This model shows how to use the General Passive Network block of the SimRF™ Equivalent Baseband library to model a bandpass filter whose S-parameters are specified in a Touchstone® data file. The data file 'defaultbandpass.s2p' contains 50-ohm S-parameters of a 2-port passive network from 100 MHz to 10 GHz. The model in this example accounts for the filter response over a subset of these frequencies, namely 500 MHz to 1.5 GHz.
This example model has the following sections:
- A White Noise input signal.
- An RF subsystem that uses complex baseband-equivalent modeling to simulate a bandpass filter. For more information on how SimRF™ simulates the filter, see Creating a Complex Baseband-Equivalent ModelCreating a Complex Baseband-Equivalent Model.
- A Transfer Function subsystem to compute the transfer function of the filter.
Fig. 1 Bandpass filter example model:
The cascade of blocks from the SimRF Equivalent Baseband library uses the passband frequency domain parameters that we specified in the file and in the block dialog boxes to create a time-domain complex baseband-equivalent model, which is both fast and compatible with system-level modeling.
In this section of the example, we visualize the output of this model, which is itself a complex baseband-equivalent signal, by connecting the output of Output Port block to the input of the Transfer Function subsystem and Vector Scope blocks.
Note that the center frequency of a complex baseband-equivalent signal is always 0 Hz irrespective of the actual center frequency. You can map any complex baseband-equivalent frequency back to the actual frequency by adding the center frequency to the complex baseband-equivalent frequency. A convenient way of doing this addition is to use the User-defined input domain of the Vector Scope. The second Vector Scope block in this model illustrates this.
For the bandpass filter in this example, the following figure shows the frequency response of the baseband-equivalent model ranging from -500 MHz to +500 MHz in frequency. Because the center frequency (as defined by the center frequency parameter of the Input Port block) is 1 GHz, the actual frequencies are 500 MHz to 1.5 GHz.
Fig. 2 Frequency response labeled using complex baseband-equivalent frequencies:
Fig. 3 Frequency response with X-axis relabeled to correspond to passband real frequencies:
Fig. 4 Zoom in to center of frequency response with X-axis relabeled to correspond to passband real frequencies:
To see the frequency response centered at another RF frequency, change the center frequency parameter f_c in the Model Explorer.