Agilent Spectrum Analyzer CW Power Measurements and the Effects of Noise - Article Spectrum Analyzer CW Powe free download

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The following article appeared in the December 2012 edition of NCSLI Measure: The Journal of Measurement Science, with all rights under copyright of NCSL International. Spectrum Analyzer CW Power Measurements and the Effects of Noise The frequency discrimination capability of spectrum analyzers makes them a key component in the electronic test and measurement industry. They are used in various applications to measure the power of an electrical signal.. Although they do not have the same inherent amplitude accuracy as other measurement devices such as broadband power sensors [1], they have superior dynamic range that can extend, in some cases, to the environmental thermal noise limit. Previous work has described the theoretical model of spectrum analyzer power measurements in the presence of noise, but practical guidelines for actual measurements are less forthcoming. This paper examines how to configure a spectrum analyzer to measure a low-power continuous wave (CW) signal so that the trade-off between measurement time and accuracy is optimized. It presents equations describing both the bias and the variance of spectrum analyzer measurements due to noise. Introduction The topic of spectrum analyzer power measurements and noise has been previously addressed by articles and application notes such as items [2] and [3]in the list of references at the end of this document. This paper builds on these works and examines in greater detail the practical implications for actual measurements. Spectrum analyzers are capable of a wide range of measurements, but this paper shall simply consider the case of measuring the power of a 1 continuous wave (CW) signal of known frequency in the presence of noise. It is important to remember the assumption of a CW signal, as different results will be obtained for other signals such as those with pulsed or spread-spectrum characteristics [4]. The model of a spectrum analyzer signal + noise measurement will be reviewed, along with the statistics of noise and signal + noise measurements for various averaging algorithms. Next, a basic block diagram of a spectrum analyzer will be presented and the impact of each component on the measurement will be discussed. Finally, recommendations for configuring the spectrum analyzer will be summarized along with equations describing both the measurement bias and variance due to noise. Noise Model A spectrum analyzer is fundamentally a voltage detector. The sophistication of the circuitry producing the detected voltage, as well as the post-processing performed upon it, can be quite impressive, but a CW signal can simply be represented as scalar for our purposes. The detected voltage has a phase component, but scalar spectrum analyzers are unable to detect this and any phase information is lost after the signal passes through the spectrum analyzer's envelope detector. Any n