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Which oscilloscope?

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Steve Conner

Mon Jul 09 2012, 07:14AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Telequipment was a British company that was bought out by Tektronix. They had their own range of scopes, which were OK but not to the same standard of performance and build quality as Tektronix. At least that's my opinion, having owned several of both makes.

Ash Small

Mon Jul 09 2012, 09:07AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

That's a fair point about sampling rate. I've not used mine above 1MHz yet, also the signal starts to distort at less than this, and my probes are only rated for 100kHz. I also suspect that it has 'soggy electrolytics'.

I bought it with the intention of building a 5kW, 25kHz H-bridge (a fairly low frequency to keep switching losses and core losses low). I should start winding the 50kV secondaries sometime soon (the difficult bit). I've nearly finalised the design for the formers/bobbins.

I've got used to the distortion at higher frequencies, which I assume are mostly to do with the probes, and can compensate to a point when interpreting the waveform, so it's still quite a useful cheap 'scope.

The main reason I decided on this one was that it was local so I could collect it myself, rather than have it sent by post and possibly get damaged. I am looking for a better one though, at the right price, and will see if I 'get lucky' at the Poole Hamfest next month.

I do have some better probes somewhere, but I've not found them yet. I was given them about 20 years ago as part payment for a job I did, and they are in a box somewhere.

PhilGood

Mon Jul 09 2012, 09:16AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Patrick wrote ...

for noobs, sampling rate seems to be the goofing point.

Paul_J. wrote ...

Hi ash,
Yes, I was referring to the 10Mhz range. To make accurate measurements, you should only measure signals 1/10 the bandwidth.

To clarify something:

After reading many digital scopes descriptions, I noticed some are said to have a bandwidth of 1/10 the sampling rate, while some others pretend to have a bandwidth of 1/3 the sampling rate.

Can you calculate true bandwidth (I mean the max frequency where you will get accurate measurements) from the sampling rate ?

Or does it mostly depends on the shape of the signal you are measuring ? (I would say a square signal requires less samples than a complex signal to display properly)

Steve Conner

Mon Jul 09 2012, 10:00AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Analog bandwidth and sampling rate are two separate variables that the scope designer can control independently.

At one extreme, my little battery-powered Tek 222A has an analog bandwidth of several MHz but a sampling rate of only 1MS/s. It uses equivalent time sampling (ETS) to get an effective sample rate of 10MS/s on repetitive signals.

At the other extreme are the newer "Real-time" DSOs typified by the now obsolete Tek TDS2xx series, where the sampling rate is made much higher than the analog bandwidth (the TDS210 had 60MHz bandwidth and 1GS/s) and they don't bother with ETS.

I believe they use some sort of noise shaping and oversampling to achieve the high sample rate cheaply, so while the ADC might run at 1GS/s, it probably only has 1 or 2 bits.

The analog bandwidth of the Seeed Studio pocket DSOs doesn't seem to be specified at all, which would make me uneasy about using one for serious work. Even in the digital age, analog voodoo is still needed to design a good scope input amplifier. Tek and HP have been making their own custom analog ICs for this since the 1970s, because they couldn't get good enough performance from discrete components.

If you still want one, they can be ordered direct from Seeed Studio.

PhilGood

Mon Jul 09 2012, 03:29PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks Steve for the clear and detailed explanation

And thanks for directing me to Seed studio, I didn't know them, they have some interesting products and their forum gives me more than I need infos and user feedback

Hmm... the SeeedStudio DSO Quad implementation is $40 more than the noname chinese one, do you think they use better quality hardware ?

According to some user tests, true analog bandwith seems to be around 2MHz.

After a quick look at the forum support section, I can already see many issues... like :

Seeedstudio forum wrote ...

Another issue involved here is that when you increase the T/Div, then the sample rate decreases so as not to over-run the fixed sample buffer. So unless the T/Div is set at the faster T/Div, then the sample rate will decrease...this is why it is recommended to use the fastest T/Div possible in conjunction with single-sweep to keep the sample-rate as high as possible. Then scroll across the stopped buffer to see all of the waveform.

I might stick to an analog scope, seems a good digital one is a 4 digits price

EDIT:

I've been waiting till offer ends, expecting a last minute bid, but it seems it didn't sell. Wasn't it a good oportunity ?

PhilGood wrote ...

PS: What do you think of this one : Tektronix 5441, very good condition, 170â‚¬

Seems really cheap for such a scope (well actually 170â‚¬ is not the price but the higher bid, bids end in 12 hours)

Ash Small

Mon Jul 09 2012, 09:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

It does seem to be a good scope. If I had the cash I'd get an analogue 'scope like this one.

It will probably be re-listed, quite possibly at a cheaper starting price.................

PhilGood

Tue Jul 10 2012, 01:23AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks Ash :) I'll keep an eye on it. And a good point is it's not far from where I live, I could go fetch it by car.

__________________________________________

NEW - ATTEN AT7328 20 MHZ DUAL CHANNEL ANALOG OSCILLOSCOPE

Less than $200 for a new 20MHz dual channel analog scope seems quite cheap. But for sure quality is nothing comparable to Tektronix or other good brands. Specs here:

I first found it on Ebay for Â£173

Then I googled for it and I saw prices go from less than $200 to $400 for the same new item. Weird eh ?

$187

- â‚¬207

- $310

- $370 on Ebay.com

- $399

How comes there can be such differences for the same new product ? Could it be some defective serie that is sold half price ?

GeordieBoy

Tue Jul 10 2012, 03:15PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

In my opinion one of the biggest shortfalls of these cheap low-end digital storage scopes can be that they often do not perform proper sinc interpolation on the data before displaying it.

The ARM DSO nano's I have seen reviewed just join the sample points with straight lines which means that sinewaves look very distorted and triangle-like unless they are greatly oversampled.

It's a shame that the post-processing of the sampled data isn't done properly before it is displayed because it would then be able to display decent looking waveforms up to a few MHz. As they stand the useful bandwidth is limited.

-Richie,

Steve Conner

Tue Jul 10 2012, 08:18PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Looked at from another perspective, I've seen people turn sinc interpolation off when it is available because they feel that it adds artifacts like the classic Gibbs ripples. Put another way, it might make sine waves look better, at the cost of making square waves look worse.

In the original Tek real-time DSOs, I think sinc interpolation was part of the oversampling architecture and couldn't be turned off. However the limited analog bandwidth didn't let enough HF energy through to really show the Gibbs effect, and the result was a display not unlike a traditional analog scope. I think Tek planned it that way.

GeordieBoy

Tue Jul 10 2012, 08:53PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

I hear what you're saying Steve, and I totally agree that some people prefer linear interpolation for displaying digital signals because they perceive sinc interpolation to be adding ringing and overshoot to fast edged digital signals. (It should be a switchable option, alongside linear interpolation, and probably that "sample points only" option that nobody ever seems to use!)

Whenever possible I prefer to capture the entire spectrum of digital signals if I can, so that I can actually see the "analogue behaviour" that is the rise and fall time clearly and comfortably within the bandwidth of the scope. Then the sinc interpolation gives you a nice clear picture of the true waveform being measured. Obviously this can become very expensive or not viable when looking at very high speed logic signals though.

-Richie,

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