Noise Sources and Performance Analysis Of PLL System

Phase noise performance of PLL system depends on:
i). Reference source
ii). R Divider
iii). N Divider
iv). Phase Detector
v). Loop filter Resistor Noise
vi). Active devices in loop filter
vii). VCO
vii). Power Supply
Reference source (i.e. TCXO or OCXO) is piezoelectric material which is prone to any type of physical disturbance. As reference source acts as clock for phase frequency detector (PFD) of PLL, any phase noise performance degradation of reference source has direct impact on PLL output. Hence stabilization of reference source under intense vibration environment is crucial for maintaining phase noise performance of PLL. PLL phase noise performance is also dependent on type of loop filter design. The selection of type (i.e. passive and active) and order of loop filter is based on requirement and limitations.
Fundamental of PLL Passive loop filter
While designing a passive loop filter for any PLL; preferred approach is to use 2nd order filter. Selection of 2nd order filter will give lower resistor value and higher capacitor value near the VCO for better reference spur removal. Hence 2nd order loop filter is most preferred choice of passive loop filter design. If the effect of spurious is not under control after adding this loop filter, then additional RC low pass filtering stage can be added to reduce the reference spurious. Loop bandwidth (\(\omega_{c}\)) is another critical aspect in PLL design which depends on loop filter components, having narrow loop bandwidth is good at the expense of high lock time hence, a trade-off is required between the two. Higher order loop filters with narrow loop bandwidth approach is most effective in reducing spurious which leads to improvement in phase noise of PLL system.
Fundamental of PLL Active loop filter
When charge pump generated by passive PLL is not sufficient enough to drive VCO then we need to go for active PLL design. It has a disadvantage of added in-band phase noise which is added by the active devices i.e. op-amp. However, it is generally recommended to use at least a third order loop filter because added pole reduces the additional voltage, current and flicker noise of the active devices. There are different fundamental approaches to implement active loop filter topology; one of those is shown in Fig.2 [3].