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Chebyshev Quasi-Lowpass Filter: QLPFC



QLPFC models represent lumped-element Chebyshev quasi-lowpass filters. The insertion loss ripples between zero and a specified maximum in the passband. The stopband attenuation increases rapidly beyond the upper passband edge, is monotonic, and is maximally flat at infinite frequency. This type of filter offers both simplicity and good selectivity.


Name Description Unit Type Default
ID Element ID Text LPFC1
HN Half the number of reactive elements   3
FL Lower frequency edge of passband (when Qu is infinite). Frequency 0.5 GHz
FH Upper frequency edge of passband (when Qu is infinite). Frequency 1.5 GHz
AP Maximum passband attenuation (when Qu is infinite). DB 0.1 dB
*RS Expected source resistance (at port 1) Resistance 50 ohm
*RL Expected load resistance (at port 2) Resistance 50 ohm
*QU Average unloaded Q of reactive elements in the filter   1e12

* indicates a secondary parameter

Parameter Restrictions and Recommendations

  1. 0 < HN < 27

  2. 0 < FL

  3. 0 < FH

  4. 0 < AP Recommend AP greater than or equal to 0.001 dB.

  5. 0 < RS

  6. 0 < RL

  7. 0 < QU. Recommend QU less than or equal to 1e12.

Implementation Details

The model is implemented as a short-circuit admittance matrix, whose equivalent transfer function squared magnitude is that of a Chebyshev filter:

where Cn is the Chebyshev polynomial of the first kind, and

and a lowpass-to-quasi-lowpass frequency transformation has been applied:

_FREQ is the variable containing the project frequency, and the admittances are:


This element does not have an assigned layout cell. You can assign artwork cells to any element. See “Assigning Artwork Cells to Layout of Schematic Elements” for details.

Recommendations for Use

Note that this model behaves as if it has ideal impedance transformers at its ports, so there is no attenuation due to mismatched source and load impedances. The model expects that the source impedance will equal RS and that the load impedance will equal RL, but RS need not have any special relationship to RL for ideal transmission (as would normally be the case).


[1] Rolf Schaumann, Mohammed S. Ghausi, and Kenneth R. Laker, Design of Analog Filters: Passive, Active RC, and Switched Capacitor, (Prentice-Hall, 1990), pp. 44-48.

[2] Louis Weinberg, Network Analysis and Synthesis, (Robert E. Krieger Publishing, 1975), pp. 507-529.

[3] Max W. Medley, Jr., Microwave and RF Circuits: Analysis, Synthesis and Design, (Artech House, 1993), pp. 312-317.

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