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Oscilloscope Questions

post #1 of 11
Thread Starter 

Anyone here that understands these beasties? The digital storage oscilloscope?

 

I am thinking of teaching myself basic electronics enough so I can understand some of the amp and DAC designs all of you have put together. I had a boatload of EE courses when in college. but have forgotten everything eons ago. :(

 

I do have a "boat anchor" of a scope I purchased for $300 on Ebay, an old HP 54111D. It has been reconditioned and calibrated. I am trying to understand its features in comparison to the scopes available today. 

 

Is there anyone here that can answer some questions about DSO scopes in general? Like having to do with sample rate, bandwidth, nominal vs effective vertical resolution, repetitive vs single-shot bandwidth, and so forth.? I do have specific questions regarding this material.

 

Thanks!

 

BG

post #2 of 11
check out Dave's EEEVBlog forum. His #13 blog has a good DSO intro.

http://www.youtube.com/watch?v=JTG6jWL0ZqA
post #3 of 11
Thread Starter 
Quote:
Originally Posted by Jaimo View Post

check out Dave's EEEVBlog forum. His #13 blog has a good DSO intro.

http://www.youtube.com/watch?v=JTG6jWL0ZqA

 

Thanks for the link. I found his two part video to be helpful.

 

I have done some additional research at the expense of allot of time. I find what he states is basically true with some minor changes and additions. I also do not understand his preoccupation with sampling frequency needing to be 10 times that of the scopes analog bandwidth when anything over 4 times provides negligeable benefit. I think he is also unduly enamored by the scope's sampling buffer size. I think these two parameters, sample rate and buffer size, are not as significant as other parameters that would help define the accuracy of the scope. Also as a side note the use of both channels of a scope can actually halve its actual bandwidth. And there are the functional aspects, importantly the trigger options of the scope. The trigger options can dramatically reduce the need for buffer size.

 

I guess that is why he is pushing the Rigol brand name. It has some interesting specs, that of sampling rate and buffer depth. But there is much more that goes into an accurate and usable scope than these two parameters. This is what differentiates the $350 Rigol from the $3500 Tektronix. For that matter, good probes cost over $125 apiece. Still, for audio applications, the $350 Rigol may be more than adequate. That is probably the audience he is catering to.

 

FWIW

 

BG


Edited by r010159 - 3/31/14 at 3:59pm
post #4 of 11
post #5 of 11
Thread Starter 
Sorry about the tone of the previous post. I am finding out there is allot of crap out there. For the money, Rigol is impressive. I am looking closely at their new series of 200Mhz scopes. If they have done their job, these scopes provide a good value. But of course, as a skeptic, I am skeptical until I read some well-done reviews on it.

Thanks for the link to the tutorials! smily_headphones1.gif
Edited by r010159 - 3/31/14 at 4:05pm
post #6 of 11

A good analog scope is also worth considering - if you don't need a DSO, look for a calibrated Tek scope from a reputable supplier but beware, there's loads of junked gear being passed of as "calibrated"

 

By sheer coincidence, a message from Rigol just popped into my inbox - seems that the DS1000Z is a finalist for some EDN ACE award.

 

http://news.ubm.com/index.php?s=2429&item=124893

 

It will be interesting to see how this scope family does.

post #7 of 11
Thread Starter 
Quote:
Originally Posted by Jaimo View Post
 

A good analog scope is also worth considering - if you don't need a DSO, look for a calibrated Tek scope from a reputable supplier but beware, there's loads of junked gear being passed of as "calibrated"

 

By sheer coincidence, a message from Rigol just popped into my inbox - seems that the DS1000Z is a finalist for some EDN ACE award.

 

http://news.ubm.com/index.php?s=2429&item=124893

 

It will be interesting to see how this scope family does.

 

This is very interesting indeed! And the price for its features and bandwidth is excellent. I do not need a scope to be THAT accurate for my purposes. I am not designing a probe to be sent to Mars with this scope. :wink_face:.

 

What accuracy in milivolts do I need for my purposes? And do I require 8-bit vertical resolution instead of the 6-bit nominal (8-bit effective at 10 Mhz) with my "boat anchor" of an old HP scope?

 

Thanks!

 

BG

 

EDIT: I found an excellent description on what of an oscilloscope affects signal fidelity, which is the goal of a good scope. http://cp.literature.agilent.com/litweb/pdf/5989-5732EN.pdf IMO this document explains to a significant degree the difference in accuracy between a $350 Rigol and a $3500 Tektronix. Hint: the accurate interleaving of the ADC chips. Another worthwhile doument is http://www.ztecinstruments.com/applications/oscilloscope-measurement-fundamentals.php Hint: oversampling minimizes the effect of noise introduced into the signal by the scope.


Edited by r010159 - 4/1/14 at 4:02am
post #8 of 11
Quote:
Originally Posted by r010159 View Post
 

Is there anyone here that can answer some questions about DSO scopes in general?

 

Yes, but it's not particularly efficient for us to teach you a complex technology one post at a time.

 

Go read The XYZs of Oscilloscopes, by Tektronix. Then if you still have questions, you'll have a solid basis for posing them. Specific questions yield better answers than broad ones.

 

(Sorry for the Google search link instead of a direct link, but the official Tek page wants to send you through a marketing signup form, and I'm worried that links to the PDF on other sites will go stale.)

 

Quote:
Originally Posted by r010159 View Post
 

What accuracy in milivolts do I need for my purposes?

 

I don't know.  What are your purposes?

 

You only said you want to understand DAC designs and such, but you can understand a DAC without any oscilloscope at all.

 

Oscilloscopes are for troubleshooting, and I'm not going to be able to guess what troubleshooting problems you're going to run into. Until I know the problem you face, I can't recommend a specific solution.

 

I will observe that millivolt resolution probably isn't necessary for a DAC, since it should be running on 3.3 or 5V digital signaling, so even half volt resolution will be enough for some measurements. Millivolt resolution in a DSO matters when you're dealing with very low voltage signaling, as with high-speed LVDS schemes.

 

Quote:

And do I require 8-bit vertical resolution instead of the 6-bit nominal (8-bit effective at 10 Mhz) with my "boat anchor" of an old HP scope? 

 

6-bit resolution is pretty poor. That only lets you distinguish 64 levels, which gives jumps big enough to easily see on even a very small CRT. Even the cheapest DSOs these days will give 8-bit resolution, so it's not exactly a luxury spec item. 8-bit resolution was standard by the early 1990s.

 

That said, the scope on your bench measures more signals than the scope in the box in the warehouse. If you can't afford to upgrade, and your current scope does the job, keep with it.

 

Scope technology has come a long way in the quarter century since your old HP scope was current, but audio DAC technology is pretty much 1987 level technology, too. Sure, audio DACs have been refined a lot since then, but if all you need to know is whether there's a glitch on the leading edge when the frobnitzim compander IC blatnicates, your old scope can probably still answer the question.

 

My scope is about a decade old now, and it still answers all the questions I expect it to. We have a scope double that age at work with the same outlook: it keeps answering the questions, so we don't replace it.


Edited by tangent - 4/1/14 at 4:53am
post #9 of 11
Thread Starter 
Quote:
Originally Posted by tangent View Post
 

 

Yes, but it's not particularly efficient for us to teach you a complex technology one post at a time.

 

Go read The XYZs of Oscilloscopes, by Tektronix. Then if you still have questions, you'll have a solid basis for posing them. Specific questions yield better answers than broad ones.

 

[deleted]

 

I don't know.  What are your purposes?

 

[deleted]

 

I will observe that millivolt resolution probably isn't necessary for a DAC, since it should be running on 3.3 or 5V digital signaling, so even half volt resolution will be enough for some measurements. Millivolt resolution in a DSO matters when you're dealing with very low voltage signaling, as with high-speed LVDS schemes.

 

[deleted]

 

My scope is about a decade old now, and it still answers all the questions I expect it to. We have a scope double that age at work with the same outlook: it keeps answering the questions, so we don't replace it.

 

Thanks for your reply.

 

My main use would be debugging of audio circuits and timing glitches introduced by errors in the software of an embedded system. So my scope may be enough for these purposes.  Also I sometimes will need to evaluate the quality of a square wave. This is what may require 8-bit resolution. The average effective resolution of my scope is only 7-bits with a 6-bit worst case scenario.

 

<proverbial lightbulb pops on>

 

I think I know the reason for oversampling. It is to handle the noise introduced by the processing of the signal by the oscilloscope. This probably can come from several sources including jitter. The downside is a less accurate representation of the signal since the oversampling involves averaging. Good scopes introduce less noise in the processing of the signal which means less oversampling required which means a more accurate reproduction of the signal.

I will read that XYZs of Oscilloscopes now.


Edited by r010159 - 4/1/14 at 5:17am
post #10 of 11

No, oversampling is there to reduce the risk of aliasing artifacts. But you'll learn that in the XYZs of Oscilloscopes, and more.

post #11 of 11
Thread Starter 

I do not mean to provide you with information that you already know, but I find all of this very interesting. 

Yes, I understand about antialiasing and avoiding the use of a theoretical brick-wall filter. But a ratio of sampling rate to bandwidth of 4:1 would serve this purpose. I am talking about scopes that has a 10:1 ratio.

The following reasons may explain this:

The factors that reduce the accuracy of an oscilloscope can be mostly lumped into high- and low-frequency errors. Noise is generally the cause of high-frequency errors, while low-frequency errors are caused by drift stemming from temperature, aging, bias currents, etc. High-frequency errors can usually be removed by oversampling and averaging. Low-frequency errors often require the calibration of the instrument, either internally or through a factory calibration.

 

When acquiring Period and Frequency measurements their accuracy can be very much affected by the sample rate. Both of these measurements are calculated by counting the number of samples that occur between Middle crossings. If a 10 MHz signal is being sampled at 100 MHz, this will result in exactly ten samples per period. The samples at the zero crossings may be very near the borders. If one is missed, this results in only nine samples being detected which returns a Period of 9 * (10nsec) = 90 nsec and a resulting Frequency of 11.1 MHz. This resolution is obviously not very good. It could be improved by acquiring long waveforms to capture many cycles and average out the resolution error. Another solution would be to sample the signal at 1 GHz or greater. Overall, for more accurate Frequency and Period measurements, it is best to sample at a far greater rate than the signal and capture many cycles.

 

So oversampling provides more accuracy with respect to errors (including noise).

 

Here is another interesting factoid that I have come across in my own words. Interleaved ADCs allow sample rates much higher than the sampling limit of the individual ADC chip. However, if the phase clock of each interleaved ADC is greater than 1/2 sample period delayed, a high sampling rate would increase the errors in the resulting waveform. So in this case, the lower sampling rate would provide for a more precise waveform. Therefore, the calibration of the interleaved ADCs is of paramount importance.

 

So what do you think? Interesting stuff here. :regular_smile :

 

EDIT: I just figured out that oversampling only applies to repetitive signals, not single-shot.

 

BG

 

 

 

 

 


Edited by r010159 - 4/1/14 at 7:57pm
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