The new project: diseño de una radiobaliza - Emisor -.
During the lectures of any electronic or telecom courses we speak of sinusoidal signal, we used source of sinusoidal signal and so on... but where they come from? How in reality we can produce this kind of signal? The basic idea is the sinusoidal oscillator.
If we think how to obtain a sinusoidal signal (a precise "tone" in frequency) we can tought to the parallel of L and C. But it' s important to understand that this is only an ideal circuit. We saw in the practice that we can' t have a real component called L, but we have an inductance with her parasitic resistence R. Hence the circuit is not again an oscillator, because of the presence of the R we obtain a fading sinusoid.
We need a new idea and we start to study a different structure: an amplificator of gain k and a filter H. If in input we have a sinusoid signal from a generator, what we obtain in general at the output? And what are the condiction in the amplifier and the filter to obtain that the output is a replica of the input? If we answer to these question we conclude that the product of k and the amplitude of the filter at the input frequency must be equal to 1 and the change in phased 0.
What happen if we connect the input with the output? Because they are the same (under the condiction writed above) nothing happens in the sense that the results doesn' t change.
Therefore we have removed the generetor and we find that the circuit admits for solution a sinusoidal signal: we found the oscillator sinusiodal!
For example using an OPamp (because we need small amplification factor) we can realize the amplifier and we can realize in practice the filter with a resitance a capacitor and an inductor. We have to pay attention that the conditions are met (1 in total amplification and 0 phase shift for a particular frequency f0 that depends on the value of L and C).
There is two important remarks. First, what happens if the amplify is not perfectly 1? The answer is that the sinusiod can make increasing oscillations or decreasing one. Second, if we don't have the generator (obviusly because we are trying to making it!) how can this circuit start to works? The secret is that at any temperature the amplifier itself make some termal noise, this noise is a constant in all the frequency and therefore the frequency f0 is excited and if we have a passaband filer we obtained the desired sinusoid.
We have to note that the product of the amplify factor k and the value of the amplitude of the filter must be little higher than 1.
Where we have to take our sinusoidal signal producted from our oscillator? There is two possibilities, one is at the output of the filter and the other is at the output of the amplifier. The first chose is positive if we look at the shape of the sinusoid but we have to observe that is an high impedence node and this is not good because anything that we connect modify the oscillator circuit. The other possibility is to take the signal at the output of the amplifier, this is a low impedance node but we find that the shape is not a perfect sinusiod because if the amplitude is more than 1/H(f0) we find a crescent sinusoid that is cutting because of the saturation limit of the amplier.
In conclusion we obtain an oscillator if we have an amplifier and a passaband filter appropriately designed, the output of the filter is connected to the input of the amplifeir. Under certain conditions this circuit start itself to produce sinusoid signal at the frequency of the peak of the passaband filter, we obtain purer sinusoid if the Q quality factor of the peak is higher.
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