Modulation of a signal in Amplitude
The bandwidth of a signal, that we denominated B (x) is the representation of the frequencies obtained by the transformed one of Fourier. Due to the properties of symmetry of the transformed one of Fourier, this bandwidth has an equal symmetry in the axis and, being the same for the negative frequencies
- Moduladora signal, that we are going it to indicate as x (t). It is the signal that contains the information that interests to us to transmit, as an image or sound signal. We are going to assign to this signal a bandwidth B (x) that if we showed it in the domain of the frequencies, we obtain the signal that imagines in the image superior
- Carrying signal. c (t). It is a sinusoidal signal of greater frequency coming from an oscillating intern, who has three parameters to emphasize, that they are, the amplitude of the signal, the frequency of the same and the phase angle (in which moment the signal takes place). Of them, the carrier frequency, is the parameter that is going to us to interest but in this study and must be much greater to the bandwidth of the moduladora signal, that is f (c) >> B (x)
- Modulated signal. We denominated it s (t) and is the resulting signal of the modulation process. This signal, due to the modulation, has a certain bandwidth around the carrier frequency, as we see ahead but
We show of schematic form, these three signals
The result of the previous process we see it in the following image, where we see that the moduladora signal has modified the carrying signal in amplitude (the signal of down), giving the result of modulated signal (signal inferior)
Commercial data on A.M.
Because to use a signal of greater frequency when the signal can be emitted that interests to us?
To use modulation A.M. using signals of much greater frequency to the moduladora, has among others, the following advantages
1 To adapt the signals to propagation means.
One more first higher antenna is seen, of 264 meters, that it emits to 585 KHZ, corresponding to RNE1, and with a power of emission of 600 KW.
2 lower antenna (130 meters) corresponds to RNE5, emitting in 657 KHz with 50 KW of power
How we obtain the signals in our receiver?
We will see the part where we already received the modulated signal (selected with a band-pass filter). The diode is in charge to pass only the positive half-cycles, signals of equal frequency but that they vary in the time (blue waves). Condenser C is loaded to the maximum value of the tips and unloading in the voltage drops of the same signal (line orange). It is necessary to consider that the value of R and C must be like:
Where the f (c) is the frequency of the carrying signal.
If we completed the circuit, we have something as
Of voluntary way, but with extra note for the subject, to realise one radio soft A.M. of type, using some tutorial of Internet and with the precaution to use diode of germanium and some earpiece of high impedance. This is because the energy that we are going to catch by the antenna is very low and components are needed that do not prevent the current passage (case of the diode) and which they require loss intensity of operation (auricular of high impedance).
Bibliography. Notes of the Oberta University. To see document in denominated DRIVE