We saw that if we want to recover the original sound signal we have to use a moltiplicator and then a low pass filter to eliminate the high frequency components. But how much difficulty is need to build up a moltiplicator in the receiver that is with the same parameters of that one in the transmitter?
Now think that we want the radio be a mass media? First of all we have to find a way of divide the frequency to different station. This is not a problem beacause at each station we assign a different portant frequency, we only to pay attention to separate enough the different channels. Secondly, we have to produce very cheap receiver because we want to sell a lot, and so people how to pay very few money to buy it.
Our target is to eliminate the moltiplicator at the receiver (is the most difficult, and so expensive, component to make). We find the solution, in doing that we have to complicate the transmitter! Now at the receiver we only need to replace the moltiplicator with an envelope detector: this is the amplitude modulation (AM).
At the end we observed that the envelope detector need more than 0.3 Volts to work and we have to build a pass band filter that works also as a tuner.
After seeing the theory we went to the laboratory to mesure an unknow AM signal, we use PicoScope (a software that works like an oscilloscope). Only seeing at the wave form and mesuring some amplitude and distance (in frequency) we have discover what was the frequency of the portant, what was the frequency of the tone and what was the parameter m (modulation index) that is the ratio between the max amplitude of the envelope signal and the costant volts added to obtain a positive signal.
Here there is two capture from PicoScope
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