(a)
(b)
Fig.1 (a). Basic block diagram of PLL, Fig.1 (b). Block diagram for X-band frequency generation.
A PLL is an electronic feedback system that is used to control a slave voltage controlled oscillator (VCO) that is responsible for output frequency signal generation. The slave oscillator’s phase can be controlled only within the bandwidth (BW) of the PLL system which is loop filter design dependent. High frequency Radar and EW system generally operates in X and Ku band. Direct generation of high precise frequency signal in these bands is very difficult by using PLL design techniques due to high sensitivity of the VCO in the frequency band. In this paper for the generation of X-band signal, indirect technique has been used employing the S Band PLL project with multiplier as shown in Fig. 1(b).
Phase noise is defined as random phase modulation of PLL generated system and it measures the frequency stability in frequency domain generally specified in dBc/Hz at given offset from carrier frequency. Spurious (undesired frequency pickup) plays critical role in the phase noise performance of the system. The occurrence of spurious due to reference sources (i.e. Temperature compensated crystal oscillator (TCXO), oven compensated crystal oscillator (OCXO)) are known as reference spurious, while spurious coming from other components comes under non-reference spurious. The effect of spurious can be reduced by proper selection of essential components in initial stage of designing as per system requirement. Therefore, it is very important to select components such as reference source which have direct impact on PLL phase noise and spurious performance. Thus, components with minimum phase variation under vibration in order to build a system with low vibration sensitivity are preferred.
Section II discusses noise sources in PLL system and their contribution in phase noise and phase noise performance analysis of S Band PLL. Section III explains effect of random vibration on phase noise performance of reference source like TCXO or OCXO. Section IV discusses in-depth analysis of compensation techniques, power supply purification techniques and stabilization mechanisms of g-sensitive reference source for meeting phase noise performance of PLL system under random vibration. Section V covers the precaution that has to be taken before and during random vibration test. Section VI covers results of phase noise achieved before and after implementation of suggested techniques and stabilization mechanisms in this article for meeting stringent requirements of electronic warfare avionics and military systems under harsh conditions.