaination. A 500ns slope such as 500nS from 0 to 600V is requires quite some bandwidth to display propperly. Its not a fraction of 2Mhz, its multiples! Id guess a scope of factor 20 the fastest slope would suffice for indicative measurements.


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## Qer (May 7, 2008)

david85 said:


> Alright, so PC based is loosing ground....


They never really had any ground. PC based scopes have one advantage; price. That's pretty much it, really, unless you want to do automated measurement or maybe log results over extremely long time periods. Gee, soon time for the Spanish inquisition if I'm gonna add reasons like this. 



david85 said:


> So far we have the fluke 97 shown here:
> [...]
> And then we have the Tektronix TDS210.


You're at a cross road here. 

Do you want light and portable or stationary (even though most stationary scopes can run on battery and are pretty light weight nowadays)? Tesseract likes his portable scope because he can bring it everywhere, personally I like the stationary better since they have a better user interface (knobs and a multitude of buttons instead of small, cryptic buttons and nestled menus).

Not to mention the real advantage with stationary scopes, as I see it. My multimeter sooner or later always becomes part of the spaghetti that's the contraption I'm testing, my scope always is a bit off and thus still accessible without me risking short circuits, sparks, smoke etc. On the other hand a stationary scope is an excellent place to stack piles of junk on, which makes it very stationary, while the portable scope isn't as inviting to stack stuff on but instead it's easier to misplace and you have to dig it out every time you need it.

So, you'll have to choose what hell you want to live in.


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## O'Zeeke (Mar 9, 2008)

David, check this out http://www.afrotechmods.com/groovy/oscilloscope_tutorial/oscilloscope_tutorial.htm


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## peggus (Feb 18, 2008)

steven4601 said:


> I agree with the first part. The second part needs more explaination. A 500ns slope such as 500nS from 0 to 600V is requires quite some bandwidth to display propperly. Its not a fraction of 2Mhz, its multiples! Id guess a scope of factor 20 the fastest slope would suffice for indicative measurements.



I wasn't saying that 300kHz would be sufficient for accurately display a waveform with 500ns rise time, it was just an illustration that frequencies you are interested in are higher than the switching frequency, perhaps I should have been clearer on that point.


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## helluvaengineer96 (Oct 8, 2010)

You would need to move up to the Tek TDS3k TPS2K series for battery in the Tek family. So if you want an inexpensive battery then the fluke 97.. I've used the 97 before it is adequate, the trade off will be ease of operation and screen refresh speed. I don't have the patience for the 97 so I'll be using Tek with car inverter.

Either way they are both good enough for anything in an EV. 


*To answer this question:*
"Is there any reason not to buy one used?"

Probes--- Probes get physically destroyed pretty easily.. So in buying a new scope you know you will get new probes. I plan on buying used scope and new probes.

Calibration - New is guaranteed to be calibrated... Given what we are doing.. meh.. I won't worry too much about calibration. Tho some folks are really insistent on calibration.


*Last note on probes*: most STD probes are good to 150V to 300V, after that you need to rig a cheap voltage divider or buy a high voltage probe (expensive). I am going the cheap route and using a voltage divider.. If going the voltage divider route remember you don't want to use resistors that are too high in value, the capacitance of the probe would impact results if you use too much resistance. 
At high voltage probe point recommend you solder in resisters and a dip switch.. That way you can turn off the small current draw when not probing. Make sure resistors are selected that don't allow too much current but enough to overcome probe capacitance.



david85 said:


> Alright, so PC based is loosing ground....
> 
> So far we have the fluke 97 shown here:
> 
> ...


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## JimDanielson (Oct 19, 2008)

I've been looking to buy a cheap o-scope for awhile. This rigol has great reviews, and some youtube video reviews too. http://www.amazon.com/Rigol-DS1052E...3VOA/ref=sr_1_1?ie=UTF8&qid=1292181082&sr=8-1

It has the option to sync to PC for data logging, has knobs for ease of use, color screen makes it easy to distinguish between the 2 channels, and you could always get an inverter to run a bench o-scope in the car.


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## Weisheimer (May 11, 2009)

Here is a popular portable scope with enough speed to satisfy your needs.
200 mhz bandwidth and 1 gs/s sample rate.

These folks are good to deal with too.

http://cgi.ebay.com/Tektronix-THS73...51575?pt=BI_Oscilloscopes&hash=item5196b5db77

These have a built in DVM as well.


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