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Old 8th March 2012, 08:42 PM   #21
BFNY is offline BFNY  United States
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Quote:
Originally Posted by percy View Post
Thanks BFNY. I wasnít aware of this approach at all. I guess I didnít realize that the mv/div setting of a scope actually amplifies/attenuates the signal itself before measuring it. I just took it has some kind of a visual zoom.
Yes, you got it right. This is, as above, the hard part of designing a scope. I work directly with R&D people that do this - design PC based scopes - in my current job, and have since 2003. So I'm pretty close to understanding what goes on inside.

The point is the A/D chip has one max input level - say 2V, in many cases it must be balanced. That's not very friendly, so the instrumentation "front end" has to add quite a bit of "other" capabilities - AC,DC coupling, 50ohm/1Mohm inputs, filtering, triggering, and gain/attenuation. In most cases, the bandwidth of the scope will be determined by the front end choices.

That why it's so hard to build an equivalent type instrument DIY. And yes, sound cards are also poor in the front end capabilities, but the 24 bit range gives them lot of dynamic room.
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Old 8th March 2012, 08:57 PM   #22
BFNY is offline BFNY  United States
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Originally Posted by SoNic_real_one View Post
You forgot to trow in the noise/THD factor of the scope input stage in there... I doubt that you can really "see" 119dB on a scope.
A professional sound card is way more usefull than a scope for measurements up to 50-80kHz.
Yep, did you miss the mention of a sound card for that in the post above?

Turns out when you measure effective bits, which includes noise, THD, etc, it measures great at low frequency, and poor at high frequency. The quoted number is usually in the BW of the scope, or broken out for different frequencies.

So a 500MS/s scope has really GOOD effective bits in the 10MHz range and below, and will be significantly worse at say 250MHz.

A perfect 8 bit scope would have 8 effective bits, but it's not that uncommon to see 7 or 7.2 bits for low frequencies like 10MHz - hence the "ballpark" number of 45dB above. Also, you can use averaging to increase the effective bits, and FFT techniques to extend the spur free dynamic range beyond the 45dB number.

And yes, scope front end design separates the men from the boys. When not done correctly, it can add a lot of unwanted "things" to the signal. The best companies know how to do it right.
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Old 8th March 2012, 09:08 PM   #23
BFNY is offline BFNY  United States
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Originally Posted by SoNic_real_one View Post
It just ATTENUATES the input signal to the lowest value. That will decrease the final SNR for any higher voltages.
Actually, in my experience, no, and no. Generally all commercial scopes I've worked with have both gain and attenuation. And the harder thing to do is gain.
Unless you are talking about whopper voltages, like way over 50V, and then there are other problems where bandwidth usually suffers as compared to 50 ohm bandwidth at lower voltages.
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Old 8th March 2012, 10:37 PM   #24
percy is offline percy  United States
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Can I ask a couple of questions for my general understanding of scopes and spectrum analyzers –
Does this all apply to spectrum analyzers also ? I am assuming spectrum analyzers have a similar input gain/att setting as well ?
And is it possible to export data from a digital storage oscilloscope out to a PC where a third party program can be used to do spectrum analysis on it ?

Thanks.
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Old 8th March 2012, 10:43 PM   #25
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It's all about the ADC real resolution (not equal with the number of bits). Faster ADC's like the ones in oscilloscopes will have less bits than a dedicated audio ADC. Digital scopes have usually 8-12 bit ADC. Noise floor is usually around -50...-90dB depending of the modes (slow average gets better SNR).
A high quality scope costs 10-50 times the amount that you pay for a professional sound card. So it's all about what you really need to measure.

Last edited by SoNic_real_one; 8th March 2012 at 10:53 PM.
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Old 8th March 2012, 10:51 PM   #26
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Originally Posted by percy View Post
. . . is it possible to export data from a digital storage oscilloscope out to a PC where a third party program can be used to do spectrum analysis on it ?
My Tektronix TDS1002B (a low-end digital 'scope) can export a text file of the sample values to a USB Flash drive. I have never done so, but I'm sure the file can be read by Excel, Matlab, etc - perhaps converted to a *.wav file for analysis by LTSpice, or an audio-processing program.

Dale
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Old 8th March 2012, 10:56 PM   #27
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Originally Posted by dchisholm View Post
My Tektronix TDS1002B
That low-end costs some $1000, has 8 bit resolution (except on 2mV/div). It's great for some things, but lousy for high-end audio measurements

Last edited by SoNic_real_one; 8th March 2012 at 10:59 PM.
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Old 8th March 2012, 11:17 PM   #28
BFNY is offline BFNY  United States
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Originally Posted by SoNic_real_one View Post
That low-end costs some $1000, has 8 bit resolution (except on 2mV/div). It's great for some things, but lousy for high-end audio measurements
may want to read
Any high resolution diy spectrum analyzers ?
again, especially the 2nd part of the post looking to do measurements ABOVE 2MHz to understand what we are discussing here.
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Old 8th March 2012, 11:26 PM   #29
BFNY is offline BFNY  United States
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Originally Posted by percy View Post
Can I ask a couple of questions for my general understanding of scopes and spectrum analyzers Ė
Does this all apply to spectrum analyzers also ? I am assuming spectrum analyzers have a similar input gain/att setting as well ?
And is it possible to export data from a digital storage oscilloscope out to a PC where a third party program can be used to do spectrum analysis on it ?

Thanks.
Yes, in general, spectrum analyzers have a front end with attenuation and gain. The way the measurement is done vs. scopes is totally different in "old style" SA's, while some of the newer high end units have hybrid modes, where real time scope type measurements can be done over a limited band of frequency, that the user can select. This is needed to analyze digital communication type wireless signals.

In general, if the scope has a disk drive, usb port, GP-IB, LAN, RS-232, etc. you can export the data. How it's done is a little different for all.
Most people use Matlab, Labview, or something like that for extensive post processing.
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Old 9th March 2012, 12:10 AM   #30
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Originally Posted by BFNY View Post
again, especially the 2nd part of the post looking to do measurements ABOVE 2MHz to understand what we are discussing here.
I think you need to read that again, there was no 2MHz mention:
Quote:
With many 24bit/192Khz soundcards and audio interfaces on the market these days and lots of freeware programs you dont even need to do anything special for signal analysis upto that frequency and dynamic range resolution. What I am curious is why arn't there any (or atleast I haven't managed to find any) pc based diy spectrum analyzers, especially the hardware, for higher sampling rates. I see that there are a few 16+ bit (enob) ADC chips from TI and AD that have sample rates of several (2-10) MSPS. I mean if anyone can build a 12bit 1MSPS "pc scope" then why not a 24bit 4MSPS spectrum analyzer ? What difficulty could be preventing this from happening ?
4MSPS at 24 bit means less than 350kHz bandwith...
The oscilloscope is not closer of the original post requirements either.

Last edited by SoNic_real_one; 9th March 2012 at 12:18 AM.
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