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Amplitude Modulation Example
This example shows a simple AM modulator/amplifier/demodulator chain and examines the waveforms at various points throughout the system.
Note that the modulator (and any VSS element in general) has color-coded terminal symbols. A yellow terminal indicates that a real signal must be connected. Orange terminals indicate a complex signal. A real and a complex sinusoidal source are used to drive the input signals, real for the data and complex for the carrier.
Complex Envelope vs Time Domain Analysis
VSS can operate as an envelope simulator, which means that the primary chain is analyzed by considering only the complex envelope of the signal. Please refer to the "Modulator Output" graph. The blue curve is at TP.AM_MOD CE, which is the modulator output. You can clearly see that this follows the envelope of the actual waveform. In order to produce this full waveform, we must use the CE2R (complex envelope to real) block as shown. The magenta curve shows the result at TP.AM_MOD R, after the CE2R conversion. This same technique is used at the amplifier output to compare the envelope with the modulated carrier.
Note that the modulation index (modulator) and P1dB (amplifier) parameters are set to be tunable. Adjust these values and look at the change in waveforms. Note that the default settings produce some clipping in the amplifier, which is visible in the "Amplifier Output" and "Demodulated Signal" graphs. If you adjust the P1dB setting of the amplifier, you can significantly impact this clipping.
• The Y axis in the graphs is in the same units as the amplitude parameters in SIN_R and SIN_C. In this case, the unit is Volts. You can specify the power of an AM signal as a function of the modulating signal power (Pm) as follows:
Pc = 0.5 (a^2 + b^2 * Pm)
The carrier is c(t) = a(t) * cos(wc*t + f)
a(t) = a + b * m(t)
m(t) is the SIN_R source
b/a is the modulation index
a is the average level of the envelope
In our case:
a = 1
b = 0.8
Pm = 0.5 * (1)^2 = 0.5
Amplitude SIN_R = 1
• For the modulation index, 0.8 = 80%. Use the tuner to adjust it in the circuit and its definition will be obvious.