Research On The Digital Down Conversion Technique Based On FPGAs

With the development of the worldwide techniques of spaceflight, the telemetry system is more and more demanded. The working environment of telemetry incepting system is atrocious comparatively, it requires that receiver has very high anti-interference capacity and reliability. Conventional analog intermediate frequency receiver of telemetry doesn't meet the practical requirements. With the development of microelectronics techniques, computer techniques and communication techniques, the application of digital signal processing techniques in receivers becomes more and more popular especially with the rapid development of the processors of DSP chips. Using digital down conversion to frequency conversion in telemetry incepting system becomes popular.The digital down conversion proposed in this dissertation is based on FPGA technique. At the same time, bandpass sub-sampling is adopted. The capability and technique indexes of conventional analog digital down conversion are analyzed and lucubrated. Then a new design of digital down conversion is presented.The key techniques adopted in this dissertation are bandpass sub-sampling technique,numerically controlled oscillator technique,filter technique and coordination rotation digital computer technique. In theory the sampling rate required of bandpass sub-sampling are lower than that of general Nyquist sampling. As for telemetry incepting receivers, their input signals are usually narrowband signals. Therefore bandpass sub-sampling can meet the request of telemetry incepting receivers. Those can debase signal center frequency; numerically controlled oscillator produce intermediate frequency carrier wave signal, their speed and precision will affect capability of digital down conversion firsthand. Function of filter sieve signal high frequency parts and work wih removable tool to debase signal working speed, their capability can affect capability of digital down conversion importantly. Coordination rotation digital computer is a sort of technique, which can use rotation computation to gain intermediate frequency carrier wave signal. Coordination rotation digital computer technique fit for high speed signals. This paper presents a new digital down conversion design combined with those techniques.The digital down conversion design proposed in this paper has more advantages than conventional analog down conversion. First, the whole down conversion is implemented in digital forms so it can work more stably. Second, high code rates are supported. Third, the structure is simple and easy to realize. Last, programmable logic devices are adopted so the upgrade and maintenance are convenient. The digital down conversion proposed in this paper at the same time, those also fit to be designed and realized with FPGA component. As a result it is significant to application.