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Oscilloscope versus usb card?
Oscilloscope versus usb card?
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Old 16th September 2020, 09:14 AM   #11
r_jik45 is online now r_jik45  France
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Following jjasniew's comment, and after checking more of these items on the net, there is usually a ratio of 2.5-5 between the oscillos bandwith and usb-based cards. A common 20MHz oscilloscope would require something like 100Msamples/s.
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Old 16th September 2020, 09:29 AM   #12
JRE is offline JRE  United States
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That ratio of at least 2 to 1 (sampling frequency at least twice the maximum signal frequency) is no coincidence. You will see that in every system that samples analog signals.


The reason is the Shannon-Nyquist sampling theorem. (Nyquist–Shannon sampling theorem - Wikipedia)


Accurately reconstructing an analog signal from the sampled data requires at least two data points per cycle of the signal.


As a consequence, the maximum bandwidth of a sampled signal is half the sampling rate.


The practical limit is lower, and it depends on how sharp the filters are that remove the out of bounds content. A better filter lets you get closer to the theoretical bandwidth.


Sampling rate tells you how fast the samples are taken.
Bandwidth includes the cutoff frequency of the analog circuitry and filters before the analog to digital conversion.


Any decent oscilloscope or USB data acquisition (DAQ) card will tell you both.


-------


Besides sampling rate and bandwidth, you need to keep an eye on the input voltage range.


Microphone signals can be down in the single digit millivolt peak to peak range, while speaker signals can be many volts to tens of volts peak to peak.


You need to make sure that your scope can cover the voltage levels that you will be working with.


My old analog oscilloscope has (by modern standards) a low bandwidth of only around 15MHz. On the other hand, it has a stupidly high range of voltage settings - I can measure stuff from less than 100 microvolts up to a couple of hundred volts with it.
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Old 16th September 2020, 10:14 AM   #13
jan.didden is online now jan.didden  Europe
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Quote:
Originally Posted by r_jik45 View Post
Thanks for your comment Jan.

In point of fact I was using oscilloscopes for my work. Had an old one at home too but had to discard it after years of no-use while abroad. Using one is not a problem I think.
Was using acquisition cards/softwares too at work.

Now it's for private use. Saw several usb-based scopes on the net. Wondering where to spend the money, and why specifications are not expressed the same way (Hz v. samples/s).

4 channels instead of 2. Can become handy. 2 channels in the labrador board.
OK I misread, apologies.

I got several USB scopes from TiePie engineering in Holland. The nice thing about them is that they are 8 or 10bit at the widest bandwidth, but can run 14 or 16 bit with reduced bandwidth. And that is exactly what comes handy in audio work, higher bit depth and a bandwidth less than light.

As a matter of fact I am selling my HS3-25MHz with arbitrary waveform generator ;-) . € 350 + shipping in case you (or anybody else) is interested.

Jan

Last edited by jan.didden; 16th September 2020 at 10:22 AM.
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Old 16th September 2020, 12:14 PM   #14
r_jik45 is online now r_jik45  France
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Jan, no problem, your comment was more than welcome.

Your link to TiePie is also most interesting. One can see that the ratio MHz:sample rate is in the 1:1 to 1:4 for their usb oscilloscopes.
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Old 16th September 2020, 12:18 PM   #15
r_jik45 is online now r_jik45  France
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JRE,

Got your point for both the sampling and the input voltage range.
High input values can likely be adjusted thanks to probes but having a sensitive device is important too.
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Old 16th September 2020, 12:34 PM   #16
nigelwright7557 is offline nigelwright7557  United Kingdom
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Quote:
Originally Posted by r_jik45 View Post
Oscilloscope capability is expressed in MHz (20MHz .. 100MHz, etc.), usb acquisition card working as oscilloscopes have their capability expressed in samples/s, as far as I understood.
Is there a direct relationship between samples/s and MHz?

Any member tried them?
How do you think they compare versus simple low end oscilloscopes?
It depends on how optimistic the USB scope seller is.
Some say MHz = Meg samples/second.
Usually its less MHz than Meg samples/second.

How many dots(samples) on the screen do you need to represent a sine wave for example?
For a decent sine wave I would reckon about a hundred dots.
So that means a 100 meg samples/sec scope can manage 1MHz sine wave.

While I design and sell USB scopes I rarely use one unless I want digital storage. I much prefer my analogue scope for every day use.

It has been suggested you need two points for a sine wave. Ok you can extrapolate the sine wave but what if its a triangle wave or square wave ?
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Old 16th September 2020, 12:57 PM   #17
jan.didden is online now jan.didden  Europe
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Nigel, I think it depends on the architecture. You need only two points to recreate a sine wave, assuming your anti-aliasing is up to snuff.
You really don't need to individual dots to show on a digital display, just paint the wave.

For several reasons you may want to go a hit further, like 3 or 4 points per period to ease on the low pass, but 100 points seems excessive.

Jan
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Old 16th September 2020, 12:59 PM   #18
JRE is offline JRE  United States
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Triangle and square waves contain harmonics of the base frequency. To see them accurately, you have to consider the frequency content of the harmonics rather than just the base frequency.
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Old 16th September 2020, 01:05 PM   #19
nigelwright7557 is offline nigelwright7557  United Kingdom
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Also, quite often its oscillation or distortion on that sine wave you want to see.
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Old 16th September 2020, 01:14 PM   #20
jackinnj is offline jackinnj  United States
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Oscilloscope versus usb card?
bandwidth is of little relevance if you don't have high quality probes, and while you're at it, read up on probing technique! Keysight and Tektronix have white papers on probing technique.
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