Amplitude Modulator Macro
Amplitude modulation plays a key role in many types of communication systems, the most obvious one being AM radio. In amplitude modulation, a message signal is shifted to a high frequency carrier signal whose frequency coincides with the necessary transmission requirements. The amplitude of this carrier signal is modulated depending on the strength of the message signal. The usual amplitude modulation is double sideband modulation. In this type of modulation, a DC signal is added to the message signal prior to using it to modulate the carrier signal. Double sideband modulation typically carries the form of: Vs(t) = Vo*(1 + m*Vm(t)) where Vo is the DC offset, m is the modulation index, and Vm(t) is the message signal. The modulation index should be set to a value in the range from zero to one where m=0 represents no modulation and m=1 represents a maximally modulated signal. This modified message signal is then multiplied by the carrier signal to get the final modulated signal such as: Vmod(t) = Vs(t) * Vc(t) where Vc(t) is the carrier signal which is typically a sinusoidal waveform. The macro model for the amplitude modulator is shown in Figure 1. The macro circuit consists of only a nonlinear function voltage source, E1, which performs the mathematical computations of the modulation. There are five input parameters for this macro: FS, VPeak, ModIndex, Offset, and Type. The FS parameter defines the frequency of the sinusoidal carrier signal. The VPeak parameter sets the peak amplitude of the carrier signal. The ModIndex parameter defines the modulation index, and the Offset parameter sets the offset value that is added to the input signal. These parameters correspond to m and Vo, respectively, in the Vs(t) equation above. The Type parameter determines whether the carrier signal will be a sine or a cosine function. The most interesting aspect of the macro is the use of the Type parameter. The VALUE attribute of the E1 nonlinear function... [continues]

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