Publication: Design of composite filter using t and π network
Date
2007-03-01
Authors
Tan, Zi Ding
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Abstract
Compact low-pass filter designs with a sharp attenuation response are challenging. Most conventional approaches are Butterworth or Chebyshev types, but they require a highorder design to ensure a good selectivity near the passband since they have no attenuation poles. Elliptic-function filters have attenuation poles near their passbands, making them very attractive for high-selectivity applications. However, a high-order design is also required to simultaneously ensure a flat response in the passband and a good out-of-band attenuation. In all cases, a compact planar design is practically hard to achieve due to the number and size of components to be implemented using the semilumped component approach. A planar composite lowpass filter implemented in microstrip line, designed by image parameter method will be described. This composite filter combines four filter sections and presents an attenuation pole near the cut off frequency to ensure sharp cut off. This Filter design also ensuring good matching properties in the passband. The Lumpedelement schematic of the filter has been implemented, and the lumped elements are converted into microstrip line to become a planar composite filter. The Simulations are done by using Advanced Design System (ADS). The Microstrip line simulation results of the forward transmission coefficient, S21, manage to obtain 1.5GHz as the cut off frequency of the filter, which fulfill the design specification. The simulation results also show that the filter presents a sharper cut off frequency and a almost zero-ripple pass band. The design had been fabricated and the results are measured by using a network analyzer. Measured results exhibit some losses compared to the simulation results, with +/- 0.1GHz tolerance from the cut off frequency and the insertion losses for the designs are around -1dB to -2 dB, which may caused be practical limitations. Measurement results shows that the designed filter exhibits a sharper cut off and it is quite stable in the pass band, with the rejection of the attenuated pole around 20 to 30dB. Similar filter designs have been realized for the same specifications. The design of a lowpass composite filter in microstrip with T and π network is a success but careful attentions need to be given to the parameters that need further improvements. These parameters are the 1.8GHz sharp cut off negative peak and 1.875GHz sharp matching negative peak. These filters have been fabricated on a FR4 substrate and perform a very low cost solution for RF applications.