如何识别来自PLL的相位噪声

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Introduction
Many modern communication systems utilise PLL
frequency synthesizers, commonly with specifications
on phase noise performance. This report details a
simple technique that we have found valuable in
isolating the sources of phase noise in a PLL
synthesizer, and is especially useful when only a
spectrum analyser is available for measurements. An
additional passive filter (which we nicknamed a “PLL
Widget”) is added to the PLL, greatly narrowing the
loop bandwidth, causing the output phase noise to
approximate that from the VCO alone. Inspection of
the output phase noise in the normal and reduced
bandwidth modes enables the source of the phase noise
to be tracked down to either the VCO or PLL.
The PLL widget is a small passive device and may be
plugged in place of link on the VCO control line,
enabling the bandwidth to be easily reduced during
development. The device can also be used in
measuring the frequency domain characteristics of the
PLL, including loop gain and phase margin. This is
the subject of a separate report [1].
Sources of Phase Noise
The output phase noise of any PLL synthesizer comes
from many sources including:
1. VCO phase noise
a. Basic VCO phase noise
b. VCO excess phase
2. Divider and phase detector noise
3. Noise from the resistors and active devices in
the loop filter
The Basic VCO phase noise is that measured for an
isolated VCO operating with noise free power supply
and noise free control line. VCO excess phase noise is
caused by interference in its current environment, due
primarily to power supply noise but may include other
interference from the nearby PLL components.
Sources 2 and 3 contribute to the output phase noise by
modulating the VCO control line.
Identifying Phase Noise Sources
In order to improve the output phase noise, it is
necessary to identify the sources. In well equipped
laboratories, it may be possible to measure the phase
noise of the VCO in isolation, and then in the PLL.
The added noise from the PLL can then be clearly
identified. Such a measurement is....................

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