(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.