A Design Of Matched Interpolation Filter In QAM Demodulation Chip

QAM (Quadrature Amplitude Modulation) has been widely used in various digital communication systems due to its high modulation efficiency. For example ,QAM is a standard modulation format in the transmission of digital TV signal via cable. Because of the HFC channel's imperfections, such as reflection and noise etc, QAM modulated CATV signal has distortions. On the one hand ,in order to recover the transmitted signal totally at the receiving end, the matched filter must be designed to maximize the S/N(signal to noise ratio);on the other hand, the transmitted symbol clock must be recovered in the receiver so as to recovery the symbol and perform digital signal processing followed which is related with sample rate conversion or timing recovery.Firstly, this paper concisely introduces QAM modulation & demodulation system in accord with DVB-C standard. QAM demodulation system architecture according with DVB-C standard is introduced in detail. Then the principle of optimal receiving ,Nyquist filtering and sample rate conversion are introduced in this paper. As DVB-C is concerned, a combined matched filter is designed to perform both matched filtering and sample rate conversion based on multiphase filter banks. Furthermore , the structure of the circuits is optimized by adopting I/Q reuse, folding, etc. By optimizing the circuit of the combined filter, the gate-level circuit and the layout is greatly reduced .