This document discusses the design of quasilumped element high-pass filters using microstrip lines. It explains that microstrip short sections and stubs whose length is less than a quarter wavelength can approximate lumped elements. These are called quasilumped elements. It then provides the ABCD matrix for a transmission line and discusses how a high-pass filter can be designed by transforming the element values of a low-pass filter prototype using a frequency mapping equation. An example is given of designing a high-pass filter with specific parameters using a Chebyshev low-pass prototype filter.

1. BY
ANIRBAN BHOWAL
RAHUL JAISWAL
SHARAD KUMAR

2. QUASILUMPED ELEMENT
•Microstrip line short section and stubs whose physical length is smaller than λg/4 are used for microwave realization of lumped parameters in microstrip filter structures.
•These are termed as quasilumped elements.
•They can be also termed as lumped elements if their physical length is smaller than λg/8.

3. High impedance short line sections
If l < λg/8 then,

4. Low impedance short line sections
If l < λg/8 then,

5. •A transmission line of characteristic impedance Zu has a ABCD matrix given by:
Interconversion formula

6. Quasilumped high pass filters
•Highpass filters constructed from quasilumped elements may be desirable for many applications, provided that these elements can achieve good approximation of desired lumped elements over the entire operating frequency band.
•If the size of any quasilumped element becomes comparable with the wavelength of an operating frequency, it no longer behaves as a lumped element.
•The simplest form of a highpass filter may just consist of a series capacitor,which is often found in applications for direct current or dc block.

7. Continued…
•Highpass filter can be easily designed based on a lumped-element lowpass prototype such as one shown in figure, where gi denote the element values normalized by a terminating impedance Zo and obtained at a lowpass cutoff frequency Ωc.If we apply the frequency mapping
Where Ω , ω=angular frequency variables for low pass and high pass filter respectively ωc=cutoff frequency of high pass filter Ωc= cutoff frequnecy of low pass filter

8. Transformation from low pass prototype into high pass filter
Fig. (a) represents a low pass prototype filter and fig. (b) represents high pass filter transformed from low pass prototype

9. •Series inductive element in the lowpass prototype filter is transformed to a series capacitive element in the highpass filter, with a capacitance
•And shunt capacitive element in the lowpass prototype is transformed to a shunt inductive element in the highpass filter, with an inductance

10. EXAMPLE
•GIVEN DATA:-
•Cutoff frequency:- 1.5GHz
•Passband ripple:- 0.1dB
•Source/load impedance:- 50 ohms
•A Lowpass prototype with chebyshev response is chosen. Element values are
go= g4 =1
g1 =g3 = 1.0316
g2 = 1.1474
Ωc=1

11. Calculated values

12. Design parameters
•Dielectric constant=2.2 (RT/D 5880)
•Substrate thickness=1.57mm
•Cutoff frequency =1.5Ghz