Analysis of the Load Performance of ADC in a Digital Baseband Converter for VLBI

In current VLBI systems, signals at radio frequencies(RF)are amplified and downconverted to be at intermediate frequencies(IF)and/or basebands, and are further input to analog-to-digital converters(ADCs)after being appropriately filtered.In these systems, the key components are the ADCs, which must be operated under the condition that their sampling rates are at least twice the bandwidths of received signals, as required by the sampling theorem.In the case that the input signals are the composites of many multiplexed signals, a key factor in the performance of ADCs is the degree of signal clipping, which is carried out when the magnitude of an instantaneous ADC input signal surpasses the maximum range of the ADC.Usually, since at least some clipping is present, the total noise in the ADC output includes the effects from clipping and quantization.A measure of the performance of an ADC is its signal-to-noise ratio(SNR), which is defined as the ratio of the average power of the input signal to the average power of noise in the output.This paper gives the formulas to estimate the SNR for the case that the ADC input signal is sinusoidal based on the detailed analysis of the load performance of an ADC.The paper further analyzes the effects of powers of input signals on the SNR when the input signal is of real astronomical origin, which defines the requirements on signal loading under the best quantization of an ADC.