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Analysis of Frequency Response of RF System

This example shows how to use Circuit Envelope to calculate the steady-state frequency response curve for an LC bandpass filter built using blocks from the RF Blockset™ Circuit Envelope library. The first model performs static analysis (harmonic balance) on the circuit while the second model does time domain simulation using the same circuit. This example helps you validate a circuit envelope model using a static non-linear analysis in the frequency domain, and a time domain simulation.

Frequency Domain Analysis

model = 'simrfV2_ac_analysis';

The system consists of:

  • A Continuous Wave source and a series resistor, which model a voltage source with internal source impedance.

  • Inductor and Capacitor blocks configured to model a third-order Chebyshev filter with a center frequency of 2.4 GHz.

  • An Outport block acts as a voltage sensor, measuring the voltage across a load resistor.

  • The Configuration block, which sets up the circuit envelope simulation environment. As the system is linear, the harmonic balance analysis is done with a single simulation frequency and corresponds to an AC analysis.

  1. Type open_system('simrfV2_ac_analysis') at the Command Window prompt.

  2. Double-click the block labeled 'Specify Frequency Values' to provide a vector of frequencies.

  3. Double-click the block labeled 'Calculate Frequency Response' to execute a script, simrfV2_ac_analysis_callback, that analyzes the model at the specified frequencies and plots the response.

simrfV2_ac_analysis_callback([model '/Subsystem'], 'OpenFcn');

To configure a model with circuit envelope library blocks for harmonic balance:

  • In the Model Configuration parameters dialog box, set the Stop time parameter to zero.

  • Use a Continuous Wave block to drive the system.

  • Set the Carrier frequencies parameter in the Continuous Wave, Outport blocks, and the Fundamental Tones parameter in the Configuration block to the same vector of frequencies.

Time Domain Simulation

model = 'simrfV2_transfer_function';

The system consists of:

  • A Random source generator that outputs a random continuous signal.

  • The same Chebyshev filter with a center frequency of 2.4 GHz.

  • Discrete Transfer Function Estimator block to view the frequency domain output of a time domain simulation.

  • Vector Scope to view the output.

Simulate the second model.

Compare the outputs of the first and second model.


Ludwig, Reinhold and Pavel Bretchko, RF Circuit Design: Theory and Applications. Prentice-Hall, 2000.