Sending Digital Signals

How do we go about transmitting a signal like this?

say this is 100 bits / second

The minimum frequency required to do this would be;

        a frequency of 50Hz

So the "base frequency"     =    minimum frequency required    =    1/2 the bit transmission rate

But would such a signal be recognised as a digital signal? And this could only send the signal 10101010101010 etc. We need more information.

Electrons cannot accelerate rapidly enough to produce a perfect square wave. The best we can do is approximate it by adding a few higher frequency components.

Here are a few signals plus their frequency spectra (what frequencies they contain and relative amplitudes). Notice that by adding higher frequencies we can approximate a square wave, at least enough to be recognised as such by an electronic circuit.

Notice the signal becoming more and more square as we add higher frequency components. The second example above is actually recognisable as square ( f + 3f ) so the required bandwidth to send a digital signal = 3f - f = 2f

Minimum required bandwidth = 2 x base frequency

comparing this with the equation near the top of the page we can see that;

Minimum required bandwidth = bit transmission rate

Don't worry too much about how I derived this equation though it is an important rule of thumb used by electronic engineers worth remembering.

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