{{projectinfo|Tutorial| Designing Distributed Bandpass Filters Using Coupled Transmission Line Segments |RF101.png|In this project, you will build a distributed bandpass filter using distributed coupled transmission line segments.|
*Coupled [[Transmission Lines]]
*Even Mode
*Odd Mode
Build a new circuit using the Generic Coupled T-Lines device and use two Net Markers to designate the input and output ports. Leave Ports 2 and 3 of the device X1 open as shown in the above figure.
Run a Network Analysis Test of your circuit with the [[parameters]] specified in the table below. Make sure to uncheck the "Decibels" checkbox.
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All the segment lengths are chose to be len = 37.5mm, which is a quarter wavelength at the center frequency of the filter f<sub>o</sub> = 2GHz. TEM [[Transmission Lines|transmission lines]] with ε<sub>eff</sub> = 1 are assumed. The impedances have been chosen to achieve an equal-ripple bandpass filter design.
Run a network analysis of this two-port circuit with the [[parameters]] specified below:
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The figure below shows the graph of S11 and S21 [[parameters]]. Note that a linear scale is used for frequency (the bottom axis). The filter has a center frequency of f<sub>o</sub> = 2GHz with a 3dB bandwidth of about 235MHz. The response drops to -45dB at 1.5GHz and 2.5GHz.
<table>
In this part of the tutorial lesson, you will design and test a microstrip realization of the coupled line bandpass filter you simulated in the previous part. For this purpose, you will use a substrate of thickness h = 1.6mm with ε<sub>eff</sub> = 3.4. You will assume a lossless substrate (tand = 0). Remember that in the previous section, TEM line segments with ε<sub>eff</sub> = 1 were assumed and the length of the coupled line segments were set to be a quarter free-space wavelength at 2GHz. For this part, first you need to design coupled microstrip lines with the given even and odd mode impedances. Then, you have to calculate the guide wavelengths of the couple microstrips at 2GHz. You will do these using [[RF.Spice]]'s Device Manager.
Open the '''Coupled Microstrips Designer''' dialog from the Tools Menu of Device Manager. Enter the substrate [[parameters]]: h = 1.6mm and er = 3.4. Enter the Z0e and Z0o values for the two types of coupled line segments from the previous section and calculate width and spacing of the coupled microstrip lines for each case. Then, open the '''Coupled Microstrips Calculator''' dialog from the Tools Menu of Device Manager. Enter the calculated microstrip width and spacing values to verify your design and also find the corresponding guide wavelengths λ<sub>g</sub> at 2GHz. The lengths of the coupled microstrip segments are chosen to be a quarter of the corresponding guide wavelength at 2GHz. The following table summarizes the [[parameters]] of the coupled microstrip segments L1, L2, L3 and L4.
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== Realizing the Microstrip Version of the Coupled Line Bandpass Filter==
Build your microstrip circuit using four "Coupled Mirostrips" parts with the [[parameters]] specified in the above table. You can access the part from '''Menu > [[Transmission Lines]] > Physical [[Transmission Lines]] > Coupled Microstrips'''. Connect the four parts in a cascaded fashion as shown in the figure below.
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Run a network analysis of this two-port circuit with the same [[parameters]] as in the previous section:
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