# How to use Simulink to gain the oscillation frequency of RLC circuit?

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baocheng guo on 24 Apr 2015
Edited: Sebastian Castro on 8 May 2015
I have a RLC system as below, the question is how i use the Simulink to gain the oscillation frequency because solving the equations sounds too low, lol. The signal builder is used to emit a impulse signal and the spectrum analysis is used to calculate the frequency, and I don't know is it right? because the result seems wrong. If it is wrong, how i should do to gain the frequency?

Sriram Narayanan on 4 May 2015
It is not very clear when you mentioned "How I use Simulink to gain the oscillation frequency"? Can you please explain more about your use case and share your model if possible?
baocheng guo on 8 May 2015
Thank u for ur answer, and this question is very simple.
As we know, when the circuit has a inductance (L) and capacitance (C), when give a certain voltage, such as V= Ecosw, the storage energy of L and C will equal, and it is will oscillation. The frequency of the voltage is defined as f=sqrt(L/C)/(2*pi), e.g., L = 1, C = 1,and the f could be gain as 0.159 Hz.
and the 'f' can calculate by formula, but I hope the Simulink can find this frequency auto.
best..

Sebastian Castro on 8 May 2015
Edited: Sebastian Castro on 8 May 2015
1. I think the Spectrum Analyzer block expects a discrete-time signal... So, you can either change the simulation mode of your model from the powergui block, or use a Rate Transition block on the sensed signals to get them sampled. Of course, you need to make sure your sampling rate is high enough.
2. Another thing people usually do with SimPowerSystems models is use the FFT Analysis tool in the powergui block. This will give you an FFT for those particular simulation results.
To do this, you should measure the current (or voltage) in a Scope block, turn on Structure with Time logging in the Scope, and then pull up that FFT Analysis tool from powergui.
3. I almost forgot, there is also an Impedance Measurement block in SimPowerSystems which can be coupled with the Impedance vs. Frequency Measurement option in powergui. You basically connect this block across the terminals you care about, and then you get an impedance over a specified frequency sweep.
Check that out and see if it gives you the right frequency peak(s).
- Sebastian