Xtal Osc.Key+GNDPWR OUTFigure 43.3: Simple radio transmitter for sending Morse code.resonant frequency of the LC circuit can vary between a lowest frequency of 527 kHz and an upper frequency
of, in theory, infinity (ifLorCis zero).
Other Issues
The radio receiver described here is a very simple one, but will allow you to pick up strong nearby signals.
There are many ways to improve on this circuit. For example, a carefully designed antenna can allow the radio
to pick up weaker stations. Adding more sophistication to the circuit can increase itsselectivity, allowing you
to separate stations that are close together in frequency.
43.3 The Radio Transmitter
Suppose you were stranded on a deserted island, and needed to build a simple radio transmitter to signal
passing ships or nearby islands so that you could be rescued. How could you do it?
A radiotransmitteris a device that creates modulated radio waves that can be picked up by a receiver
such as the crystal radio receiver described earlier. A very simple radio transmitter can be constructed from a
battery, acrystal oscillator, and some wire.
A crystal oscillator is a circuit at the heart of which is a small crystal of quartz—a transparent mineral
made of silicon dioxide (SiO 2 ). Quartz is chosen because it exhibits apiezoelectric effect, meaning that
applying an electric field to the crystal causes it to flex a bit, and flexing the crystal creates an electric field.
The crystal oscillator circuit is designed to flex the crystal, then feed any resulting voltage back to the crystal
again; this feedback process causes the crystal to oscillate at its natural resonant frequency, and produces an
output signal at a well-defined frequency. A crystal oscillator circuit like this is used as the time basis of a
quartz watch, for example.
Figure 43.3 shows a very simple radio transmitter for sending Morse code signals. The battery powers
the crystal oscillator, whose output is connected to an antenna. The telegraph key is used to turn power to the
transmitter on and off. While the telegraph key is held down, the circuit causes the antenna to emit a radio
wave at a frequency equal to the crystal oscillator’s output frequency. Holding down the key for a short time
transmits a “dot”, while holding it down for three times as long as a dot transmits a “dash”. These dots and
dashes form the elements of Morse code (Figure 43.4).