Low-Noise Hybrid Frequency Synthesizers Based on Direct Digital and Direct Analog Synthesis
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LOW-NOISE HYBRID FREQUENCY SYNTHESIZERS BASED ON DIRECT DIGITAL AND DIRECT ANALOG SYNTHESIS
V. V. Romashov, K. A.Yakimenko, A. N. Doktorov, and L. V. Romashova
UDC 621.317.757
Hybrid frequency synthesizers based on direct methods of digital and analog synthesis were examined. The possibilities of applying synthesizers as heterodynes of modern spectrum analyzers built on the superheterodyne plan were studied. The possibility of using a hybrid synthesizer as the heterodyne of the first cascade of spectrum analyzers was analyzed. The requirements for heterodynes of the first cascades of the mixing of spectrum analyzers are provided. A structural diagram of a broadband heterodyne generating frequencies in the range 4000–8000 MHz with a step not exceeding 1 Hz is presented. Formulas to calculate the fundamental frequency relations in the heterodyne structure were developed. A mathematical model of the power spectrum density of phase noises as a function of the offset frequency from the carrier was developed. With the aid of the created model, the noise characteristics of the proposed layout was examined. It is shown that at offset frequencies over 1 kHz from the carrier, a hybrid synthesizer based on direct digital and analog methods of synthesis has an advantage at a phase noise level 5–30 dB over low-noise heterodynes of modern spectrum analyzers. The advantages of the suggested layout by which the hybrid synthesizer is capable of improving the technical characteristics of modern spectrum analyzers are defined. Keywords: spectrum analyzer, heterodyne, phase noise, phase-locked loop, direct digital synthesizer, intermediate frequency filter, discrete side components of the spectrum, reference frequency generator.
Introduction. At the present time, spectrum analyzers built on classical superheterodyne architecture [1, 2] are widespread. The principle of operation of such analysers is analogous to the principle of operation of superheterodyne radio receivers, and is that the signal under study on the frequency mixer is mixed with the heterodyne signal. During heterodyne tuning, the difference signal selected in the narrowband intermediate frequency filter (IFF) bears information on the power of the spectral component of the signal under study, at offsets from the carrier that depend on a band of heterodyne tuning. In order to successfully suppress a “mirror channel” and achieve a small IFF bandpass range, modern analysers contain several cascades of frequency conversion, from two to four. Heterodynes of the second and subsequent stages are, as a rule, not tunable, and therefore heterodynes of the second and third stages generate frequencies on the order of single-digit gigacycles, and the heterodyne of the fourth stage, on the order of hundreds of megacycles. There are separate requirements shown for a heterodyne of the first cascade. It must generate signals with frequencies on the order of several gigahertz in a broad range (up to an octave) with small step size (to single hertz) of tuning by frequency and thereby
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