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Hi,
I´m trying to make a design of a combline filter and simulate every stage of the process. However, I can´t make a good simulation using ideal transmission line. In the figure below I show two of the stages:
- Top: filter with impedance inverters implemented with ABCD matrices.
- Bottom: filter with impedance inverter implemented with ideal transmission line.
The filter in the top yields the blue/red traces and the bottom one yields the result in black/green.
I repeated the same simulation in ADS and it works well, so I assume that is a limitation of QucsStudio or I´m doing something wrong. Does anyone knows how to fix this?
Hi Carsten,
The simulation with series stubs is not accurate, the BW of the filter is too wide. For example, the simulation of both circuits with ADS shows a very similar results, see the figure below.
I can make the bottom wider/over-coupled filter look similar to the top but it requires a lot of changes.
The Z of the 2-port ABCD is very different from the series 57 Ohm tlines.
Do you have the original L/C filter that was used to create these two filters?
Both filter are actually the same in a different stage of the design.
The one in the bottom is the result of transforming the impedance inverters (2-port ABCD) into a pi network. The series element is a stub and the shunt elements are absorbed by the resonators at both sides. That´s why they resonator stubs are different in both circuits.
Anyway, the filter is well designed, I replicated everything in ADS and the results are what I expected, but not in QucsStudio. So, my conclusion is that I´m using the TL wrongly or the ideal model isn´t correct when it´s in series.
If you are interested in the original LC prototype, you can run the attached schematic.
That´s because the filter is only transformed in frequency but not scaled in impedance. I didn´t do it because at that point I introduce the impedance inverter to make all the resonators shunt as in the first schematic I shared.
If you are interesting in the theory, I started with the method described in the chapter 8.3 in Microwave Engineering. This is Chebyshev filter of order 3 at fc = 1GHz and the LP prototype I used have the following elements:
g0 = g4 = 1
g1 = g3 = 0.8516
g2 = 1.1032
You can also find the table for frequency transformation in Table 8.6.
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