Utilizing Frequency Synthesizer Technology To Design Local Oscillator Of Spectrum Analyzer

Spectrum analysis technology is very important way in modern RF andmicrowave domain, so spectrum analyzer is the most popular and key instrument inR&D and I&M. Handheld analyzer is preferred in I&M, frequency monitor and mobilecommunication because of its small size, easily taking and performance.In modern design for spectrum analyzer, the superheterodyne technique is adoptedmostly, so the local oscillator, especial for1stlocal oscillator decides the performance ofanalyzer directly, with the development of modern wireless technique, requirement formeasurement is more and more higher, for the1stlocal oscillator of analyzer, thegeneral technique to stable oscillator frequency is hard to meet requirement, buttechnique of Phase locked loop can greatly improve phase noise performance to localoscillator, then improve measurement performance of analyzer, so PLL is preferred todesign local oscillator for modern spectrum analyzer.The technique of PLL is newer, by this technique, frequency and phase of outputsignal of oscillator can be synchronized with reference signal, thus phase noise ofoscillator is greatly improved, and has good frequency stability and resolution.PLL circuits are from simple to complicate way, many kinds of mode, but they aremade of several fundamental parts: input reference signal; phase detector; loop filter;oscillator; feedback loop (some circuits including frequency divider), etc. This articleparticularly introduces the fundamental equation of PLL, the impact to whole loop noisefor each part, etc. for some applications which very approach to practical case, itdescribes the specification and working principle of fractional N phase locked loop(FNPLL), and direct digital synthesizer (DDS) which is developed with modern digitaltechnique. For some PLL circuits working in microwave band, because the workingfrequency is much higher, in order to have good performance, multiple loops PLL arenormally used, it’s also introduced in detail in this article.At the final, this article carefully discusses to design a1stlocal oscillator forhandheld spectrum analyzer: it’s a wide band VCO circuit with multiple PLL, andworks in microwave band (4~8GHz) with DDS as a variable reference, by inputting an external high precision reference signal, to control a voltage control oscillator (VCO) towork in wide band frequency range, and achieves a better phase noise performancesignal. Besides this, simple control circuit and lower power consumption is wanted forhandheld set. It provides the working principle and simulation data for this PLL circuit,some key factors analysis to impact loop phase noise are also involved; besides these, italso provides the block diagram of actual circuit and test system; the final test data,analysis result and summary.