LC Filter Distortion - Page 1
Forum section: electronics / beginners

author: bingowashisnamo
author_rank: Newbie
timestamp: March 02, 2021, 02:11:43 am
content: I've been testing out basic circuits to build my knowledge of electronics. I hooked up a simple LC lowpass filter to see how it compared with theory. At the 34kHz resonant frequency the sine wave at the input was attenuated by about a factor of 10 from the level of the function generator. This suggests that the impedance of the LC network is about 5ohms at resonance (50ohm output impedance of the generator). The sine wave at the output was about 10 times greater than the input corresponding with the resonant peak of my RLC network where R=5ohms.What puzzles me is that when I scroll through frequency my input sine wave looks good until I hit the resonant frequency, then it gets distorted. The output sine wave at the resonant frequency looks good. Looking at the FFT of my distorted input I see a harmonic at 99kHz at 48dB which I suspect is the main cause of my distorted input signal. If I add a 0.1u cap at the input the distortion is removed. What is the mechanism(s) that is causing this distortion (or this harmonic) and why does the cap alleviate it? There are pictures below of the circuit, the measured signals with and without the extra cap, and the FFT plot. Any help is greatly appreciated.

author: RoGeorge
author_rank: Super Contributor
timestamp: March 02, 2021, 03:09:27 am
content: Quote from: bingowashisnamo on March 02, 2021, 02:11:43 amAs a side note, an RC can be a low-pass filter. An RLC (or just an LC) will make a resonant circuit, therefore it will not be a low-pass filter any more, but a band-pass or a band-stop filter (depending of how it is connected).About the distortions, they appear when there is some non-linear response somewhere in the circuit (no matter what circuit or how the circuit looks like). My guess is that you are using a ferrite coil for L1. Magnetic materials tend to be nonlinear, especially for high magnetic fields (high current through the coil). At resonance, the current through the coil increases high enough that the ferrite core starts acting non-linear.Is your L1 a ferrite coil?

author: MikeK
author_rank: Super Contributor
timestamp: March 02, 2021, 03:39:55 am
content: Where's your load? If it's intended to be a filter for a 50 ohm load then you need a 50 ohm load at the output.

author: MikeK
author_rank: Super Contributor
timestamp: March 02, 2021, 03:43:21 am
content: Quote from: RoGeorge on March 02, 2021, 03:09:27 amHuh? Amateur radio operators design and build LC low-pass filters all the time.

author: penfold
author_rank: Frequent Contributor
timestamp: March 02, 2021, 04:56:06 am
content: Just a few notes on how I would go about diagnosing that problem (even though I think the answer has been offered already):Adding the cap to the input does a few things, it reduces the output impedance of the generator as seen by your circuit effectively decoupling some of its output resistance, low-pass filters the signal and alters the impedance seen by the generator's output.The corner frequency of the low pass filter it forms with the 50 ohms is about 32kHz which sounds suspiciously close to your fundamental frequency and therefore it would effetely remove some distortion from the generator's output. But the problem with that argument is that it would imply your circuit wasn't low pass filtering very well... so is all is connected correctly, not that problem.The impedance of your filter as seen by the generator will be (50 + 5)ohms or so and resistive (so not a reactive loading problem) and at 5V would only produce about 100mA so probably not a loading issue for a descent sig gen... and adding a cap would have made that worse anyway.100mA however could be rather high for that inductance as RoGeorge pointed out... the "lumps" in the output waveform also correspond with the zero-crossing points of the input which is when the inductor current is highest sinusoidally.. and with the effects of saturation would be extra "peaky" at the tops... adding the 0.1u cap would divert ~10mA or so away from your LC part... any chance its a 100mA rated inductor?

author: RoGeorge
author_rank: Super Contributor
timestamp: March 02, 2021, 11:10:42 am
content: Quote from: MikeK on March 02, 2021, 03:43:21 amIt feels wrong to me, but it is called indeed a low pass filter, I was calling it wrong for my entire life.Thanks for pointing that out.About the rest, the explanation still stand. To test this is indeed the correct reason, try setting the generator at a much lower voltage, say 10 times lower than now. That will make the current through the coil 10 times lower, too, so chances are the magnetic material won't go into saturation, and the distortions will vanish.When adding the extra 0.1uF at the node called Input, the new resonant circuit become an RLC where the total capacitance is now half (two series 0.1uF will make a total of 50nF), therefore the resonance (and the maximum current through the coil) will be to be found at a higher frequency (because lower capacitor in the resonant LC).The prediction is you should see even higher distortions with the additional 0.1uF connected, just that it will be at a higher frequency, as seen in the attached chart.The red continuous trace is for when the additional 0.1uF is connected at the Input node.The green continuous trace is for the addition Input capacitor disconnected.Dotted lines represent the phase.

author: Benta
author_rank: Super Contributor
timestamp: March 02, 2021, 11:13:33 am
content: Quote from: MikeK on March 02, 2021, 03:43:21 amYeah, I wondered about that nonsense as well. LC or RLC make excellent filters of any kind (LP, HP, AP, BP, BS).

author: MikeK
author_rank: Super Contributor
timestamp: March 02, 2021, 04:39:32 pm
content: By the way, if the OP wants to design/analyze filters he should download the Elsie program (from Tonne Software). You can have it design filters for you, and you can manually enter components and plot the results.

author: bingowashisnamo
author_rank: Newbie
timestamp: March 02, 2021, 05:08:04 pm
content: Thank you all for your replies, this forum is great! I expect to be here often. The inductor is reaching its saturation current, losing its linearity and thus causing my input waveform to distort. Reducing the voltage does indeed remove the distortion in the waveform.As a side note I did want to design a functioning filter at all, just playing around on the breadboard. I left out the dampening resistor so I could see just how high the resonant peak could reach.Thanks everyone

author: MikeK
author_rank: Super Contributor
timestamp: March 02, 2021, 06:59:49 pm
content: Quote from: RoGeorge on March 02, 2021, 11:10:42 amI learned something new. I didn't know you could specify different values for a capacitor and have it all on the same plot. Cool!