# Am vs Fm variable capacitor receiver, what is the difference?

• sol47739
In summary, a variable capacitor is used in both AM and FM receivers to select the desired radio signal by adjusting the resonant frequency of the LC circuit. The frequency of the signal does not affect the functionality of the variable capacitor.

#### sol47739

TL;DR Summary
What is the difference between a variable capacitor in a AM receiver and a variable capacitor in a FM receiver?
What is the difference between a variable capacitor in a AM receiver and a variable capacitor in a FM receiver? I understand that Am is amplitude modulation and that the signal is carried over a changing amplitude and that the frequency is constant. And the opposite in FM signals. And a variable capacitor is a capacitor which you can change in order to match the signal you want to receive. When an antenna receives a FM wave the currents acceleration will change according to the absorbed wave and when an antenna receives an AM wave the acceleration of the charges will be the same but the amount of „electrons“ that get accelerated are larger. Thus the voltage over the capacitor changes. So I can kind of understand that you can have a capacitor and tune it in to the right amount of electrons that get accelerated in the FM case, since the amplitude is constant and thus the amount of electrons that will „build up“ on one side of the capacitor. Am I thinking correct? But how does a variable capacitor work in the AM case, when the frequency is constant?

sol47739 said:
Am I thinking correct? But how does a variable capacitor work in the AM case, when the frequency is constant?
You are confused and not thinking about it in the easiest way.

A variable capacitor is used with an inductor to form an LC resonant circuit. At resonance, energy circulates between the electric field of the capacitor and the magnetic field of the inductor. The resonant frequency is adjusted, with the variable capacitor, to select the wanted radio signal.
https://en.wikipedia.org/wiki/LC_circuit

For the AM broadcast band, the LC circuit will be resonant at about 1 MHz.
For the FM broadcast band, the LC circuit will be resonant at about 100 MHz.
The form of modulation employed decides the type of detector needed, not the method used for tuning the RF signal.

berkeman and DaveE
sol47739 said:
how does a variable capacitor work in the AM case, when the frequency is constant?
Regardless of the modulation method, it has a certain bandwidth, so a tuned circuit is needed to selectively receive signals within this bandwidth range, and the most basic form of this circuit is an LC resonant circuit, even an antenna can be regarded as It is an LC circuit, adjusting C can select the channel of different radio stations.

sophiecentaur and DaveE