Weighted sequences for HF FMCW sounding

The different aspects of implementing digital shaping of switching sequences of FMICW HF sounders are presented. Various window functions are applied to the commonly used sequences such as the M-sequence and the Barry sequence and the 20-bit sequence of Salous and Nattour [1992] which exhibited a superior performance for ionogram measurements. The window functions are compared in terms of the reduction in the mean received power and the spectral spread of the gated chirp signal. Furthermore, the length of the window is also investigated. It is found that weighting by a 1-bit window gave the best compromise. For systems which have a noise floor higher than or equal to -60 dB, the hamming window gave the best performance. However, for systems which have a better noise performance, the Von Hann window is more appropriate. The effect of the interpolation factor and the size of the DAC on the generated shaping function is also studied. The results indicate that an interpolation factor of 64 and 8-bit DAC are adequate