Synthesizer PLL phase noise is a particularly important parameter for any phase locked loop based frequency

synthesizer. Although key parameters like frequency stability, frequency range and synthesizer step size, and
frequency range are widely quoted in specification sheets for synthesizers, the phase noise is equally important.
The phase noise of a PLL frequency synthesizer is important for many reasons. It affects the performance of the
equipment in which the synthesizer is used in a number of ways.
For signal generators a clean source is needed for the tests in which the generator may be used.
If the frequency synthesizer is used in a radio communications system, then it will affect the performance of the
system. For a radio receiver used in a radio communications system it will affect parameters such as reciprocal
mixing and under some conditions the noise floor.
If the frequency synthesizer is used in a transmitter, then it can cause wide-band noise to be transmitted and this
could cause interference to other users. Accordingly for any radio communications application, the level of phase
noise is important. As the majority of the phase noise is likely to be generated by the synthesizer, PLL phase noise
characteristics are of great importance.

What is phase noise?

Phase noise is present on all signals to some degree and it is caused by small phase (and hence frequency)
perturbations or jitter on the signal. It manifests itself as noise spreading out either side from the main carrier

Some signal sources are better than others. Crystal oscillators are very good and have very low levels of phase
noise. Free running variable frequency oscillators normally perform well. Unfortunately synthesizers, and especially
those based around phase locked loops, do not always fare so well unless they are well designed. If significant
levels of phase noise are present on a synthesizer used as a local oscillator in a receiver, it can adversely affect the
performance of the radio in terms of reciprocal mixing.

Phase noise in synthesizers

Each of the components in a frequency synthesizer produces noise that will contribute to the overall noise that
appears at the output. The actual way in which the noise from any one element in the loop contributes to the
output will depend upon where it is produced. Noise generated by the VCO will affect the output in a different way
to that generated in the phase detector for example.
To see how this happens take the example of noise generated by the voltage controlled oscillator. This will pass
through the divider chain and appear at the output of the phase detector. It will then have to pass through the loop
filter. This will only allow through those components of the noise that are below the loop cut-off frequency. These
will appear on the error voltage and have the effect of cancelling out the noise on the voltage controlled oscillator.
As this effect will only take place within the loop bandwidth, it will reduce the level of noise within the loop
bandwidth and have no effect on noise outside the loop bandwidth.
Noise generated by the phase detector is affected in a different way. Again only the components of the noise below
the loop bandwidth will pass through the low pass filter. This means that there will be no components outside the
loop bandwidth appearing on the tune voltage at the control terminal of the voltage controlled oscillator, and there
will be no effect on the oscillator. Those components inside the loop bandwidth will appear at the oscillator control
terminal. These will affect the oscillator and appear as phase noise on the output of the voltage controlled
oscillator.
Matters are made worse by the fact that the division ratio has the effect of multiplying the noise level. This arises
because the synthesizer effectively has the effect of multiplying the frequency of the reference. Consequently the
noise level is also multiplied by a factor of 20 log N, where N is the division ratio.
Noise generated by the reference undergoes exactly the same treatments as that generated by the phase detector.
It too is multiplied by the division ratio of the loop in the same way that the phase detector noise is. This means
that even though the reference oscillator may have a very good phase noise performance this can be degraded
significantly, especially if division ratios are high.
Dividers normally do not produce a significant noise contribution. Any noise they produce may be combined with
that of the phase detector.
The combined noise of the loop at the output generally looks like that shown in Figure 2. Here it can be seen that
the noise within the loop bandwidth arises from the phase detector and the reference. Outside the loop bandwidth
it arises primarily from the voltage controlled oscillator. From this it can be seen that optimisation of the noise
profile is heavily dependent upon the choice of the loop bandwidth. It is also necessary to keep the division ratio in
any loop down to reasonable levels. For example a 150 MHz synthesizer with a 12.5 kHz step size will require a
division ratio of 12000. In turn this will degrade the phase detector and reference phase noise figures by 81 dB
inside the loop bandwidth - a significant degradation by anyone's standards! Provided that division ratios are not
too high then a wide loop bandwidth can help keep the voltage controlled oscillator noise levels down as well.

Noise profile of a typical synthesizer

Effects of PLL phase noise

PLL phase noise can affect different systems in different ways. However it is important that for all applications the
phase noise on the signal is known and within the required limits. However phase noise can give rise to a number
of different problems:

 Wideband transmitted noise:   When PLL frequency synthesizers are used within a transmitter, a local
oscillator source with large amounts of phase noise can be radiated away from the wanted frequency
band. This is transmitted as wideband noise and can cause interference to other users nearby.
 Increase in bit error rate:   For transmissions using phase modulation, the phase jitter or phase noise
can cause errors in the reception of the data. PLL phase noise in both the transmitter and receiver can
increase the occurrence of bit errors. It is therefore essential that the PLL phase noise is kept to
acceptable limits within both the transmitter and receiver.
 Reciprocal mixing:   This is a problem that occurs when the phase noise from the local oscillator signal is
superimposed onto a strong off channel signal. This phase noise then masks out the much lower level
weaker signal. Further information on Reciprocal Mixing can be found from the Radio Receivers menu
page.
PLL phase noise measurement and specification
Some oscillators have phase noise levels that are quoted in their specifications. Any high quality signal generator
will have the level of phase noise specified, as do many high performance crystal oscillators used as standards.
Their performance is generally specified in dBc/Hz and at a given offset from the carrier. The term dBc simply
refers to the level of noise relative to the carrier, i.e. -10 dBc means that the level is 10 lower than the carrier.
The bandwidth in which the noise is measured also has to be specified. The reason for this is that noise spreads
over the frequency spectrum. Obviously the wider bandwidth that is used, the greater the level of noise that will
pass through the filter and be measured. To prove this, just try selecting a different bandwidth on a receiver and
check what happens to the noise level. It will rise for a wider bandwidth and fall when a narrow bandwidth is used.
Technically the most convenient bandwidth to use a 1 Hz bandwidth and so this is used. When measuring this a
wider bandwidth is usually used because it is difficult to obtain 1 Hz bandwidth filters and a correction is made
mathematically.
Finally the level of noise varies as different offsets from the carrier are taken. Accordingly this must be included in
a specification. A very good oscillator might have a specification of -100 dBc/Hz at 10 kHz offset.
It has already been mentioned that the level of phase noise changes as the offset from the carrier changes and for
"simple" signal sources like crystal oscillators or variable frequency oscillators the phase noise reduces as the
frequency from the main carrier is increased. For frequency synthesizers the picture is a little more complicated as
we shall see.

Summary
PLL phase noise is a particularly important parameter for any synthesizer. It can have a significant effect on the
performance of the system in which it is used whether in a signal generator, radio communications system, or any
other application. Accordingly when designing or specifying a synthesizer, the PLL phase noise is one of the major
parameters that should be included in the specification from the outset. In this way, the PLL phase noise
performance can be incorporated into the overall design at the earliest stages and no costly rework have to be
undertaken.