Microstrip elliptic diplexer design by elliptic LPF and CQ BPF

To exploit the frequency band of 870-1200MHz as the extended upstream for CATV, extension amplifier is the main part of the system. To realize it, developing mass production suitable diplexer with very sharp skirt on both low-pass and high-pass sides is the key point microstrip diplexer is chosen to meet this challenge. Elliptic LPF by quasi-lumped components and CQ BPF by open-loop resonators are designed to get a cutoff edge with enough selectivity separately. Then diplexer is obtained by connecting the BPF to the LPF by coupled feedline with a suitable length, inside which both of BPF and LPF maintain their performances without any appreciable decrease. Both of the extension amplifier and the extension system by this diplexer are tested. Experiment results show that the developed extension amplifier is able to make use of the frequency band between 865MHz and 1155MHz to transmit 16QAM and 64QAM at 5 Msymbol/sec in high waveform quality without any noticeable effect on the conventional upstream and downstream.