Reduction of Spurious Signal Upconversion in Frequency Multipliers

Abstract

Usually many applications of radar transceivers and heterodyne frequency synthesizers assume a spurious signal power level below −60 dBc. In the case of modern synthesizers using direct digital synthesis (DDS) systems, the number of emerging spurious signal frequencies is very large, and spectral purity within −60 dBc can only be obtained in the relatively narrow tuning band of the DDS unit. For the purposes of widening this useful frequency range, the frequency multiplying operation is applied commonly. Then, during the process of frequency multiplication of the baseband signal containing inband spurious signals, the effect of the upconversion of spurious signals occurs. The paper contains an analysis of the undesirable effects of the conversion of spurious signal frequencies accompanying the process of frequency multiplication. A method of reducing the level of upconverted spurious signals is proposed. The numerical calculations and measurement results are provided. For the case of a frequency multiplier with a multiplying factor equal to N, the power ratio between the desired output signal and upconverted spurious signal drops by an additional 1/N2. It has been found that the application of the presented method during the design process of the frequency multiplier allows this ratio to be improved by 6 dB.