Multisim - How do I find the Resonant Frequency (FR) on an LRC circuit?

[Harley]

Hello everyone! Thank you for viewing my question.

I have an assignment question and have completed all of the questions and I am satisfied with my results. The issue I am facing however, is the 'build your circuit on Multisim to verify your answer' part.

My given values for this task are:

Vs = 14 V
F = 50 Hz
R = 12 Ω
L = 102 mH
C = 0.01 µF

I have calculated FR using the following method:

Have I used the correct method? And how can I measure this in Multisim to prove/see if my answer is correct?

Thank you!

 

[(*steve*)]

Do you know what happens at the resonant frequency?

 

[dorke]

The method is correct but your answer isn't.
You entered the wrong numbers for C=0.01e-6

 

[Harley]

The method is correct but your answer isn't.
You entered the wrong numbers for C=0.01e-6

Thanks for replying, you are right! I didn't notice that, I've done it as 1x10^-6 instead of 0.01x10^-6 Thanks! I'm just going to put that down to a 'middle of the night brain fart' haha. I've corrected my work throughout, still not sure how to display the RF in Multisim though. I did it once last year but it was so long ago I don't remember what to connect or how to connect it.

 

[Harley]

Updated results (doh!)

 

[hevans1944]

Have I used the correct method? And how can I measure this in Multisim to prove/see if my answer is correct?

Your method is correct, as are your updated results, even though in (C) you have FR = 468.333 Hz but used the correct value in the calculations. Now calculate XL, XC, and Z at the resonance frequency.

I am "pretty sure" Multisim will allow you do a frequency-sweep simulation. Just set the sweep limits a few percent below and above the calculated resonant frequency. Measure the voltage across R, which will be an indication of the current in the circuit. The width of the peak will be proportional to R, small R => smaller width. This proportionality is called Q (for quality, historically) and represents the power lost in R versus power applied to the series RLC circuit at the resonant frequency.

BTW, you appear to be a motivated fast-learner. Probably be teaching us a thing or two this time next year. Sure wish I knew how to do pretty-print and spiffy schematic drawings.

 

[KJ6EAD]

The bode plotter in multisim works well for this kind of exercise.

 

[dorke]

It is called "AC Analysis frequency response".
Here you have a How to Video

 

[Harley]

@hevans1944 @KJ6EAD @dorke Thank you very much I will try everything you've suggested and report back!

 

[Harley]

 

[hevans1944]

Try narrowing your frequency sweep range to more closely bracket the resonance peak. The magnitude plot should look "curvy".

 

[KJ6EAD]

Try narrowing your frequency sweep range to more closely bracket the resonance peak. The magnitude plot should look "curvy".

Make sure the frequency step is small enough as well. Probably 1Hz would look good.

 

[Harley]

Great, thanks I'll give it a go

 

[Harley]

Is this any better? Not quite a curve but a little more so than the last one. (Side note, thank you so much for your help with this, we've not been shown how to do this before).

 

[Ratch]

Is this any better? Not quite a curve but a little more so than the last one. (Side note, thank you so much for your help with this, we've not been shown how to do this before).

Try a frequency range, between 700 to 80,000 Hz. Also, plot more points, especially near the resonant frequency, so that you can see the true shape of the current curve as shown below.

Ratch

 

[dorke]

A few things:
1.You should learn how to modify the default labels to have meaningful names ,like so:

2.Your simulation is fine,try this setting and display it in "full screen size".



3.This is the result:

 

[hevans1944]

Hey, @dorke, hows come you asked for 200 points but the display only plotted 20? Nice that it "filled in the blanks" when plotting the sweep though.A linear magnitude scale would provide a more dramatic-looking resonant peak, as @Ratch showed in post #15. @Harley, please note the sharp reversal of phase as the sweep passes through the resonant frequency. This type of circuit is sometimes used as a shunt "notch" filter, without the resistor, to suppress line frequency interference, although the twin-tee notch filter is easy to build and eliminates the need for a bulky iron-core inductor.

Try to stay ahead of your instructors. A lot of stuff worth learning about is omitted or glossed over in formal courses, which is why I encourage students to experiment outside the class room at home, and why I included the tid-bit about notch filters. Filter design is a fascinating subject in itself, but hold off on that until you get more maths squared away.

 

[dorke]

Hop,
In Multisim the number of points chosen for calculations aren't necessarily the same as the points displayed.
There is an option to chose the points displayed and lines connecting them.

Absolutely ,a linear vertical mode would dramatize changes of the plot,
although for filters the vertical decibel (dB) scale is the "standard" display used .

So ,
Here is the same circuit as a notch filter (sometimes called "a parallel frequency trap ").
Linear and with points and connecting lines.

 

[hevans1944]

Thanks, @dorke! This is way cool stuff. Maybe I will come out of my cave and fire up one of the SPICE simulators I happen to have installed, just so I can keep up with the younger generation. Sure beats trying to compose equations and use Excel spreadsheets for graphing.

Oh, wait! I don't trust spreadsheets either.

,,, for filters the vertical decibel (dB) scale is the "standard" display used .

Absolutely. IIRC, the "width" of the filter band pass (or band reject) is usually specified at the -3db points from the peak or the +3db points above the bottom of the notch. More information here.

 

[Harley]

Thanks everyone you're all very informative, sorry I'm so bad at this stuff they don't teach you any of this in lessons they just expect you to know it. I'll give it a go