High Q _SC Frequency Tunable Bandpass Filter Design

Tunable filter is an important component of VCO. High quality factor tunable filter can reduce the phase noise of VCO. Affected by the varactor Q values, the spectrum-based complex quality (Qsc) factor of the microstrip tunable filter deteriorates quickly. According to the definition of the Qsc factor, the main task of this thesis is to design a tunable bandpass filter which conform to the high Qsc (peak>300) in2.6GHz-3GHz.Combined with the requirement of the tunable filter, first we use the spectrum-based complex quality factor to judge the influence of the tunable filter on the VCO phase noise. Then the Qsc-factor is analyzed, and the narrow band low loss bandpass filter is selected. In view of the requirements of this thesis, a center frequency and bandwidth manageable hairpin-comb bandpass filter is employed. The design steps are summed up in the end of chapter two, which build a foundation for the design of tunable hairpin-comb bandpass filter. According to the principle of tunable filter, the effect of the varactor to the circuit performance is pointed out. Next we discuss the methods to determine the size of resonators based on the interaction between varactor and hairpin-comb bandpass filter. To verify this theory, HFSS and ADS software are combined to design a tunable filter. Finally circuit theory and network parameters are discussed to form the negative resistance circuit. The equivalent formula of negative resistance circuit and calculating formula of input impedance are deduced to get a negative resistance equivalent circuit. Matlab is used to analysis the circuit and network. This design chooses a PHEMT chip-ATF34143with several appropriate feedback elements to act as a negative resistance circuit, which not only reduces the insertion loss but also increases Qsc-factor significantly. According to the simulation results, the peak of Qsc of negative resistance tunable filter is higher than300in2.689GHz-3.245GHz, and the maximum of up to787. The filter has a tuning range of2.746-3.082GHz and tuning bandwidth of11.5%.