# Frequency analysis with oscilloscope

#### 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.

#### 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.

#### WBahn

Joined Mar 31, 2012
26,398
There are limitations placed on new members and one of those is the ability to edit posts. I'll make the changes for you.

• Giorgior27

#### jpanhalt

Joined Jan 18, 2008
10,968
Last edited:
• atferrari and Giorgior27

#### 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
$$15.92 kHz$$ ≅$$10^5 rad/s$$ ⇒ $$\frac {V_o}{V_i} = 0.622 = 4.12 dB (5 on matlab)$$
$$111.41 kHz$$ ≅ $$7*10^6 rad/s$$⇒ $$\frac {V_o}{V_i} = 0.129 = 17.8 dB (20 on matlab)$$

#### MrAl

Joined Jun 17, 2014
7,891
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.

#### 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
7,891
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.