Microstrip Parallel-Coupled Line Bandpass Filter with Multispurious Suppression
|關鍵字:||平行耦合微帶線;帶通濾波器;多重高階諧波抑制;microstrip parallel-line;bandpass filter;multispurious suppression|
The inherent zero of a microstrip coupled stage near twice the design frequency (2fo) is found tunable by varying its coupling length. This zero is used to suppress the unwanted response of paralle-coupled line filters at this frequency. The above idea is extended to design over-coupled middle stages of the filter for the spurious |S21| peaks at 3fo and 4fo, so that the upper stopband can be greatly enhanced to 5fo. The passband preserves a response as good as the traditional design. Measured results have a good agreement with simulation data and show that the idea works very well. Corrugated coupled lines are also devised to design microstrip bandpass filters with multispuriois suppression. Quarter-wave corrugated stages are tuned to allocate inherent transmission zeros at 2fo and 4fo. Stages with proper coupling lengths are arranged to cancel the unwanted peaks at 3fo, 6fo and 7fo, and the tapped input/output scheme is employed to tackle that at 5fo. The measured data of designed bandpass filters show rejection levels better than 30dB in the upper stopband. Three circuits are fabricated and measured to demonstrate the idea. Periodic stepped-impedance resonators (PSIRS) are proposed to design bandpass filters for multispurious suppression. Denoted as PSIRN, a PSIR of □/2 long at design frequency fo consists of N periods of hi-Z and low-Z sections. A PSIRN coupled section shows transmission zeros at various frequencies. The zero can be tuned by changing the impedance ratio of the hi-Z and low-Z sections, and be adopted to suppress the spurious peaks. Responses of the PSIR filters show good rejection in the upper stopband.