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].