Phase noise

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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

 


Typical phase noise profile of a signal source

 


 

Note on Phase Noise:

Phase noise consists of small random perturbations in the phase of the signal, i.e. phase jitter. An ideal signal source would be able to generate a signal in which the phase advanced at a constant rate. This would produce a single spectral line on a perfect spectrum analyzer. Unfortunately all signal sources produce some phase noise or phase jitter, and these perturbations manifest themselves by broadening the bandwidth of the signal.

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.

 

 

 

When looking at the noise level there are three elements to the specification:

  • The phase noise level itself:   this is expressed in dB relative to the carrier, i.e. dBc. This method is adopted because the phase noise normally varies in line with the carrier level. The level of phase noise relative to the carrier is also the important factor. Where the phase noise varies with the carrier level, the specification can state that the phase noise is - n dBc at a given carrier level.
  • The offset from the carrier:   An essential part of the phase noise specification is the offset from the carrier at which the phase noise was a certain level. This is because the noise level varies according to the frequency offset from the carrier, the frequency offset must be given. Typically the phase noise rises much faster closer in towards the carrier and falls away until it ultimately reaches a noise floor.
  • The measurement bandwidth:   The noise power is proportional to the bandwidth and therefore it is necessary to state the bandwidth that has been used. Obviously the wider bandwidth that is used, the greater the level of noise that will pass through the filter and be measured. Technically the most convenient bandwidth to use is 1 Hz because it is easy to relate the level to other bandwidths. As a result this phase noise specification format has been almost universally adopted. Spectrum analyzers are unable to measure in a 1 Hz bandwidth directly because this would require a very narrow filter bandwidth - therefore they measure the signal in a wider bandwidth and mathematically adjust the level to that of a 1 Hz bandwidth. Using current signal processing technology this is simply a further calculation that is added into the routine.

Thus a typical phase noise specification for a signal generator or other oscillator may be -100 dBc / Hz at a 100 kHz offset. For a complete phase noise specification several points will be specified to give an indication of the phase noise at different points, typically at points varying by a factor of ten: 10 Hz, 100 Hz, 1 kHz, etc.

 

Low Phase Noise Frequency Synthesizer Design

- a simple graphical and understandable approach to understanding where phase noise is generated within a PLL frequency synthesizer and designing it to meet a requirement

In this section

Phase noise in PLL frequency synthesizers if of great importance because it determines many factors about the equipment into which it is incorporated. For receivers it determines the reciprocal mixing performance, and in some circumstances the bit error rate. In transmitters the phase noise performance of the frequency synthesizer determines features such as adjacent channel noise and it contributes to the bit error rate for the whole system.


 

Phase noise in a PLL synthesizer

Phase noise is generated at different points around the synthesizer loop and depending upon where it is generated it affects the output in different ways. For example, noise generated by the VCO has a different effect to that generated by the phase detector. This illustrates that it is necessary to look at the noise performance of each circuit block in the loop when designing the synthesizer so that the best noise performance is obtained.

Apart from ensuring that the noise from each part of the circuit is reduced to an absolute minimum, it is the loop filter which has the most effect on the final performance of the circuit because it determines the break frequencies where noise from different parts of the circuit start to affect the output.

To see how this happens take the example of noise from the VCO. Noise from the oscillator is divided by the divider chain and appears at the phase detector. Here it appears as small perturbations in the phase of the signal and emerges at the output of the phase detector. When it comes to the loop filter only those frequencies which are below its cut-off point appear at the control terminal of the VCO to correct or eliminate the noise. From this it can be seen that VCO noise which is within the loop bandwidth is attenuated, but that which is outside the loop bandwidth is left unchanged.

The situation is slightly different for noise generated by the reference. This enters the phase detector and again passes through it to the loop filter where the components below the cut-off frequency are allowed through and appear on the control terminal of the VCO. Here they add noise to the output signal. So it can be seen that noise from the reference is added to the output signal within the loop bandwidth but it is attenuated outside this.

Similar arguments can be applied to all the other circuit blocks within the loop. In practice the only other block which normally has any major effect is the phase detector and its noise affects the loop in exactly the same way as noise from the reference. Also if multi-loop synthesizers are used then the same arguments can be used again.


Effects of multiplication

As noise is generated at different points around the loop it is necessary to discover what effect this has on the output. As a result it is necessary to relate all the effects back to the VCO. Apart from the different elements in the loop affecting the noise at the output in different ways, the effect of the multiplication in the loop also has an effect.

The effect of multiplication is very important. It is found that the level of phase noise from some areas is increased in line with the multiplication factor (i.e. the ratio of the final output frequency to the phase comparison frequency). In fact it is increased by a factor of 20 log10 N where N is the multiplication factor. The VCO is unaffected by this, but any noise from the reference and phase detector undergoes this amount of degradation. Even very good reference signals can be a major source of noise if the multiplication factor is high. For example a loop which has a divider set to 200 will multiply the noise of the reference and phase detector by 46 dB.

From this information it is possible to build up a picture of the performance of the synthesizer. Generally this will look like the outline shown in Fig. 6. From this it can be seen that the noise inside the loop bandwidth is due mainly to components like the phase detector and reference, whilst outside the loop the VCO generates the noise. A slight hump is generally seen at the point where the loop filter cuts off and the loop gain falls to unity.

By predicting the performance of the loop it is possible to optimise the performance or look at areas which can be addressed to improve the performance of the whole synthesizer before the loop is even built. In order to analyse the loop further it is necessary to look at each circuit block in turn.