A high speed transmission system using QAM and direct conversion with high bandwidth converters

Ultra high speed data converters have applications ranging from data acquisition systems, radar, satellite and terrestrial communications and radio astronomy telescopes. The latest data converters offer signal bandwidths of up to 7GHz along with sample rates at 5GSps and more. This capability means that efficient and flexible wideband data transmission system for high data producing hardware, such as imaging cameras, can be designed and constructed. This system could be easily extended to use on assets requiring encrypted data transfer, such as UAVs. This paper describes a system design which uses a 12 bits 3GSps DAC from e2v (EV12DS130A) as the transmitting element and a 10 bits Quad 1.25 GSps ADC (EVIOAQ190A) in 3GSps single channel configuration as the receiving element. The transmission is based on a 256-QAM. The processing is done by a Stratix II FPGA from Altera on the transmitter end; the receiver is based on a Virtex 6 FPGA from Xilinx. A transmission system based on a parallel architecture for the software was developed which can reach data rates up to 6Gbps. The modulation processing consists of 8 QAM256 modulators in parallel using NCO (Numerically Controlled Oscillator) as sine wave generators and LVDS transmitters to interface the FPGA and the DAC. The reception stage also interfaces ADC and FPGA through LVDS receivers and uses a multi-stage correlation method to demodulate the data. In this paper, the choice of modulation standard will be discussed along with the architecture of the scheme adopted. Finally demonstration of test results using various data patterns will be presented. The extension of this technique to communication standards like DOCSIS (Data Over Cable Service Interface Specification) is also considered along with the use of frequency hopping encryption.