Author Topic: Which oscilloscope for $100  (Read 12953 times)

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Online Fungus

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Re: Which oscilloscope for $100
« Reply #75 on: May 29, 2017, 12:11:41 am »
Because it's a "Toy" rather than a useful piece of test equipment.  ::) Maybe you like the user interface though its pre-modification MP3 player buttons?

I hate the user interface but it's a useful tool if that's all you've got or can't use a real 'scope because you're outdoors.
« Last Edit: May 29, 2017, 12:25:48 am by Fungus »
 

Online Gyro

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Re: Which oscilloscope for $100
« Reply #76 on: May 29, 2017, 12:21:35 am »
Certainly a Quad might be useful out in the field when nothing else is available. It remains to be seen whether the OP is outdoors or has a bench. From his original scope suggestions I would hazard a guess that he has a bench (and a computer).
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Offline timb

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Re: Which oscilloscope for $100
« Reply #77 on: May 29, 2017, 12:32:08 am »
Err, because it's Single channel, 200kHz (at best!) 1Msps of course. :palm:  That's a catastrophically bad deal compared to what else you could buy for the same money. As you said yourself "It's a toy"!

Ok, if you want to stay in the DSO domain then the VDS1022 is [Edit: 2 channel] 25MHz (verified) 100Msps. A working second hand Analogue scope of ebay would get you 50-100MHz for the same price.
But now you are looking only at bandwidth. I'm looking at doing single shots, low frequency signals, measurements, storing screenshots, etc. IOW: everything an analog scope sucks at. The right choice depends on the intended use of the OP.

To be fair, you can do single shot *and* "screenshots" with an analog scope by simply using a digital camera. (Okay, it's a pain and the ass and I wouldn't want to do it every day, but in a pinch it *does* work.)
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Offline David Hess

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Re: Which oscilloscope for $100
« Reply #78 on: May 29, 2017, 12:42:43 am »
Name something an analog scope is better at (and don't come up with something which is due to people staying stuck in old habits)!

Does that include a comparison against digital storage oscilloscopes which should be able to do something but are broken by design?

Given the choice, I would recommend a reliable DSO but if the cost needs to be as low as possible for a given level of performance, then a used analog oscilloscope is going to deliver more.

1. RMS noise measurements (1) - Rigol's RMS measurement function is completely broken as far as noise measurements and I assume this is because of making measurements on the already processed display record but I wonder if Keysight's DSOs which do the same thing have the same problem.  This makes points 2 below more difficult to evaluate.

2. Noise - Dave made a video which discussed whether analog oscilloscopes had less noise but he did not make any quantitative measurements.  Comparisons between my own measurements on analog oscilloscopes and various quantitative reports from EEVBlog indicate that modern DSOs usually do have a lot more noise than they should.

3. Add and invert differential measurements on an analog oscilloscope do not suffer from multiplied quantization noise. (2)

4. You can get a lot more bandwidth for a given price from a used instrument and used analog oscilloscopes are more repairable than used digital storage oscilloscopes; most of the later lack service documentation.

(1) It is a little counter intuitive that RMS noise measurements can be made on an analog oscilloscope but it is surprisingly easy to do using the tangential method and accuracy and repeatably are good.  This is time consuming compared to an automatic measurement on a DSO but many of them cannot make this measurement accurately at all even though they should be able to.

(2) Some very old DSOs do not either because they do the subtraction in the analog domain before quantization.  A differential probe solves this problem and has other advantages but they are not inexpensive.
 
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Offline nctnico

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Re: Which oscilloscope for $100
« Reply #79 on: May 29, 2017, 02:14:16 am »
You can circumvent using pre-processed data by choosing the timebase so that the actual samples are used for a measurement. Besides that many DSOs have FFT which could tell you something about the frequency spectrum of the noise as well. When doing differential measurements using 2 channels I always had more trouble to get the vertical range setup in a way I still got some visible signal without overdriving the input. Also DSOs have averaging, hi-res or (even better) input filtering which cleans up noise from a signal nicely.

I'm not so sure about cheap high bandwidth either. I've owned my fair share of high bandwidth analog scopes but the really high bandwidth ones went for fairly high prices. Nowadays a Tektronix TDS500 or TDS700 series is a good choice if you want a compact 500MHz DSO for relatively little money. The service manuals including schematics are available and many people know how to repair these. The same goes for many older Lecroy DSOs but these are larger and heavier.
« Last Edit: May 29, 2017, 02:16:20 am by nctnico »
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Offline David Hess

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Re: Which oscilloscope for $100
« Reply #80 on: May 29, 2017, 04:12:11 am »
You can circumvent using pre-processed data by choosing the timebase so that the actual samples are used for a measurement.

If I understand what you wrote correctly, a long record length would make this worse.  Won't that be a marvelous opportunity for marketing to advertise their shorter record lengths and lower sample rates which will allow accurate RMS measurements to be made.

Or they could just fix it.  It is not like we have not known how to make reliable RMS measurements on DSOs (and sampling instruments) for decades.

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Besides that many DSOs have FFT which could tell you something about the frequency spectrum of the noise as well.

This would be more useful if one could take for granted that DSO FFT functions worked to make absolute quantitative noise measurements.  Almost all fail and offhand I do not know of any which work correctly.  This EDN article discusses the problem.

I do not have any DSOs with an FFT function that I trust for this so at low frequencies, I use a 7A22 to make absolute RMS spot noise measurements.  Then with some math I can usually derive the 1/f noise corner frequency.  None of the budget DSOs I have tested could make this measurement with their FFT function.

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When doing differential measurements using 2 channels I always had more trouble to get the vertical range setup in a way I still got some visible signal without overdriving the input.

That is definitely a problem and one of the reasons that better oscilloscopes place the overdrive limit outside of the visible display.  One of the tricks to getting good results on an analog oscilloscope is to use the variable function on one channel to exactly match the gain of both channels; to me it is eerie to see the visible noise suddenly decrease to a minimum when CMRR (common mode rejection ratio) is maximized.

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Also DSOs have averaging, hi-res or (even better) input filtering which cleans up noise from a signal nicely.

Averaging and high resolution acquisition mode are very powerful functions.  Input filtering is common on all oscilloscopes to one extent or another.

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Nowadays a Tektronix TDS500 or TDS700 series is a good choice if you want a compact 500MHz DSO for relatively little money. The service manuals including schematics are available and many people know how to repair these.

The only service manual schematics I am aware of are for the old TDS series are for the TDS520.  Back when I was looking for an inexpensive used DSO, I stayed with the 2230/2232 and 2440 series for this reason despite their disadvantages.  The known problems with many of the TDS series like leaking capacitors was a factor also.

Despite their low used cost, I tend not to recommend the 2230/2232 or 2440 series to beginners.  They meet my minimum requirements for a DSO but their age makes for low reliability and beginners are not likely to be in a good position to maintain them.
 
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Offline nctnico

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Re: Which oscilloscope for $100
« Reply #81 on: May 29, 2017, 04:30:49 am »
The Tektronix TDS500/TDS600/TDS700 are basically all the same when it comes to the things that break often. AFAIK there are some TDS600 series services manuals out there as well. I've worked on a few of these which I bought with a defect and the TDS520B component service manual always got me where I needed to be. However with any repair: you have to have a talent to fix stuff when it comes to complicated problems because a schematic doesn't tell you which component is bad.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline daveyk

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Re: Which oscilloscope for $100
« Reply #82 on: May 29, 2017, 04:37:28 am »
The Tektronix TDS500/TDS600/TDS700 are basically all the same when it comes to the things that break often. AFAIK there are some TDS600 series services manuals out there as well. I've worked on a few of these which I bought with a defect and the TDS520B component service manual always got me where I needed to be. However with any repair: you have to have a talent to fix stuff when it comes to complicated problems because a schematic doesn't tell you which component is bad.

I'm still going to piss around with that TDS544A. I ordered an entire front end for it; payed $54. If that don't fix it, then the acq board still has problems. I replaced all caps and scrubbed the board with simple green and ISO. So now I know who to ask for help - lol.


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Offline james_s

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Re: Which oscilloscope for $100
« Reply #83 on: May 29, 2017, 03:41:11 pm »

I've used a Tek TDS340 and it has significant limitations compared to an analogue scope: short memory (so can't scroll along a waveform looking for glitches) and no peak-detect (so averaging can hide transients and noise).

I'm not sure I follow. How are those things limitations compared to an analog scope? Analog scopes don't have *any* memory, and I'm not sure peak detect is a concept that has an analog counterpart. A CRT provides the same averaging that hides transients and noise. That's why DSOs tend to look noisy and glitchy.

Also I would second the comment that the DSO Nano things are toys, and marginally useful at best. Can't really compare the price though as the nanos are new, and you're not going to get a decent *new* scope for $100, analog or digital.

There are plenty of $100 or less scopes out there that are reasonably good though. You can get 100MHz in either analog or digital form within that budget. Much beyond 100MHz and analog starts to have a significant price advantage.
 

Offline David Hess

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Re: Which oscilloscope for $100
« Reply #84 on: May 29, 2017, 10:05:34 pm »
I've used a Tek TDS340 and it has significant limitations compared to an analogue scope: short memory (so can't scroll along a waveform looking for glitches) and no peak-detect (so averaging can hide transients and noise).

The TDS340 manual and specifications say that it has peak detection.
 

Offline tggzzz

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Re: Which oscilloscope for $100
« Reply #85 on: May 30, 2017, 01:50:50 am »
I've used a Tek TDS340 and it has significant limitations compared to an analogue scope: short memory (so can't scroll along a waveform looking for glitches) and no peak-detect (so averaging can hide transients and noise).

The TDS340 manual and specifications say that it has peak detection.

Maybe I've misremembered; it has been a while since I used it and I was only interested in risetime. I'll check when I next have access to it.
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Offline james_s

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Re: Which oscilloscope for $100
« Reply #86 on: May 30, 2017, 02:18:39 am »
The TDS340 does have peak detection, I have one right here. As to whether it works well I can't really say, it's not a feature I've used much.
 


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