The Crystal Radio

Lesson 4

I hope I get to make a Crystal Radio!!!  It’s on my list.  For now I will answer the following questions.

How do radio waves carry information?

There are two common ways to put information in a radio wave.  They are called A.M. and F.M. 

For AM the information is put in by varying the amplitude (the height of the wave is the amplitude).   Here you change the height of the wave (amplitude) to code the message.

With FM the amplitude is kept the same but the frequency is varied (how many crests cross a certain spot in a second).  Here you change the frequency of the wave to code the message.

How do radio receivers and transmitters work?

Transmitters takes a string of data and in-coding it into a sine wave after that it can also be amplified and be sent through the air.  The receiver receives the message and takes the radio wave and decodes it and makes it into words or whatever the message was.

A radio receiver picks up the carrier signal produced by the transmitter at
the receiving antenna. The signal is typically thousands of times smaller
than it was at the transmitter. It's like a rock was dropped in the middle
of a very large pond we measured the wave a very long distance away. The
receiving antenna does the exact opposite of the transmitting antenna: it
converts the EM radiation back to an electrical signal. In most areas,
there are literally thousands of different carrier waves hitting a
receiving antenna at difference frequencies and modulation type. The radio
selects a specific frequency using a tuner. The tuner is an electrical
circuit designed to select only certain frequencies and reject all others.
The tuning control on a radio adjusts the center frequency of the tuner.

Once the carrier we want is selected (which corresponds to a particular
radio station), the carrier signal is demodulated. Demodulation is the
process of recovering the information from the carrier. Since the
transmitter modulated the carrier with audio signals, the demodulator
recovers the same audio signal. The audio signal at this point is also very
small - we couldn't hear it. The audio is passed into an audio power
amplifier which makes it strong enough to drive a speaker.

The speaker is like a linear motor. Applying an electrical signal on the
speaker wires causes the speaker cone to move. This motion changes the air
pressure which our ears are capable of hearing.

from here.

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