Build RLC Oscillator Circuit: Real-Life Examples & Falstad Simulation

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Discussion Overview

The discussion revolves around the functioning and construction of RLC oscillator circuits, particularly in the context of real-life applications and simulations using the Falstad circuit simulator. Participants explore the theoretical underpinnings, practical challenges, and specific oscillator designs such as the Hartley oscillator.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether a simple RLC circuit can produce sustained oscillations in a real circuit, noting that simulations may not reflect reality.
  • Another participant argues that in a real RLC circuit, the capacitor will charge to the supply voltage, leading to a halt in oscillation after initial damped activity.
  • Some participants suggest that additional components, such as transistors or elements with negative differential resistance, are necessary for sustained oscillation.
  • A participant discusses the importance of positive feedback in oscillators, indicating that it can help overcome resistive losses and sustain oscillation.
  • There is a query about the Hartley oscillator's design, specifically the grounding of the center tap of inductors and its impact on circuit behavior.
  • Another participant mentions that feedback in oscillators is typically achieved using reactive elements to satisfy the Barkhausen stability criterion.
  • One participant provides clarification on the Hartley oscillator's feedback mechanism, explaining the role of the tapped coil and the necessity of tuning for specific frequencies.

Areas of Agreement / Disagreement

Participants express differing views on the ability of a simple RLC circuit to sustain oscillations, with some asserting that it cannot while others explore the conditions under which it might. The discussion includes multiple competing perspectives on the design and functioning of oscillators, particularly regarding feedback mechanisms and component requirements.

Contextual Notes

Participants note that the simulation may not accurately represent real-world behavior, and there are unresolved questions about the specific values and configurations needed for oscillation in circuits like the Hartley oscillator.

Who May Find This Useful

This discussion may be useful for individuals interested in circuit design, particularly those exploring oscillator circuits, feedback mechanisms, and the practical implications of theoretical concepts in electronics.

Raint
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Hello I am trying to understand how exactly an oscillator circuit work and how to build one.

So I was palying around in falstad and made this:


the following is link. tried to get around spam filter so.. write without spaces
h t t p s : / / w w w . f a l s t a d . c o m / c i r c u i t / #

and add:

%24+1+5.0E-6+10.20027730826997+54+5.0+50%0Ac+208+176+208+288+0+1.0E-5+-1.5725275170777988%0Ar+320+176+320+288+0+100.0%0Aw+320+288+208+288+0%0AR+320+288+512+288+0+0+40.0+5.0+0.0+0.0+0.5%0Al+208+176+320+176+0+1.0+-0.011516250308585715%0Ag+320+176+496+176+0%0AO+208+176+160+176+0%0Ao+0+64+0+35+20.0+0.025+0+-1%0Ao+0+64+0+35+20.0+0.05+1+-1%0A

As it is visible from circuit simulator, only a capacitor, inductor and resistor is used to create non-ending sinus waveform from DC.

Question: would this actually work in real circuit? And if so, then why every example of oscillator circuit online includes at least one transistor and few more capacitors and inductors?

thank you.
 
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Despite what you simulator is showing you, this doesn't actually happen.

What does happen is that the capacitor charges up to the same DC voltage as the supply and then all further activity stops.

At best you might get a slight "ringing" or damped oscillation when power is first applied but this reduces in amplitude to zero, usually after just a few cycles.

You do need a transistor, a FET, an integrated circuit or even a tunnel diode to produced sustained oscillation.
 
It seems this website does not have correct electric circuit simulation algorithm. An RLC circuit produces damped oscillations, or an aperiodic puls, depending on the value of the resistance.
 
You can use a positive feedback (so output and feedback input are in phase) gain of greater than 1 to overcome your resistive losses. The oscillation will grow til its held back down by nonlinearities of your amplifier.
 
Or an additional element with negative differential resistance.
 
Thank you for explaining that. it makes sense that capacitor would only charge till the voltage of Voltage source. I'm trying to understand oscillator schemes. For example Hartley oscillator. Understanding part does not go that well.

For example, in Hartley oscillator. the middle of two inductors is connected to ground. Why? how does it change the circuit? Also I tried changing values of capacitors and inductors in circuit simulator. once i did, the oscilliation broke down. So apparently they must be in right proportions. But how?

Does anyone know where I could find materials about oscilator online?
 
DragonPetter said:
You can use a positive feedback (so output and feedback input are in phase) gain of greater than 1 to overcome your resistive losses. The oscillation will grow til its held back down by nonlinearities of your amplifier.


I understand how feedback in phase could do the trick. but getting that feedback is the hard part. I guess, a good way to do it is to somehow connect transistor base to the oscillations in LC. But not sure how.
 
Usually, in most types of oscillators, the feedback is achieved by potentiometrically dividing the voltage, but, instead of Ohmic resistors, one uses reactive elements, so that the Barkhausen stability criterion is satisfied for a particular (carrier) frequency.

The schematics given are deformed so that this basic scheme may seems obscured, but, it should be possible, through topologically equivalent deformations (i.e. only elongating and twisting the wires without breaking them, or reconnecting them if they cross), to bring it to the textbook example.
 
Have a look at the links in this post:
https://www.physicsforums.com/showpost.php?p=3908733&postcount=6

The center tap of a Hartley oscillator is not grounded. Notice that this is a single coil with a tapping on it, so the two halves are magnetically coupled together. The tap is usually one third of the way up from the grounded end.

Feedback in this oscillator is from the emitter via the coil (which is a step-up transformer) back to the base of a transistor or to the gate of a FET.
Tuning the coil with a capacitor ensures that this feedback only happens at one frequency, so that is where the circuit oscillates.

However if you build this circuit, you will need an oscilloscope to view the output, or a receiver in the same room so that you can listen to it. the receiver will not need to be connected to the circuit as there is always some radiation.
 

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