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# Resonance or?

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#### Electronman

##### New Member
Hi,

The LC circuits are used to widely to generate resonance at a certain freq for RF and other usages.
I have a question regarding to this.
If we have an oscillator made by a chip like 555 or a TTL chip which generates oscillation at a certain freq the can we use just an inductor to its output at and get similar results to LC CIRCUITS? because L and C have 90 degrees of phase difference then they have the MIN or MAX efficiency at a certain frq. But if the oscillator chip generates the oscillations and freq then just an L is enough to propagate the signal, then maybe we do not need a cap??? At which situation the efficiency of the output is higher???

A 555 or a TTL chip has a square-wave output.
An LC circuit is resonant and its oscillator produces a sine-wave output.
An inductor without a capacitor is a high impedance at high frequencies and would not do anything at the output of the square-wave oscillator if it was in parallel with the output. If in series with the output then the square-wave will be reduced in level and will be filtered a little.

It might be possible to use a 555 with an LC circuit to make an oscillator. I've never seen the configuration, but lots of ideas are out there.

As AudioGuru said, the 555 outputs a square wave. This can excite an LC circuit just fine though.

Hi,

I have seen a 555 use an inductor to do the timing rather than a cap, but
it's more of a novelty thing and certainly not as cheap or simple as using
a cap. There might be some magnetic field sensitivity too, which can be
good or bad depending on what you are doing with it.

There is also the LC filter on the output of the 555 idea, to help to filter into
a more sinusoidal wave shape. For the best wave shape, the resonate
point of the LC combination should match the running frequency of the 555,
and the 555 should be connected to put out as close to a square wave as
possible.

I'm looking for the inductor circuit. Can you point me to it?

Usually, folks want to go in the other direction because a cap is smaller/lighter and easier to integrate in a chip or on a board. I wonder if Electronman is trying to confuse a 555 which is a phase locked loop with an active filter which is normally built with an opamp and an RC feedback loop?

So what If the output of the oscillator tuned to be a sine and not a square?

The output of a 555 is a rectangular wave. I have never seen one drive a tuned LC circuit to produce a sine-wave.

If you want a sine-wave than why not make a sine-wave oscillator?

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No, I meant If the square wave is just the problematic thing which causes us to not use an JUST L instead an LC for an electromagnetic resonator?

An inductor without a capacitor does not resonate so the output will probably not be a sine-wave.

An inductor without a capacitor does not resonate so the output will probably not be a sine-wave.

I meant what about if we have a sine generator source (made by an op amp or any sine generator) then connect its output to an inductor to have radio waves instead of using a tank, makes sense?

Most opamps cannot go as high as radio waves.
If you have a sine-wave generator that goes high enough then an inductor will do nothing. An inductor is not a radio transmitter.

An LC tank is used to smooth the square-waves output of a transistor into a sine-wave.

Now I am not able to understand the real reason for using an LC instead of an just L for propagating electromagnetic waves?

Because a LC circuit is a band-pass filter.

Hello again,

An LC filter might be used on the output of a 555 or other square wave
generator to filter the squarish wave into a sine so you can get more of a
pure output spectrum rather than a bunch of junk with your required
signal. The inductor may radiate if it is large enough relative to the
wavelength, but many times it is just used for the filter.

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