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.
Highlights
The main essence of radiolocation and sensor techniques can be reduced to the following steps: generation of the desired signal with appropriately selected parameters, sending it to the tested object and inferring about the characteristics of the object by analyzing changes in the parameters of the received signal
In a relatively narrow frequency range, the direct digital synthesis (DDS) system synthesizes the output signal without the presence of additional spurious signals, i.e., for a certain reference frequency, one can select a tuning band in which the output signal spectrum will be devoid of spurious frequencies
When all the available tuning band is used, many additional components appear in the output signal spectrum with frequencies
Summary
The main essence of radiolocation and sensor techniques can be reduced to the following steps: generation of the desired signal with appropriately selected parameters, sending it to the tested object (space) and inferring about the characteristics of the object by analyzing changes in the parameters of the received signal. From the point of the use of generated signals in radiolocation and sensor techniques, the ideal situation is where one can generate a signal with high spectral purity (max −100 dBc/Hz at 1 kHz in the X band) with the possibility of relatively fast and quasicontinuous (i.e., with a step of Hz) tuning in the preferably wide band, e.g., equal to several GHz. Currently, the most effective and perspective method seems to be the direct digital synthesis (DDS, or the so-called direct mapping method) using a high-quality clock signal. In a relatively narrow frequency range, the DDS system synthesizes the output signal without the presence of additional spurious signals, i.e., for a certain reference frequency, one can select a tuning band in which the output signal spectrum will be devoid of spurious frequencies (purity level of −60 dBc and below). When all the available tuning band is used, many additional components appear in the output signal spectrum with frequencies
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