An oscillator with multiple positive feedback

[GGAPBE96]

If an oscillator has multiple positive feedback paths,
it oscillates at plural frequencies or at a certain frequency??

 

[LvW]

If an oscillator has multiple positive feedback paths,
it oscillates at plural frequencies or at a certain frequency??

Do you have a specific circuitry in mind ?
All oscillators are designed in such a way that they oscillate at one frequency only, even if there are multiple feedback branches.

 

[GGAPBE96]

Do you have a specific circuitry in mind?

Concretely speaking,I'm thinking of crystal oscillators with the risk of parasitic oscillation.

 

[LvW]

If there are parasitic oscillations in crystal oscillators, they are caused in most - if not in all - cases not by a secondary (unwanted) feedback path but by the different modes the crystal can operate.

 

[GGAPBE96]

If there are parasitic oscillations in crystal oscillators, they are caused in most - if not in all - cases not by a secondary (unwanted) feedback path but by the different modes the crystal can operate.

I believed parasitic oscillation was derived from an unwanted feedback path..
I can't understand the words "different modes the crystal can operate".
The two oscillations are compatible??

 

[FvM]

I can't understand the words "different modes the crystal can operate"

Most mechanical and EM oscillators have "modes". A crystal at least has different overtones, I guess, you have heard about it before.

You didn't suggest yet a reason, why "multipath" should be related to "plural" frequencies. I also don't know a crystal oscillator topology, that is particularly involving multipath feedback, you should give a hint to explain. Apart from crystal oscillators, in a linear system, multiple feedback pathes superimpose to a total feedback path, that has to fulfill the oscillation condition.

 

[biff44]

anything can happen! Usually, one of the feedback frequencies has a little more small signal gain, or is a little more close to 2Π degrees, so it builds up first and dominates the others by compressing the available gain. However, if something changes, like a slight change in temperature, etc, another frequency can take over.

Usually if you see multiple frequencies of oscillation, they are very far removed from each other, like tones at 200 KHz and other simultaneous tones at 400 MHz, etc.