Variable Cutoff Frequency Pre-Modulation Filter for PCM/FM Transmission System

The purpose of this study is to design, analyze and calibrate a pre-modulation filter with a variable cutoff frequency property for the PCM/FM transmission system. The effect of the pre-modulation filter reduces the energy contained in the sidebands of the modulated carrier. Most existing filters are based on a constant cutoff frequency specification. The onboard units of the launch vehicle which have been performed by the functional and environmental tests should not be revised by any hardware modifications from a reliability point of view. In case of the VCU in this paper, it is necessary to change the PCM bit rate and the cutoff frequency of the pre-modulation filter with software programming in order to improve the picture quality in the allowable bandwidth even if the unit has already been manufactured. In addition, the link margin around the satellite separation event is very low because the flight range is more than 2000km. Under the same RF power condition, the reduction of the transmitting data rate will result in an increase of link margin. For this, the pre-modulation filter should have the variable cutoff frequency property with software programmable function according to 0.7 times the data rate. The variable cutoff frequency pre-modulation filter consists of a digital FIR filter, a DAC system and a tunable 2nd order LPF to meet the magnitude frequency response of an analog 7th order Bessel LPF. The attenuation requirements of three sub-blocks are generated by the frequency response analysis and the linear phase features are also taken into account for each stage. And the analog conversion part including a DAC system and a tunable 2nd order LPF is verified and calibrated using a tone generator.