# 4-Pole Filter & Reso%



## Tod (Oct 21, 2007)

Hi my Friend,



> In a multi-pole filter, there is a tendancy at the 'corner' frequency for the reactive components to cause a bump or 'resonant rise' in the response when the damping is insufficient.



Is this a passive filter your talking about? And would that circut have a coil devise (Henerys)? At 100 hz center freq what would you think the corner freq(s) would be? In my tests I saw nothing to indicate any bumps so maybe they were already worked out. :?: 



> BTW The term Q is also often used instead of resonance (Q stands for quality because it originated as a measure of quality for inductors) . As you raise the Q of the 'circuit', the 'bump' at the corner frequency should get higher and steeper and if not limited in some way it would eventually break out in oscillation at the corner frequency.



I do remember a few years ago reading about this (Q standing for quality) along with all the details. What can be confusing is that nowdays I've seen "Q" stand for (or used for) bandwidth, at least seemingly so. 

BTW I do trust my tests fairly well so that what you see is the way it's working. Actually it appears that the Reso% could be an EQ with a rather broadband bandwidth (2 or 3+ octaves). 

God Bless you too, your technical input and understanding is not only refreshing but has a tendency to keep one on their toes. :D 

Tod


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## Big Bob (Oct 22, 2007)

Hi Tod,



> Is this a passive filter your talking about? And would that circut have a coil devise (Henerys)? At 100 hz center freq what would you think the corner freq(s) would be?



I'm sorry that sometimes my posts seem to confuse the issue more than shed any light on the subject :lol: So, let me try to clarify a little (but I'll probably only make it worse :wink: 

Clarification #1: Now days when we talk about filters we might very well be talking about some software synthesized, mathematical contraption that tries to emulate the behavior of an analog circuit. I was describing the analog circuit itself, not NI's attempt at recreating it. In fact I tried to indicate that with my opening, long-winded sentence.



> In my tests I saw nothing to indicate any bumps so maybe they were already worked out.



Nor did I and I and that concerns me a little as to NI's implementation. BTW I also tried to say that in my opening sentence but I guess it didn't come across. :? 



> I do remember a few years ago reading about this (Q standing for quality) along with all the details. What can be confusing is that nowdays I've seen "Q" stand for (or used for) bandwidth, at least seemingly so.



Clarification #2
I know it can be confusing but believe it or not it's really the same thing being talked about :o . The simplified origin of all this goes something like this. Inductors (coils if you like) have two basic electrical properties. One is that they oppose the flow of alternating current much like a resistor opposes the flow of direct (or alternating current). This opposition is called the inductive reactance of the coil and it is proportional to the frequency of the alternating current and the inductance of the coil (measured in a unit called the henry as you mentioned). The second property of an inductor is its resistance. Inductors are merely coils of wire, often wound on a ferrite core (to contain the magnetic field better). But real-world wire always has some resistance. So, when an inductor is used in a circuit, it offers both resistance and reactance to the flow of alternating current. The resistive energy component is always dissipated as heat wheras the reactive energy component is alternately stored in the magnetic field and then released back to the circuit. Now, finally we get to Q. The efficiency of an inductor is related to how low the winding resistance can be made relative to the inductive reactance. The ratio of inductive reactance to resistance (at any given frequency) is more or less the fundamental definition of Q.

However, it turns out that once Q was defined this way, it would keep popping up as a very useful parameter for expressing all kinds of things releated to filters in general (even those without any inductors :o ). For instance, for a bandpass filter, higher Q circuits will exhibit a narrower bandwidth with steeper slopes each side of the center frequency.

If we were sitting together in a room together with pencil and paper at hand, I think just a few sketches (and far fewer words) would be required to illustrate how Q (or resonance) should affect the performance of various types of filters. I say 'should' because we can't always rely on the software emulation to be done correctly.

When I get a little time, let me try to find some line drawings of filter responses or (if nothing else) to figure out how I can sketch some and then post it for you. You know what they always say, one picture is worth a thousand words :roll: 

In the meantime, you have a lovely day ol' buddy.

God Bless,

Bob


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## Tod (Oct 22, 2007)

Hi Bob,

The whole reason I got curious about the resonance a while back is because I was trying to get rid of some low end noise in some samples I had (I think it was violas). Useing the 4-Pole-HP filter I was able to diminish it quite a bit however, it also took out some of the nice low end on the violas. So I started to adjust the "Reso" and found that by adding a small amount (10%) it helped bring back some of that nice low end. The problem is that it also slightly brought back the lower stuff and about 10% was maximum that I could go with it. 

After checking NI's explanation in the manual I decided to test it out to see exactly what it was doing. 

NI's explanation in the manual:


> *Resonance:* Emphasizes the frequencies just above the cutoff frequencies.


Based on http://img46.imageshack.us/img46/7398/4polereso10sinewavtestbyr8.png (my tests) I think their explanation is somewhat misleading and I'm not so sure a parametric right after the filter might not do a better job. However I haven't tried that yet.

I did try to find that article/thread I read back some years ago concerning the "Q" (quality) of inductors but couldn't find it. Hehe, I thought this was rather cute though:  

http://www.ngsir.netfirms.com/englishhtm/RL.htm

Tod


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## Tod (Oct 23, 2007)

Thanks Thonex, some good info there. I didn't realize they had filters like that. :shock: 

Tod


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