Suppression of Harmonics in Microstrip Filters Using a Combination of Techniques
Existing and newly adapted techniques for suppressing harmonic responses in microstrip filters were combined. Stagger tuning using additional stubs to shift the resonances has been adapted to quarter-wave resonators and extended to the seventh-order spurious response. Suitable resonator overlap provided transmission zeroes to cancel spurious peaks. Coupling capacitor pads acting as shorting stubs have been developed. Spurious responses were suppressed to better than $-{\hbox{33 dB}}$ , up to 6.3 GHz, in a measured filter with 1-GHz center frequency and 42% bandwidth. Suitably placed resistors have depressed the spurious level further to $-{\hbox{50 dB}}$ up to 5.6 GHz, with only 0.07-dB extra passband loss.