An anti-jamming scheme on the base band instead of the intermediate frequency (IF) is proposed to improve the anti-jamming capability of direct-sequence spread-spectrum (DSSS) receivers. For timefrequency domain technology, both the residual jamming energy and the unexpected loss of signal energy resulting from inaccurate jamming rejection would reduce the capability of signal acquisition. The scheme of detecting jamming by FFT and rejecting jamming by notch filters on the base band is exploited to improve the accuracy of jamming rejection. Monte Carlo simulation results show that the proposed scheme could enhance the anti-jamming capability of DSSS receivers.

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System calibration is fundamental to the overall accuracy of the ultrasonic temperature measurement, and it is basically involved in accurately measuring the path length and the system latency of the ultrasonic system. This paper proposes a method of high accuracy system calibration. By estimating the time delay between the transmitted signal and the received signal at several different temperatures, the calibration equations are constructed, and the calibrated results are determined with the use of the least squares algorithm. The formulas are deduced for calculating the calibration uncertainties, and the possible influential factors are analyzed. The experimental results in distilled water show that the calibrated path length and system latency can achieve uncertainties of 0.058 mm and 0.038 μs, respectively, and the temperature accuracy is significantly improved by using the calibrated results. The temperature error remains within ±0.04°C consistently, and the percentage error is less than 0.15%.

A new algorithm for time delay estimation (TDE) is proposed, based on zero crossing (ZC). The basic model of zero crossing TDE is discussed, and the mean squared error with additive white Gaussian noise is derived by a theoretic equation. It is shown that the accuracy of TDE is a function of signal-tonoise ratio (SNR), signal frequency and numbers of ZC points. The ZC algorithm approaches a comparable accuracy with FFT method for high SNR, though with low computational complexity and low processing latency. The proposed zero crossing estimator is suitable for real-time applications.