Potentiometer in series with a capacitor will act as a variable capacitor

I wonder if a 47pf capacitor in series to a potentiometer to ground, will act as a variable capacitor.
In the sense that the "effectiveness" of the shunted capacitance to ground (in an oscillator circuit tank), will be less with a series pot in place?

The potentiometer varies the effective capacitance, but it also adds losses to the oscillator circuit and at worst case stops the oscillations.

FvM said:

The potentiometer varies the effective capacitance, but it also adds losses to the oscillator circuit and at worst case stops the oscillations.

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The crystal oscillator has a shunt capacitor to ground ~22pf. I am thinking of adding another parallel capacitor-potentiometer, to this main oscillator capacitor. This way there will always be a capacitor from the crystal to the ground (main osc capacitor which will maintain the oscillation) and another capacitor (varied with the pot) from the crystal to the ground.

Do you think this configuration will kill the oscillation?

I wonder if a 47pf capacitor in series to a potentiometer to ground, will act as a variable capacitor...

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In what way? In simple words, no.

A capacitor is basically a charge storage device. In technical terms, you measure the displacement current (Xc- the reactance) and the phase shift and decide whether the device is a capacitor.

Once you decide the frequency response, you can call it a capacitor, inductor or a resistor.

In an oscillator the phase shift is the most crucial factor. Without the proper phase shift, there will be no oscillator. Further, a tank circuit (ideally) is lossless- a resistor is a lossy device.

A small resistor will decrease the Q and reduce the effectiveness- a large resistor may altogether stop the oscillation.

neazoi said:

I wonder if a 47pf capacitor in series to a potentiometer to ground, will act as a variable capacitor.

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If so, mankind would not even have spent one minute inventing the varicap.

c_mitra said:

A small resistor will decrease the Q and reduce the effectiveness- a large resistor may altogether stop the oscillation.

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So it agrees with FvM's post?
Will it tune the oscillator as if it was a varicap? The osc is a crystal based, so the major component that defines Q is the crystal.

So it agrees with FvM's post?...

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Yes.

The osc is a crystal based, so the major component that defines Q is the crystal...

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Not quite; The Q is defined by the tank circuit. It is rather difficult to work with a very high Q system and we often deliberately reduce the Q (loading).

The crystal sometimes act like an inductor and sometimes it act as a capacitor.