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anhnha
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I am wondering why LC tank only oscillates at resonant frequency not other frequencies?
Is there a physics explanation for that?
Is there a physics explanation for that?
Yes. If you write down the equation for the current and voltage, you get a second-order differential equation with the resonant frequency as the solution (see https://en.wikipedia.org/wiki/LC_circuit for the explanation of the derivation).anhnha said:I am wondering why LC tank only oscillates at resonant frequency not other frequencies?
Is there a physics explanation for that?
anhnha said:I am wondering why LC tank only oscillates at resonant frequency not other frequencies?
Jarrodmccarthy said:I'm not familiar with the symbols but they might be transistors.
Oscillation - it is possible. It looks like a Flip-Flop with tuned drain circuits to me. Since the LC combination has no phase shift at resonance, it does not help any. What helps, is that the MOSFET that is "on" only has a given amount of current (current source at the bottom of the figure) and after some time the current through the inductance does not increase and the voltage across it will decrease. This will make the other MOSFET start conducting, sending a current through its drain circuit and "stealing " current from the first MOSFET. This behavior will "kick" the drain circuit into oscillation mode and couple this oscillation to the other MOSFET.anhnha said:So, could you explain why this cross coupled oscillator below oscillate? It is only connected with a DC voltage source.
I guessed they were transistors so thanks for specifying MOSFE.Vanadium 50 said:They're MOSFETs. And circuits with transistors are not simple LC circuits.
Svein said:Oscillation - it is possible. It looks like a Flip-Flop with tuned drain circuits to me. Since the LC combination has no phase shift at resonance, it does not help any. What helps, is that the MOSFET that is "on" only has a given amount of current (current source at the bottom of the figure) and after some time the current through the inductance does not increase and the voltage across it will decrease. This will make the other MOSFET start conducting, sending a current through its drain circuit and "stealing " current from the first MOSFET. This behavior will "kick" the drain circuit into oscillation mode and couple this oscillation to the other MOSFET.
There are several variants of LC tank oscillator, and this is not one of the most used. The dominant circuits are the Colpitts oscillator (https://en.wikipedia.org/wiki/Colpitts_oscillator) and the Hartley oscillator (https://en.wikipedia.org/wiki/Hartley_oscillator).
I think those resistances are different so there would be a potential difference giving preference to one MOSFET or the other.anhnha said:Thanks for the detailed answer.
I have some problems understanding the oscillator. The first one relating to the boldfaced part above. I don't get what you meant here.
Second problem is that how can the oscillation start up? Two transistors are exactly the same, so which one will be ON first? Will both transistors be OFF permanently?
Hi, the resistances are same (not exactly because tolerance)Jarrodmccarthy said:I think those resistances are different so there would be a potential difference giving preference to one MOSFET or the other.
If both start OFF, they will not draw current, which makes the drain HIGH, turning the other one ON. If both start ON, they will pull the drain low, turning the other one OFF. Which one? Random.anhnha said:Second problem is that how can the oscillation start up? Two transistors are exactly the same, so which one will be ON first? Will both transistors be OFF permanently?
As I remarked above:anhnha said:I simulated the oscillator. If Vdd is step voltage then the oscillator oscillates but if Vdd is constant then it doesn't oscillate. Could you explain why?
Such a circuit will usually only oscillate when you do not want it to (cf. Murphy's law). Try this circuit instead:Svein said:It looks like a Flip-Flop with tuned drain circuits to me.
The LC tank circuit is a type of electrical oscillator that consists of an inductor (L) and a capacitor (C) connected in parallel. At the resonant frequency, the reactances of the inductor and capacitor are equal and opposite, resulting in a cancellation of the reactance. This allows a continuous flow of current through the circuit, creating an oscillating electrical signal.
If the LC tank circuit is not at resonant frequency, the reactances of the inductor and capacitor will not cancel each other out and the circuit will not oscillate. Instead, the circuit will behave like a regular series or parallel RLC circuit, depending on the configuration of the components.
The resonant frequency of an LC tank circuit is determined by the values of the inductor and capacitor. The higher the values of these components, the lower the resonant frequency will be. Conversely, lower values of inductance and capacitance will result in a higher resonant frequency.
Yes, the resonant frequency of an LC tank circuit can be changed by altering the values of the inductor or capacitor. This can be done by physically changing the components or by using variable components, such as a variable capacitor, to adjust the resonant frequency.
LC tank circuits are commonly used in electronic devices for a variety of purposes, including as oscillators in radio frequency circuits, as filters in signal processing, and as tuning circuits in radio receivers. They are also used in more complex circuits, such as inductively coupled plasma (ICP) systems for spectroscopic analysis.