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    A New System Noise Measurement Method Using a 2-bit Analog-To-Digital Converter

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    We propose a new method to measure the system noise temperature, TsysT_{\rm sys}, using a 2-bit analog-to-digital converter (ADC). The statistics of the digitized signal in a four-level quantization brings us information about the bias voltage and the variance, which reflects the power of the input signal. Comparison of the variances in {\it hot} and {\it sky} circumstances yields TsysT_{\rm sys} without a power meter. We performed test experiments using the Kagoshima 6-m radio telescope and a 2-bit ADC to verify this method. Linearity in the power-variance relation was better than 99% within the dynamic range of 10 dB. Digitally measured TsysT_{\rm sys} coincided with that of conventional measurement with a power meter in 1.8-% difference or less for elevations of 10βˆ˜βˆ’88∘10^{\circ} - 88^{\circ}. No significant impact was found by the bias voltages within the range between -3.7 and +12.8% with respect to the threshold voltage. The proposed method is available for existing interferometers that have a multi-level ADC, and release us from troubles caused by power meters.Comment: to appear in the Publications of the Astronomical Society of Japan, Vol.62, No.5; 12 pages, 6 figure
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