What is the role of a diode in an am radio?

Answer 1

Whenever a signal with components at several different frequencies flows through a diode,

the diode causes "mixing" among the frequencies ... for every pair of frequencies in the original

signal, two new frequency components are created: one at the sum of the original two, and one

at their difference. That is, if the original signal's wave has frequencies 'A' and 'B' in it, and it's

passed through a diode, then the output wave will have four frequencies in it: A, B, A+B, and A-B.

The original signal that enters an AM radio is the one that comes down from the antenna. If it's

an AM radio signal, then it consists of three frequencies: Carrier, Carrier+music, and Carrier-music.

There are 3 pairs of frequencies in this set. For each pair, the diode creates new signal components,

at the sum and difference of the original 2 frequencies. So with 3 different frequencies before the

diode, we have all of the following frequencies coming out after the diode:

1). Carrier

2). Carrier+music

3). (Carrier) + (Carrier+music)

4). (Carrier) - (Carrier+music)

5). Carrier

6). Carrier-music

7). (Carrier) + (Carrier-music)

8). (Carrier) - (Carrier-music)

9). Carrier+music

10). Carrier-music

11). (Carrier+music) + (Carrier-music)

12). (Carrier+music) - (Carrier-music)

To see what we've got now, let's go through the list and simplify the expressions:

1). Carrier

2). Carrier+music

3). 2xCarrier + music

4). -music

5). Carrier

6). Carrier-music

7). 2 x Carrier - music

8). music

9). Carrier+music

10). Carrier-music

11). 2 x Carrier

12). 2 x music

Looking through the list, we see that 9 of the 12 products are up around the frequency of the

radio carrier. It's easy to filter them out and throw them away, leaving only the following

three component signal frequencies, in the audio range:

4). -music

8). music

12). 2 x music

Mathematically, positive and negative frequencies are the same signal, so components #4 and #8

can be added to produce a component with double the amplitude (volume) of either one, which

only leaves the component at double the frequency of the music. It's in the range of legitimate

music frequencies, so we can't get rid of it completely. But it's weak to begin with, when it comes

out of the diode, and it's competing with the double-amplitude component at the real-music-frequency,

so it doesn't trash the desired signal too severely. What we're left with is the components

equal to the frequencies in the original music program, with some unavoidable distortion

on account of the presence of the low-level second harmonic.

The purpose of the diode was to perform the mixing of frequencies, so that we could separate

them by filtering, discard the ones that were used only to transport the music to us through

space, and keep the ones we want to send to the speaker or earphones.