Frequency analysis with oscilloscope

Thread Starter

Giorgior27

Joined Jun 26, 2018
5
Hi all,
I have to do the frequency analysis of elementary RC and RL and then i have to do that in lab using the oscilloscope.

Where

\(V_{1pp}=2V \text{sine wave}\\R=1.2k \Omega\\ C=12nF\)

I did computation

\(G(s)=\frac{6.9444*10^3}{s+6.9444*10^3\)

Using matlab I plotted the Bode diagram of G(s).

Then I was wondering if there is a way to obtain bode diagram with the oscilloscope.

In Lab I measured \(V_1 and V_2\) for different frequency and i obtained 9 values. The only thing that I was able to think is to convert those values in dB and then to plot another graph in log scale using matlab. But i think it's not the only way. Maybe is there a function on the oscilloscope?

I would like to compare the theoretical bode diagram with the real one obtained through measurements.

EDIT: I'm not able to use Latex, even if i tried to follow the guide in this section. I'm sorry for that

MOD NOTE: Edited to clean up tex code.
 

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WBahn

Joined Mar 31, 2012
26,398
You are putting your LaTeX code inside plain tags and then inside code tags. Not surprising that it doesn't work.
 

Thread Starter

Giorgior27

Joined Jun 26, 2018
5
Oh ok i'm sorry, now i got it. I'm completely new to LaTeX.
I tried to edit again my first post but i can't now, why? I would like to modify it just to make it more readable.
 

jpanhalt

Joined Jan 18, 2008
11,088
Last edited:

Thread Starter

Giorgior27

Joined Jun 26, 2018
5
Thanks, I'll try that in lab.
I have another doubt about my result.
The transfer function in the first post is wrong and the correct one is:
\( G(s) = \frac {69.444*10^3}{s + 69.444*10^3} \)

The bode diagram obtained from this tf on matlab is attached.

I tried a simulation on LTSpice to find Vi and Vo at different frequencies and the results are a bit different from that bode diagram.
For example at
\( 15.92 kHz \) ≅\(10^5 rad/s \) ⇒ \(\frac {V_o}{V_i} = 0.622 = 4.12 dB (5 on matlab) \)
\(111.41 kHz\) ≅ \(7*10^6 rad/s \)⇒ \(\frac {V_o}{V_i} = 0.129 = 17.8 dB (20 on matlab) \)
 

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MrAl

Joined Jun 17, 2014
7,959
Thanks, I'll try that in lab.
I have another doubt about my result.
The transfer function in the first post is wrong and the correct one is:
\( G(s) = \frac {69.444*10^3}{s + 69.444*10^3} \)

The bode diagram obtained from this tf on matlab is attached.

I tried a simulation on LTSpice to find Vi and Vo at different frequencies and the results are a bit different from that bode diagram.
For example at
\( 15.92 kHz \) ≅\(10^5 rad/s \) ⇒ \(\frac {V_o}{V_i} = 0.622 = 4.12 dB (5 on matlab) \)
\(111.41 kHz\) ≅ \(7*10^6 rad/s \)⇒ \(\frac {V_o}{V_i} = 0.129 = 17.8 dB (20 on matlab) \)
Hi,

That looks better.
 

Thread Starter

Giorgior27

Joined Jun 26, 2018
5
Hi, what looks better? Sorry but I didn't understand. I wanted to say that plotting the Bode diagram based on the transfer function is a bit different than plotting the bode diagram of the results obtained by LTSpice. So i was wondering if i'm doing something wrong.
 

MrAl

Joined Jun 17, 2014
7,959
Hi, what looks better? Sorry but I didn't understand. I wanted to say that plotting the Bode diagram based on the transfer function is a bit different than plotting the bode diagram of the results obtained by LTSpice. So i was wondering if i'm doing something wrong.
Hi again,

Oh i meant that your second attempt was better because it was numerically more accurate.

Yes, now that you mention it, there are actually TWO different kinds of Bode diagrams: The true Bode diagram and a pure frequency analysis graphing which is now also called a Bode diagram because of modern computers.

There is a technique you can use to create a true Bode diagram that is based on approximations to the frequency response. While doing a true frequency analysis plot is more accurate and more suitable for computer analysis rather than hand analysis, it is now often also referred to as a Bode diagram probably because now computers and simulators are so common.

To be sure you should probably find out if you are supposed to be doing a true Bode diagram or a frequency plot is acceptable.
The true Bode diagram will have more straight lines for the most part with some exceptions, while the frequency plot will have a continouos curve.
 
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