APPLICATION NOTE
5
The REFLECTS circuit shown in Figure 2 is used to measure the
->Input reflection coefficient, S11
->Output reflection coefficient, S22
The reflection coefficients are the input voltage multiplied by 2 minus AC unity. The E device, E1, has a gain of 2.
As in the transmission coefficients measurement circuit, the interface pin labeled CKT is used to connect to the
external circuit. The pin, SRE, is a hidden pin like STR described above, (see Figure. 6). The REFLECTS symbol
also has a DCbias property. On active circuits, the DC level can be set on voltage source, V1, by changing the
DCbias property for the REFLECTS part instance in OrCAD Capture. By default, this property is set to zero.
Figure 6: Pin Properties for the Reflects symbol showing hidden S_RE pin
Using the sub schematics
The sub schematics can be used for both passive and active circuits. One can also create and use symbols from
these sub schematic in design. Design example referred in this application note uses XMITS and REFLECTS
symbols created from these sub schematics. See Creating and Editing Parts in the OrCAD Capture User's Guide
for reference to creating hierarchical parts.
Passive circuit implementation
The circuit shown in Figure. 7 is a fourth-order Butterworth bandpass filter with a center frequency of 250 MHZ.
The first circuit measures S11 and S21. The second circuit measures S12 and S22. For simulation, the AC
analysis settings are 500 linear points from 200 MHz to 350 MHZ. The results of the analysis are shown in Figure
8. Extract the attached Test_Case.zip to find the passive folder with this example.
