Phase noise

“ Noise due to phase modulation ”

Phase noise of signal sources is a cause of EVM, adjacent channel power, adjacent channel rejection and blocking degradation in communication systems.

Communications systems often employ an oscillator and mixing to perform up-conversion or down-conversion in both receivers and transmitters. The oscillator phase noise is then mixed into the communications channel bandwidth.

Modulation schemes have become more complicated over the years moving from analog to digital to multi-carrier. Phase noise in analog systems was the cause of residual modulation and adjacent channel issues. With more modern digital and multi-carrier systems it degrades EVM.

What is phase noise?

Ideally frequency sources used in a communication system should be comprised of a pure sine wave carrier. This would be represented in the frequency domain by a single line. All real sources have unwanted amplitude or phase modulated noise components. These phase modulated components are known as phase noise.

The amplitude components are likely to be of a lower level than the phase modulation component. This is due to the compressed amplifiers found following signal sources and the inherent degradation in phase noise due to frequency multiplication. Therefore in communication system signal sources the predominant form of noise is phase noise.

For RF oscillators this noise is most commonly expressed in terms of single sideband phase noise, L(f). This is a relative measurement of the noise measured in a 1 Hz bandwidth, at a frequency offset from the carrier, relative to the carrier power. The units are dBc/Hz.

Phase noise measurement

Spectrum analysers may be used to measure phase noise, taking the resolution bandwidth filter into account, although this can prove inaccurate. Some spectrum analysers have a built in phase noise measurement capability to perform this calculation, often including a more accurate noise bandwidth calculation. This function is usually found in a noise marker menu. The output from a spectrum analyser displays the summation of the amplitude and phase noise of the source under test and the analyser's own phase noise. This makes measuring low phase noise oscillators difficult as the performance is often limited by the spectrum analyser. For low phase noise sources, alternative methods must be utilized, such as frequency discriminators or phase noise test systems.

PLLs and phase noise

In synthesizer phase locked loops, there are a number of noise sources that contribute to the PLL's noise profile. The three main sources are:

1. Reference source, low frequency offsets
2. PLL dividers, middle plateau
3. VCO phase noise, high frequency offsets

These sources are acted upon by the loop, such that the loop low pass filters the reference source and PLL divider noise and high pass filters the VCO noise.

There are other potential noise contributions; the loop itself will contain resistors and possibly active elements such as op-amps. Both of these have noise contributions.

Special care must also be taken with the power supply to the reference sources and VCO as these are both susceptible to power supply noise which may degrade the PLL phase noise performance.

Integrated phase noise

Phase noise is often displayed in an integrated form. The integrated single sideband phase noise is convertible to RMS phase error, RMS frequency error or residual FM and jitter. An approximate conversion to EVM is also possible. The integrated phase noise over an adjacent channel will contribute to the adjacent channel power and adjacent channel selectivity.