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This contributor finds it easiest to understand the following explanation.

Fasten your seat-belt; it may be a bumpy ride:

The basic principle to keep in mind is this:

When you pass more than one frequency through any non-linear device, the frequencies

get "mixed". What comes out the other end is the original frequencies PLUS the sum-

frequency and the difference-frequency of every possible combination of the original ones.

Remember this.

A non-linear device can be anything that conducts current better in one direction

than it does in the other direction. Like a semiconductor diode. Or a sharp-pointed

wire touching some varieties of naturally-occuring minerals, which are called "crystals"

when they're used in simple hobby radio kits.

Start at the radio transmitter:

-- The transmitter generates a "carrier" wave, at the frequency where it'll show up

on your radio dial.

-- The carrier wave and the voice or music waves are passed through some non-linear

device, where the frequencies get mixed.

-- What comes out is the (carrier frequency alone), (music frequency alone), sum of the

(carrier + music) frequencies, difference of the (carrier - music) frequencies.

-- The transmitter sends out the narrow cluster of (carrier alone), (carrier + music), and

(carrier - music) frequencies. The (music frequency alone) is filtered out and discarded.

Now go to the crystal radio:

-- The wire antenna catches some of the radio waves as they fly by, and conducts them

down to the "radio" set.

-- Flowing down the wire are separate signals at different frequencies. There

are the (carrier frequency alone), the sum (carrier + music) frequency, and the

difference (carrier - music) frequency.

-- All of them are sent through the "crystal", where the frequencies mix. The original

frequencies come out, along with the sum and difference of each pair.

-- Out from the crystal come these frequencies:

The originals: (carrier alone), (carrier + music), (carrier - music)

The sums: (carrier + carrier + music), (carrier + carrier - music), (2 x carrier)

The differences:

(carrier - carrier - music) = -music

(carrier - carrier + music) = music <==== ! ! !

(carrier + music - carrier + music) = 2 x music

Almost hidden in this pile of many electrical signals is one that's a copy of the original

music signal ... exactly what came from the microphone back at the studio, and got

mixed with the carrier for transmission.

All of the other signals that come out of the crystal are filtered out and discarded,

and just the "music" signal alone goes to the headphones ... "for your listening

and dancing pleasure", as they used to say in the days when the crystal radio was

about the best that everyone had.

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Wiki User

13y ago

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