Notch Filter - Page 1
Forum section: electronics / projects

author: Bryan
author_rank: Frequent Contributor
timestamp: September 27, 2018, 08:01:04 am
content: All:Putting together a notch filter to alleviate some line noise being introduced from a LED fixture into my X10 setup. Being playing around with this schematic in LTSpice. I am wondering what is the purpose of the 330K resistor. No expert in LTSpice, and maybe I have it setup wrong but I see no difference in attenuation or the waveform if the resistor is removed from the schematic?Thanks

author: Bryan
author_rank: Frequent Contributor
timestamp: September 27, 2018, 08:02:02 am
content: and the schematic...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2018, 10:44:57 am
content: Leakage so the capacitor discharges when unconnected.C3 is a really odd value, I suppose it'll work but it's not a traditional notch filter circuit, meaning its performance and impedance may not be as expected. Not that mains wiring has any controlled impedance...Tim

author: mikerj
author_rank: Super Contributor
timestamp: September 27, 2018, 01:19:24 pm
content: T3sl4co1l has answered your main question, but what is the designed load impedance for this filter? Without adding that to your simulation the second half of the filter (L2, C5,C4 in the schematic) is doing nothing at all.Is this going to be filtering mains voltage levels? There could be some pretty hefty currents circulating through L1/C3 if there is any significant energy around the resonant frequency (~16kHz)

author: Bryan
author_rank: Frequent Contributor
timestamp: September 27, 2018, 06:49:32 pm
content: Thanks everyone. it's intended purpose is to attenuate noise ~120KHZ being introduced into the mains from a lousy mfg. designed power supply (pretty common these days) from a LED fixture. It's based on a design from a common commercial X10 filter sold, but is supposed to be better?.From my simulations on LTspice a higher capacitance at C2 increases the attenuation, but I am sure at a expense. I thought perhaps R1 purpose was as a bleeder, but wondering why 330K, which got me thinking it had a alternate purpose. Guess it realy doesn't matter as long as it's a high value.http://www.nomad.ee/micros/x10filter.shtmlhas some schematics on various filters both commercial and DIY to deal with the X10 issue of mains noise.I added 600R ohm to the output in LTspice?The one I will probably settle on is below. I modified the component values slightly as the inductor values at the current levels ~3 amps were more comonly available.

author: ogden
author_rank: Super Contributor
timestamp: September 27, 2018, 07:46:10 pm
content: Quote from: Bryan on September 27, 2018, 06:49:32 pmThere's no point to filter just 120KHz interference while passing-through it's harmonics and other nasty hi-frequency stuff, so you don't need notch filter. First thing to try - clamp-on ferrite chokes, one on input and another on output both as close as possible to supply. If it does not help then better just throw that piece of  in the garbage and get proper stuff

author: Bryan
author_rank: Frequent Contributor
timestamp: September 27, 2018, 08:30:08 pm
content: X10 relies on communication at 120khz so not sure how the harmonics would be a issue?. Agree about chucking it...X10 is old technology from the 70;s when there was few switching supplies and the mass of junk out there that radiates noise. I should probably update the controllers to something more modern, but it's simple and for the most part works.

author: ogden
author_rank: Super Contributor
timestamp: September 27, 2018, 09:52:43 pm
content: Quote from: Bryan on September 27, 2018, 08:30:08 pmCould be issue, could be not. Depends on how good receiver is. Problem with notch filters are - ideal components does not exist. You will struggle to place notch at target frequency and it will not be as deep as your simulation (that uses ideal components BTW). Simulate with real components. Also try to increase/decrease inductors and capacitors by 5% or whatever tolerance you plan to get and see where notch moves. Could be so that in result your filter cost more than that chitty LED panel supply.With low pass filter it's is simple - just move cutoff frequency far enough and component tolerances will not matter much.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 27, 2018, 10:42:50 pm
content: I'd also vote for more generalized filtering. While 120kHz may be the frequency of concern and by making a notch filter and taking it out you would likely solve your problem, less noise is always nice! Odds are good that your noisy power supply is spewing out all sorts of frequency content, and some of it could effect noise on other things or cause an issue somewhere down the line, so a more generalized low pass filter set to be as low as you want to buy caps for would both solve your 120kHz problem and eliminate some of that other spurious nastiness.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2018, 11:17:39 pm
content: You can save somewhat on inductors by notching just one frequency, but a full filter isn't much more. Can also buy an off the shelf filter module, given some considerations (grounding, fusing..).Tim

author: Bryan
author_rank: Frequent Contributor
timestamp: September 28, 2018, 08:26:04 am
content: Thanks everyone, yes probably should consider a low pass filter, and the attenuation is the same if not more at 120Khz. At least with the component values specified in one commercial product.

author: Bryan
author_rank: Frequent Contributor
timestamp: September 28, 2018, 07:34:36 pm
content: Out of curiosity is there way of having a look at the line noise with a scope. I believe I can use a transformer from what I understand, but the amplitude of the noise will be reduced by the factor of the transformer (perhaps isolation transformer instead?) and high frequency noise would be attenuated, which is ok as I am more interested in viewing under 200Khz.

author: ogden
author_rank: Super Contributor
timestamp: September 28, 2018, 08:16:42 pm
content: Quote from: Bryan on September 28, 2018, 07:34:36 pmKind of sensitive, safety-related subject. Good place to start:https://www.eevblog.com/forum/beginners/mains-and-the-oscilloscope/. Then there's few google hits worth to read

author: Bryan
author_rank: Frequent Contributor
timestamp: September 28, 2018, 08:27:56 pm
content: Yes, have read a fair bit about scopes and mains and the ground issue, so would be careful. Off topic but probably the best I found in understanding is Uncle Dougs videoshttps://youtu.be/vz70GSBhYjAhttps://youtu.be/5SjVm-rF39A