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Hands-On Review of Micsig's TO202A Touch-Screen Tablet Oscilloscope

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mbarlev8
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Does it have a tracking function?
mbarlev8   11/27/2016 2:16:22 AM
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Hi,

By tracking I mean taking a measurement (e.g. Period or Pulse width) and instead of presenting a single number measurement, presnting it as a graph vs time. So, for example if I have a FM modulated signal by tracking FREQUENCY measurement I could see the graph of the modulating signal.

David_Ashton_EC
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Re: Why the 200 mhz BW ?
David_Ashton_EC   5/15/2016 4:18:41 AM
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@Adam....back from the airshow....I WANT an autogyro.....



 But I digress, sorry.  I watched the video and I think what this boils down to is the roll-off of the antialiasing filter.  It would be nice to see the tests in the video done on your scope - I suspect the -3dB bandwidth would be ahead of the stated bandwidth by more than the 13% (113 MHz vs 100 MHz) of the Tek scope tested there (based on your readings given in your article).  Have you got the gear to do this?  But based on the figures you gave - measured 346mv, actual value 350 mv = 0.1 dB down, at 264/200 MHz = 32% more than rated frequency, that is pretty damn good.  (I want a Micsig as well, if I win the lotto I'll buy one of each :-)

As in your photos, the waveform in the video link gets more and more stepped the higher you go, till it approaches being triangular.

David_Ashton_EC
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Re: Why the 200 mhz BW ?
David_Ashton_EC   5/14/2016 7:41:22 PM
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@Adam...thanks....just on my way out (to an air show :-) but will have a look later.

Aeroengineer
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Re: Why the 200 mhz BW ?
Aeroengineer   5/14/2016 7:15:37 PM
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David,

 

Go to the video link that I posted to the about 1 minute mark and watch from there for the next minute or two.  It is all summed up there. 

 

It is a measurement to the 3dB power point on the scope.  This IS the definition of scope bandwidth. Once the actual magnitude of the signal shown on the scope is at this point (which is ~71% of the actual value) you have reached the defined bandwidth of the scope.  Why is this the defined measurement, I am sure that it relates to the general practice to define a filters bandwidth at this point as well.

 

There have been a lot of misrepresentations that came into being as the digital scopes came into being with the 1:10th -1:5th numbers being part of this. 

 

These 1:10th to 1:5th numbers are rules of thumb that only apply to certain situations and are not unique to digital scopes, but to analog scopes as well.

David_Ashton_EC
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Re: Why the 200 mhz BW ?
David_Ashton_EC   5/14/2016 6:23:01 PM
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@Adam...great explanation of nyquist, but I'd take issue with you on the -3db bandwidth, you say that

Summing it up, sample rate, Nyquist, and bandwidth are interrelated, but it comes down to the actual definition of o-scope bandwidth being related to the 3dB signal drop from actual as the cutoff for that definition.

but in the article you say

This image shows the signal generator delivering a 265MHz sine wave signal with a peak-to-peak value of 350mV, while the scope is measuring a frequency of 264.9MHz and a peak-to-peak value of 346mV. With the listed analog bandwidth of 200MHz, the indicated signal peak-to-peak value should only be 247mV.


Which rather refutes your comment above.  I reckon the reason they use the 10:1 sampling to given BW figure is more to do with the waveshape that can be reliably displayed.  Above this, you may get good amplitude but not enough samples to show a decent waveshape, you may miss some (eg) overshoot or higher harmonics on the signal.

All that said, the performance of the Micsig is pretty darned good!

Aeroengineer
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Re: Why the 200 mhz BW ?
Aeroengineer   5/14/2016 3:48:59 PM
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No this is not exactly how it works.  Lets give an example with a scope that was a 100MSps device, and a signal that was 50MHz.

 

Let's break it down into two separate events.  On the one hand we have a signal that is operating at 50MHz, it has no knowledge of the sampling rate of the scope.

 

Similarly if we say that we have a scope that is just taking samples at 100MSps and it has no knowledge of the signal (this is because the trigger is not yet come into the equation).

 

How do you get these two things into proper phase?  Because you are right at the Nyquist limit, you only have just enough samples to describe the signal.  The problem is, what is the phase of these two signals in relation to eachother?  There is nothing to sync the phases.  So you could just happen to be sampling the signal when it is at its nodes, or you could sample at its anti nodes.  How do you know where you are sampling?  This will have a great effect on the magnitude of the signal you are sampling.

 

Once you throw the trigger in there, you can begin to align the phases.  The trigger will control the phase of the data capture from the device.  It will physically alter the phase.  How much phase and how fast depends on the scope and the trigger methodologies.

 

When you are at the Nyquist limits, you need the trigger to align the scope to the signal to have a hope of getting the correct data.  This is not necessarily easy, especially when the signal rate is not an integer fraction of the sample rates that are available.  Here you can begin to get alising and other effects that will cause you to have issues with getting the correct magnitude of the signal measurement.

JamesM951
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Re: Why the 200 mhz BW ?
JamesM951   5/14/2016 2:42:02 PM
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"Depending on how a trigger is set up (and how fast it is) you could theoretically be sampling when the signal is at a node each time, and hence you would show no signal."

For a digital scope, I would have expected them to capture everything into a buffer & then use the selected trigger point to start the display. The "trigger" is just a post processing display control & has nothing to do with the actual capture. Of course, that's just how I would have designed it.   

Aeroengineer
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Re: Why the 200 mhz BW ?
Aeroengineer   5/14/2016 10:43:58 AM
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David,

 

This truely comes down to the definition of o-scope analog bandwidth.  This is the point at which the signal magnitude is 3dB below the original value. 

 

As you have mentioned, though, there may be other reasons for wanting to sample the specific signal at a higher rate depending on what you are looking for.  You may be looking for signal distortion on a 100MHz signal.  Essentially this signal distortion represents a frequency component that is greater than the 100MHz and you will need to be able to capture that.  So there are reasons that a scope with 200MHz analog bandwidth may not be high enough for measuring a signal that its primary frequency is much less than that.

 

On the other hand, there are reasons that you can use a scope with a lower rated bandwidth to be able to get usable trouble shooting information.  If you are not as concerned with the magnitude of the signal, but just interested to know that the signal is actually present, then you can use the scope to go probing for much higher frequency EMI on the board.

Aeroengineer
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Re: Nice scope!!
Aeroengineer   5/14/2016 10:38:01 AM
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Thanks David.  It was nice and clear.  The great thing is that the group is in the process of adding more trigger improvements to the device as well as working to upgrade the FFT functionality.

Aeroengineer
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Re: I want one
Aeroengineer   5/14/2016 10:36:25 AM
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It was a really fun scop to use.  There is a learning curve in the beginning, but it lasted four about 30 minutes and then the device became very useful very quickly.  The giant screen for this price range is what really sets it apart.  The analog performance with this screen, I can tolerate a few small niggles.

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