This example shows how to use power ports and measure the signal power using the spectrum analyzer.
model = 'simrfV2_powerportdefinition_1.slx'; open(model)
In this model, the simulink signal is interpreted as a
Vrms^2 signal referrerd to 1 Ohm. This is the implicit convention used in the Communications System Toolbox.
When measuring the signal power with the spectrum analyzer, refer the measurement to 1 Ohm. In RF Blockset, use the power option for the ports at the input and output of your system. Notice that the ports automatically add source and load impedances, and scale the signal to refer it to the specificed impedance. In this way, the available power of the input signal is referred to 50 Ohm. The RF Blockset amplifier is thus processing a signal with the same power as the Simulink signal.
When probing internal nodes, use the voltage option in the "sensing" port otherwise you will mismatch your circuit. Also, make sure that you adjust the reference impedance in the spectrum analyzer to 50 Ohm.
model = 'simrfV2_powerportdefinition_2.slx'; open(model)
In this model, the simulink signal is interpreted as a Voltage signal. When measuring the signal power with the spectrum analyzer, refer the measurement to 50 Ohm.
In RF Blockset, use the voltage option for the ports at the input and output of your system. Notice that you need to add source and load impedances, otherwise the amplifier will be mismatched. You also need to add a factor of 2 to make sure that the voltage at the input of the RF Blockset amplifier has the same value as the Simulink signal. The factor of 2 takes into account the voltage division between the source and input impedances.
Simulate both the models and observe.