View source for Proof Without Words: Heterodyne (q)


Hi Tom,
 the simplest way to build an radio antenna, a lot of people do not
really know is use a light bulb and a colored sheet of plastic.

Basically, take your source of audio waves you want want to send, for
example from a microphone, even better one, would be the sound card
output of your computer. Connect the source to the light bulb and put
a colored sheet of plastic over it. Sometimes, it is better to use a
red LED, for this sort of stuff.

The color of the plastic is the carrier frequency and your signal will
be the modulated signal.  And roughly speaking you have a built a
narrow band FM modulator. So, roughly, you have also built a
heterodyning device.

In fact, this simple device can also help students picture how radio
waves really work.

Make a sine audio wave using ur computer that is about 40 hz(small
enough so the you can see as it happen, and large enough, not to be
cut off). Now, take the audio output of your computer and connect it
to a light blub covered with a colored sheet of plastic or even
better, an LED. Now, Plot the wave on the screen. And now, play it to
the sound card output, which is connected to the light blub. The
frequency of fickering of the light blub is match the one the students
see on the screen. Sometimes, it is use Audio tools(from
[[SoundForge]].net) that show how these events happen synchroneously.

Sometimes, it better to replace the source of the light blub with a
switch connected to the voltage source. If you periodically turn the
switch off and on, the light blub will behave as the switch tell it to
behave. Even though the switching happens at one frequency, the light
blub internally also contains a color component that has a frequency
of its own.

You can also show students the purpose of modulation.  You have take
another computer, take a light blub and wrap it in another color, blue
for example. Now, you can show them how, you can simulateously
modulated another signal, by band shifting it to the blue specturm
without it interfering with the infrared one.

To be more effective, you can play the 2 audio signals simulateously
using the speaker. And show your students that without band shifting
the singals, the signals will interfere and contribute to noise.
-------------------------

To be more fun, you could build an evenlope detector using a
photoelectric diode.  Fiber optic cables are useful, in this case
because they can transfer signals, from one end to another,without
degrading the intensity of the signal. Now, take that demodulated
signal from the evenlope detector, connect it to the microphone input
of another computer and play it there. I should warn you that you
cannot use an envelope detector to successfully disentangle modulated
signals with different carriers, unless you use a filter. One way to
build a simpe filter would be once again, a sheet of plastic of a
certain color. Once can you could use the math  sinAsinB = 0.5(cos(A -
B) + cos(A+B)) to explain why the filter works.

 And if you want to even more fun, you can make ur students build a
low baud wireless network between 2 computers. MATLAB will be of great
use to you for these sort of things. MATLAB comes with sndrec(sound
recorder function) and play( sound play function). It can also read
and save files locally. May be, you can motivate your students to send
messages between 2 computers, using these techique. :-)

Ofcourse, i randomly ended up (re-) discovering these things when i
was trying to build a no cost random number generator which can
reproduce results of PEAR PSI experiments. Although i never succeeded
with reproducing PEAR PSI results, i did have a lot of fun, in between
;-)

-suresh