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Misuse of the term "resonance"

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carbonzit

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I guess this is a complaint, or at least a cautionary note: Seems like I've read postings here that incorrectly use the term "resonance".

Specifically, I've read things like "the resonant frequency of this low-pass filter is ...". (Sorry, don't have the actual postings at hand, will have to hunt for them.) The implication is that a low-pass filter (or high-pass filter for that matter) can have a resonant frequency.

It can't. A resonant circuit, technically speaking and by definition, must have both inductive (L) and capacitive (C) elements, and it is resonant when XC=XL. Since low-pass, high-pass, and other filter types only contain either C or L elements, they cannot resonate.

I also read a posting that talked about how a large electrolytic capacitor could "resonate" at a certain frequency, with a chart to illustrate. In this case, however, the explanation is that the capacitor itself contains a significant amount of inductance, so again there are both C and L elements.

I think that the writers of such things are confusing other frequencies (like cutoff or corner frequency) with resonant frequency.

This may seem like a nitpick, and maybe it is, but in fields like electronics, it's important that we all speak the same language, and that we are fairly precise about what we mean. Otherwise, confusion arises, and that's not a good thing.
 
An active bandpass filter exhibits frequency response that resembles that of an LC parallel resonant circuit. The bandpass peak frequency is referred to in the industry (at least by **broken link removed** :D) as the resonant frequency.
 
The center frequency of an active bandpass filter is sometimes called the resonant frequency because of it's similarity to an LC bandpass filter. But it's probably not correct to refer to the corner frequency of a LP or HP filter as its resonant frequency.

It's true a passive resonant circuit must have Ls and Cs, but an active filter can behave as a resonant circuit with only Rs and Cs and its transfer function can be made to look the same as a filter with Ls and Cs.
 
It's true a passive resonant circuit must have Ls and Cs, but an active filter can behave as a resonant circuit with only Rs and Cs and its transfer function can be made to look the same as a filter with Ls and Cs.

Well, sure: I didn't mean to discriminate against the likes of gyrator (simulated inductor) circuits, which I suppose for the purposes of this discussion can be considered to be L elements.

A bandpass filter? I'm skeptical; again, strictly speaking, they don't have resonant Fs, even though a graph of one's response may look like it. Guess that's a judgment call ...
 
It depends on your definition of resonance.

I do take issue with this statement:
Since low-pass, high-pass, and other filter types only contain either C or L elements, they cannot resonate.
I'm so old, I studied passive LC filter design in college. See this site for an example. It's true that a lowpass filter does not resonate, but it definitely can have L's and C's. In fact, to make canonical (Butterworth, Chebyshev, etc.) filters without active elements, the filter must have both L's and C's (except for first order filters, of course).
 
It depends on your definition of resonance.

I do take issue with this statement:
carbonzit said:
Since low-pass, high-pass, and other filter types only contain either C or L elements, they cannot resonate.

I'm so old, I studied passive LC filter design in college. See this site for an example. It's true that a lowpass filter does not resonate, but it definitely can have L's and C's. In fact, to make canonical (Butterworth, Chebyshev, etc.) filters without active elements, the filter must have both L's and C's (except for first order filters, of course).

OK, so I should have said:
Since low-pass, high-pass, and other filter types typically only contain either C or L elements, they cannot resonate.

Better? (replace with your modifier of choice if you like)

[Hey--nested quotes work!]
 
carbonzit,

This may seem like a nitpick, and maybe it is, but in fields like electronics, it's important that we all speak the same language, and that we are fairly precise about what we mean. Otherwise, confusion arises, and that's not a good thing

I agree with you. Calling the pass frequency of a filter its resonant frequency instead of its center frequeny, because it sometimes has a similarity to a true resonant circuit, is like calling margarine "butter" because you spread it on bread.

You are also correct about being precise. That is why I don't use the term "current flow" when I really mean charge flow.

Ratch
 
carbonzit said:
Since low-pass, high-pass, and other filter types typically only contain either C or L elements, they cannot resonate.

I take issue with that statement as well. most lo, hi, band pass filters I have ever seen all have C's and L's :)

an example....

rf_lowpass_filter-png.52222


**broken link removed**

cheers
Dave
 

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I take issue with that statement as well. most lo, hi, band pass filters I have ever seen all have C's and L's :)

an example....

rf_lowpass_filter-png.52222


**broken link removed**

Hmm, maybe that's because you work out there in RF-land, where everything is weird (every piece of wire becomes an inductor, etc.).
 
Last edited:
Hmm, maybe that's because you work out there in RF-land, where everything is weird (every piece of wire becomes an inductor, etc.).

Well Mr Zit, wait until you get into the microwave region, then you will see weird!

At microwave frequencies, a bit of "pipe" with some screws in it can be a filter, usually a bandpass filter.

JimB
 
Well Mr Zit, wait until you get into the microwave region, then you will see weird!
At microwave frequencies, a bit of "pipe" with some screws in it can be a filter, usually a bandpass filter.
JimB

hey Jim,
yeah, ya just gotta love pipe cap filters :) have them in both my 5.7Ghz and 10GHz transverters

Dave
VK2TDN
 
While we are at it: my nitpicking is with the use of the term "phase" when it is clearly "polarity" that should be used. Damn the details. E
 
I use an active second-order highpass filter with a high Q so that its resonance makes bass-boost then a sharp dropoff for my stereo system.

I use an active second-order lowpass filter with a high Q so that its resonance makes treble boost then a sharp dropoff on telephone confrencing systems.

They don't use inductors.
 
I use an active second-order highpass filter with a high Q so that its resonance makes bass-boost then a sharp dropoff for my stereo system.

I use an active second-order lowpass filter with a high Q so that its resonance makes treble boost then a sharp dropoff on telephone confrencing systems.

They don't use inductors.

You're wrong. Plain and simple wrong.

Of course, you'll never admit it ...
 
You're wrong. Plain and simple wrong.

Of course, you'll never admit it ...
So who made you an authority on that?

It's common to refer to peaking in filters as "resonance" whether the filter is LC or active. Just because this use of the word doesn't fit in your narrow definition doesn't make it wrong, "plain and simple" or otherwise.

You and "Hopelessly pedantic" can live in your own little world of narrow definitions if you like, but that doesn't mean they are absolutely right, or that the rest of us have to be confined by them..
 
Rooms and halls have resonances and they don't have inductors. Even a pop bottle has a resonance. Musical instruments?
 
Rooms and halls have resonances and they don't have inductors. Even a pop bottle has a resonance. Musical instruments?

Yes, they do, being complex mechanical systems with the equivalents of L and C elements. But your filters, mentioned earlier, do not have resonance, so that term is misused there.

Please explain to us how those filters have resonance. Show your work.
 
Show your work.
I designed my bass boost circuit long before this Australian guy published his circuit on the web but they are the same. Maybe he copied mine.
The circuit is an active second-order highpass filter with positive feedback higher than a flat Butterworth filter so it has a resonant peak.
 

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