AM radio - Amplitude Modulation

Amplitude Modulation

fig 1 : A tunable crystal radio circuit (wikipedia, public domain).

Amplitude modulation is a way to transmit and receive a signal over radio waves.

It doesn't require much material or knowledge in comparison to other methods. An antenna, a battery, a tuning circuit.

Early AM radio receivers did not even require a battery. They were called crystal radios.

How does it work?

We start with 2 signals. The carrier frequency and the input signal. The amplitude of the carrier gets modulated by the input signal. The result of this can be mathematically expressed in an equation of the form (input+1)*carrier.

The modulated AM signal then is transmitted from the emitting antenna to any receiving antennas.

The receiving radio stations will have a tuned circuit that resonates to the carrier frequency.

The received signal is then demodulated and played through a speaker or earphone.

Demodulation

There are few different approach to demodulate an AM signal.

Easiest way is just to cut the signal in half, keeping only the positive part of it, using a diode. The result can be further cleared of some of the noise by using a capacitor or low pass filter.

In the canvas demo below, I used a capacitor in parallel with a high impedance earphone, such as it ressembles early crystal radios. Distortion is still present but this is basically as good as it gets for a radio without batteries I guess.

First: Input signal.
Second: Carrier frequency.
Third: AM signal.
Fourth: Demodulated / Rectified signal.
Fifth: Filtered signal to the earpiece.

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