A New System Noise Measuring Method Using a 2-Bit Analog-to-Digital Converter

Abstract

We propose a new measuring method for the system noise temperature, $T_{\rm sys}$, using a 2-bit analog-to-digital converter (ADC). Statistics of the digitized signal in a four-level quantization gives us information about the bias voltage and the variance, which reflects the power of the input signal. A comparison of the variances in hot and sky circumstances yields $T_{\rm sys}$ without a power meter. We performed experimental tests using the Kagoshima 6 m radio telescope and a 2-bit ADC. The linearity in the power–variance relation was better than 99% within the dynamic range of 10 dB. Digitally measured $T_{\rm sys}$ was in agreement with that of a conventional measurement with a power meter, although the temperatures differed by 1.8%, or less, for elevations of 10$^{\circ}$–88$^{\circ}$. No significant impact was found by the bias voltages within a range of $-$3.7% to $+$12.8% with respect to the threshold voltage. The proposed method is available for existing interferometers that have a multilevel ADC, and release us from troubles caused by power meters.