Products > Test Equipment

Zooming *IN* [actually: signal reconstruction]

<< < (5/8) > >>

noisyee:

--- Quote from: balnazzar on October 21, 2022, 05:43:12 pm ---I'd like to know which of these hypotheses did the scope in Dave's video violate. Maybe the front end was ineffective at discarding the high frequency components due to roll off from being at maximum (3 db) frequency, as he seems to hint? If so, would you elaborate upon this particular point? Thanks.

--- End quote ---

It seems Dave feeding some not so ideal sine wave to the scope so it cause artifact by interpolation.
To my own experience, you can hardly benefit from interpolation if you push to the 2.5X ratio limit since in reality pure sine wave don't exist, needless to say the front end itself will distort your signal. At 2X analog BW, attenuation is usually less 20dB. 2nd harmonic can easily cause aliasing and ultimately cause that kind of twisting effect in the video by interpolation.
Dave's rule of thumb is more practical while other statement is true mathematical.
A good practice is always use a little bit "over-killed" instrument if possible. You will save a lot of time by no needing to carefully verify your test setup.

noisyee:

--- Quote from: Bud on October 21, 2022, 09:47:23 pm ---
--- Quote from: balnazzar on October 21, 2022, 05:43:12 pm ---Nyquist’s most famous theoremproposes that a signal can be reconstructed perfectly from discrete samples if the following two rules are observed:
1. The highest frequency component sampled must be less than half the sampling frequency and
2. Samples must be acquired in equally spaced intervals

--- End quote ---
There is a flaw in this statement because it gives no guidellines as to how low the amplitude of a high frequency component is considered practically sufficient for satisfactory visual reproduction of the waveform on the oscilloscope screen. Based on the  rules quoted above, a 20GHz s scope would not be able to perfectly reconstruct my 1kHz square wave because the spectrum of my square wave is infinite.

--- End quote ---
That's correct because ideal square wave have zero rise and fall time while 20 GHz scope doesn't. In signal integrity, frequency is not that important than rise and fall time.
This statement is mathematical correct but not particularly useful.


--- Quote from: Fungus on October 21, 2022, 10:18:13 pm ---
--- Quote from: Bud on October 21, 2022, 09:47:23 pm ---Based on the  rules quoted above, a 20GHz s scope would not be able to perfectly reconstruct my 1kHz square wave because the spectrum of my square wave is infinite.

--- End quote ---
PS: If it's an 8-bit ADC then you can only ever show 128 harmonics of a square wave so a 20Mhz, 8-bit DSO would show the same as a 20GHz, 8-bit DSO.  :)

--- End quote ---
I don't think so because:
a. harmonics of a square adds together to form the edge which is far larger than 1LSB
b. you can see less than 1 LSB signal in FFT because of the noise floor can do the dither for you

Fungus:

--- Quote from: bdunham7 on October 22, 2022, 12:25:16 am ---
--- Quote from: Fungus on October 21, 2022, 10:18:13 pm ---PS: If it's an 8-bit ADC then you can only ever show 128 harmonics of a square wave so a 20Mhz, 8-bit DSO would show the same as a 20GHz, 8-bit DSO.  :)

--- End quote ---

I don't think that makes sense mathematically.  At the rising and falling edges all of the (odd) harmonics add, although they don't add at their peaks so it gets complicated and hard to visualize or make up simple hand-wavy explanations..  You don't see the individual harmonics, you see their sum.  So harmonics 127 + 129 add up to more than 1 LSB, as do harmonics 247-265, even though individually they wouldn't.

--- End quote ---

Yeah, you're right. Those tiny harmonics can add up to a lot if there's a large number of them and they're all in phase (which they will be on the edge of a step input).

Mental model adjusted.

2N3055:

--- Quote from: balnazzar on October 21, 2022, 09:30:03 pm ---
--- Quote from: Fungus on October 21, 2022, 06:53:46 pm ---
Dangerous not in physical sense, of course. I poses a danger to measurements, up to the point that you see a completely different signal. That's a bit worse than just inaccurate.

--- End quote ---

This is where (eg.) the Rigol MSO5000s 8GHz sample rate is a big advantage. It's 6x Nyquist where most scopes aim for 2.5x.

It's not always about "noise". Other numbers matter, too.

--- End quote ---

True.

...But if it had been just a bit less noisy...  ;D
[/quote]

Really, you should read a bit before making completely inaccurate statements.

Max ZOOM ratio (acquired any way) is ratio of available memory. Scopes with same memory and different sample rate will drop at same sample rate after you just change several timebase positions.
Actually we can argue that scope with higher sample rate will see it's higher sample rate only on few fastest timebase positions.


Fungus:

--- Quote from: 2N3055 on October 22, 2022, 01:22:41 pm ---Max ZOOM ratio (acquired any way) is ratio of available memory. Scopes with same memory and different sample rate will drop at same sample rate after you just change several timebase positions.
Actually we can argue that scope with higher sample rate will see it's higher sample rate only on few fastest timebase positions.

--- End quote ---

So it's best to start out with as high a sample rate as possible...?

Another point in favor of the MSO5000.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod