Misuse of the term "resonance"

[carbonzit]

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.

That's mostly gobbledygook, which is pretty standard for you. It's a high-pass filter, so tell us, in words we can all understand that actually explain something, how it can have a resonant peak. What is a resonant peak, in your understanding, anyway?

 

[audioguru]

My Bass Extension circuit has a Q and a peak exactly like an LC filter. The amplitude of the resonant peak is adjusted with the gain control on the highpass filter because it adjusts the amount of positive feedback through the feedback resistor. If the gain exceeds about 2 then it oscillates.

In google there is a link to an article about a guitar pickup resonant peak with different loads. Its LC resonant peak looks exactly like my bass extension circuit's resonant peak excent its frequencies are reversed.

 

[carbonzit]

My Bass Extension circuit has a Q and a peak exactly like an LC filter. The amplitude of the resonant peak is adjusted with the gain control on the highpass filter because it adjusts the amount of positive feedback through the feedback resistor. If the gain exceeds about 2 then it oscillates.

In google there is a link to an article about a guitar pickup resonant peak with different loads. Its LC resonant peak looks exactly like my bass extension circuit's resonant peak excent its frequencies are reversed.

Well, OK, I'll admit that at least that top curve looks like a classic resonant peak, like that of a fairly highly selective L-C filter. But then again, it doesn't look much like the response of a typical low-pass filter either, which one certainly wouldn't expect to be that "peaky".

Normally, that "peak" would be the FC of the filter. Where does that peak come from?

 

[audioguru]

A second-order Sallen and Key Bessel highpass filter has a droopy response because it is two cascaded first order filters.
A second-order Sallen and Key Butterworth highpass filter adds positive feedback to bolster the response near the cutoff frequency. Then it is flat down to near the cutoff frequency with a sharp drop at lower frequencies. You can see ringing at the boosted frequency of a second-order Sallen and Key Butterworth filter because it is caused by the positive feedback. When the positive feedback is increased then there is a peak in the frequency response near the previous cutoff frequency.

 

[MRCecil]

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.

AG,

Actually, I think you are confusing an underdamped filter response with resonance around the corner frequency. What you stated in your post #13 sounded exactly like a Chebyshev response, which does employ degrees of underdampening. From your graphic, that seemed apparent. Unfortunantly, the phase response was not included to confirm this.

I set up a simulation of a simple but grossly underdamped 2-pole highpass Chebyshev that closely approximates the response of your circuit at the highest peak of ~10.5db. With a phase shift of just 79deg, it would appear to be a relatively stable system lacking the possibility of becoming a resonator.

 

[audioguru]

AG,

Actually, I think you are confusing an underdamped filter response with resonance around the corner frequency.

They are the same.
Your resistor and capacitor values are much too low for a TL072 opamp to drive. But your response curve looks the same as mine.

 

[MRCecil]

Oh Boy. LOL!

 

[Warpspeed]

Resonance is the condition of magnified circulating energy, and it has both a measurable frequency and a measurable "Q".

Now a tuned bandpass filter has both these characteristics, but most other types of filters do not. It is a highly special case.

A smart and crooked lawyer (if paid sufficiently), could probably argue the case for you.

Bill Clinton said "I did not have sex with that woman".
And a bunch or resistors, capacitors and op amps are not real resonance.......

 

[Roff]

Resonance is the condition of magnified circulating energy, and it has both a measurable frequency and a measurable "Q".

Now a tuned bandpass filter has both these characteristics, but most other types of filters do not. It is a highly special case.

A smart and crooked lawyer (if paid sufficiently), could probably argue the case for you.

Bill Clinton said "I did not have sex with that woman".
And a bunch or resistors, capacitors and op amps are not real resonance.......

The inductor said, "I did not have sex with that capacitor!".
But we all saw the current flowing back and forth between them...

 

[Warpspeed]

I could tell you some shocking instances of conjugate coupling, and the resultant erotic resonance..

But common decency, and the moderators here would not be amused.

 

[crutschow]

Resonance is the condition of magnified circulating energy, and it has both a measurable frequency and a measurable "Q".

Now a tuned bandpass filter has both these characteristics, but most other types of filters do not. It is a highly special case.

A smart and crooked lawyer (if paid sufficiently), could probably argue the case for you.

Bill Clinton said "I did not have sex with that woman".
And a bunch or resistors, capacitors and op amps are not real resonance.......

Au contraire. If it looks like a duck, quacks like a duck, and walks like a duck, then it's a duck.

If I put two circuits in a black box and one has exactly the same frequency response as the other, then how can you say that only one exhibits the "real resonance". That's just a personal opinion, not a technical fact.

What do you mean "magnified circulating energy"? An LC circuit does not magnify the energy.

 

[Warpspeed]

What do you mean "magnified circulating energy"? An LC circuit does not magnify the energy.

It sure does !
If your circuit Q is 100, then for one watt in, and one watt out of your tuned filter, there will be 100 circulating volt amps.

And those are real volts and amps, and they can be huge in a radio transmitter or induction heater when tuned to resonance.

 

[crutschow]

It sure does !
If your circuit Q is 100, then for one watt in, and one watt out of your tuned filter, there will be 100 circulating volt amps.

And those are real volts and amps, and they can be huge in a radio transmitter or induction heater when tuned to resonance.

They may be real volts and amps but they provide imaginary power not real power. There is no magnifying of energy or real power, only magnification of reactive power.

The reason it may help in a transmitter or induction heater is that is increases the efficiency of the power coupling from the source into the load. But it certainly creates no new real power.

 

[Warpspeed]

They may be real volts and amps but they provide imaginary power not real power. There is no magnifying of energy or real power, only magnification of reactive power.

The reason it may help in a transmitter or induction heater is that is increases the efficiency of the power coupling from the source into the load. But it certainly creates no new real power.

Sorry but you are quite wrong.

Volt amps are indeed real, just because they circulate around a closed circuit does not mean they don't exist at all.

The reason transformers (and alternators) are rated in VAs and not in watts, is that all those volts and amps can easily overload and destroy a transformer, even though the output load in watts may be minimal. Reactive power, particularly resonant reactive power can rise to destructive levels, it sure is real.

Try fitting a fuse to a very high Q tuned circuit, you can blow it easily with minimal applied power at resonance. Where did all those circulating amps come from ?

 

[audioguru]

A load is rated in the number of Watts it consumes.
A transformer does not consume Watts, it passes VAs.

 

[crutschow]

Sorry but you are quite wrong.

Volt amps are indeed real, just because they circulate around a closed circuit does not mean they don't exist at all.

The reason transformers (and alternators) are rated in VAs and not in watts, is that all those volts and amps can easily overload and destroy a transformer, even though the output load in watts may be minimal. Reactive power, particularly resonant reactive power can rise to destructive levels, it sure is real.

Try fitting a fuse to a very high Q tuned circuit, you can blow it easily with minimal applied power at resonance. Where did all those circulating amps come from ?

I appreciate your being sorry, but I am not quite wrong.

Apparently you are not familiar with the term "imaginary power" which means reactive power. Imaginary doesn't mean the volts and amps don't exist, it just means the volts and amps are 90 degrees out of phase (hence in the imaginary quadrant of the complex number plane) and are not carrying real power (watts).

Reactive current can cause problems when it is dissipated in the resistance of a circuit (such as the transformer resistance) where it does causes real power dissipation. But it is the portion of the voltage and current that is not 90 degrees out of phase that generates that power.

To summarize, in a resonant circuit you can indeed have large values of current and large values of voltage, but they do not represent real power only reactive power.

If reactive power rises to destructive levels it is because the high current or high voltage from that reactive power over-stresses some part in the system.

It should be noted that a resonant circuit can store energy in the capacitive and inductive reactance which allows the current and voltage to increase beyond that of the source but that is simply storing the energy applied to the circuit, not magnifying the energy anymore than a battery magnifies energy.

Perhaps we are just quibbling mostly over semantics.

 

[Warpspeed]

A load can be rated for both watts and VAs.
Power factor tells you the proportion of watts to VAs.

And you might be interested that all the losses within a transformer that cause transformer heating are in phase and measured in watts.

 

[Warpspeed]

I appreciate your being sorry, but I am not quite wrong.

Apparently you are not familiar with the term "imaginary power" which means reactive power. Imaginary doesn't mean the volts and amps don't exist, it just means the volts and amps are 90 degrees out of phase (hence in the imaginary quadrant of the complex number plane) and are not carrying real power (watts).

Reactive current can cause problems when it is dissipated in the resistance of a circuit (such as the transformer resistance) where it does causes real power dissipation. But it is the portion of the voltage and current that is not 90 degrees out of phase that generates that power.

To summarize, in a resonant circuit you can indeed have large values of current and large values of voltage, but they do not represent real power only reactive power.

If reactive power rises to destructive levels it is because the high current or high voltage from that reactive power over-stresses some part in the system.

It should be noted that a resonant circuit can store energy in the capacitive and inductive reactance which allows the current and voltage to increase beyond that of the source but that is simply storing the energy applied to the circuit, not magnifying the energy anymore than a battery magnifies energy.

Perhaps we are just quibbling mostly over semantics.

Yes you are quite right.
Imaginary power might suggest it is completely illusory and does not really exist
But we both know that is not the case.
Perhaps "potential power" be a better name for it, but "imaginary power" is what the mathematicians have decided to name it.
But circulating reactive power is very real, and it can offer a circuit designer a real challenge.

 

[crutschow]

Perhaps "potential power" be a better name for it, but "imaginary power" is what the mathematicians have decided to name it.

The naming goes back to the development of the complex number system and how to deal with the square root of (-1) which undefined in the real number system. The mathematicians decided to define the √(-1) as i (e.g. i² = -1) and all numbers that involve i "imaginary". It was perhaps an unfortunate term to use but that's what stuck. Since AC circuits are described using complex numbers (where j ≡ i) and reactive current involves the imaginary part of the complex number, then reactive current is also call imaginary.

 

[Mr RB]

What's wrong with the square root of -1?