In order to broaden applications of microwave photonic filter (MPF), we review existing techniques to improve the out-of-band suppression ratio of the MPF implemented in incoherent or coherent operational regime. A single bandpass MPF is proposed based on carrier-suppressed single sideband (CS-SSB) injected distributed feedback (DFB) laser. Four-wave mixing effects have been reduced through optical carrier suppression. The out-of-band suppression ratio of the single bandpass MPF increases from 20 dB to 32.3 dB. The methods to improve the out-of-band suppression ratio of the MPF are discussed.