Author Topic: Spec of a scope: "flatness" - What is it exactly?  (Read 427 times)

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Offline eTobeyTopic starter

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Spec of a scope: "flatness" - What is it exactly?
« on: May 29, 2024, 09:13:53 am »
I can not find information about this spec (just lot of mechanical flatness).

Anyone can explain or has some links for it?
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Online gf

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Re: Spec of a scope: "flatness" - What is it exactly?
« Reply #1 on: May 29, 2024, 09:38:04 am »
If you mean specs like in the attached screenshot, they refer to the frequency response.
 
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Offline pdenisowski

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Re: Spec of a scope: "flatness" - What is it exactly?
« Reply #2 on: May 29, 2024, 10:08:14 am »
I also assume they mean the frequency response - in this case, how uniform the passband is.  I discuss this in one of my videos

https://youtu.be/FhT8TpuI7ek?t=531

« Last Edit: May 29, 2024, 10:09:51 am by pdenisowski »
Test and Measurement Fundamentals video series on the Rohde & Schwarz YouTube channel:  https://www.youtube.com/playlist?list=PLKxVoO5jUTlvsVtDcqrVn0ybqBVlLj2z8
 
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Offline eTobeyTopic starter

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Re: Spec of a scope: "flatness" - What is it exactly?
« Reply #3 on: May 29, 2024, 12:52:16 pm »
I have now made 2 different measurements, which i thought would yield the same result. I run a sweep (5-55Hz), and looked at the maximum amplitudes. Then i made a FFT (1Hz - 500Hz), but from 22Hz to 70 Hz there is a huge bump. What is this?
"Sometimes, after talking with a person, you want to pet a dog, wave at a monkey, and take off your hat to an elephant." (Maxim Gorki)
 

Online ebastler

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Re: Spec of a scope: "flatness" - What is it exactly?
« Reply #4 on: May 29, 2024, 01:29:18 pm »
What was the actual signal amplitude you sent to the scope during that sweep? Looks like about 20 mV p-p with a 500 mV offset?!

I think you mixed somthing up with the peak hold function. You don't want that applied to the time series signal, but select it as the FFT mode! (So the FFT uses the strongest frequency components it encounters at various times, while the sweep is ongoing.)

You also need to set a rather slow sweep time on the signal generator. Essentially the FFT must complete at least one full spectrum calculation for each data point you want in the final swept spectrum. 
 

Online gf

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Re: Spec of a scope: "flatness" - What is it exactly?
« Reply #5 on: May 29, 2024, 02:27:08 pm »
I think you mixed somthing up with the peak hold function. You don't want that applied to the time series signal, but select it as the FFT mode!

Indeed. Never ever use peak detect acquisition mode in conjunction with FFT.
Peak detect acquisition is not sampling in the sense of the Shannon-Nyquist samplling theorem.
 


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