Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out
Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

PHASE NOISE

VIEWS: 69 PAGES: 3

									                                                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 is now
seen degrading 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.




                  Single sideband 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. 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 analyzer may be used to measure phase noise, taking the
resolution bandwidth filter into account, although this can prove
inaccurate.

NoisedBc-10.Log10(RBW Hz)
Spectrum analyzer resolution bandwidth to 1Hz bandwidth
conversion.
Some spectrum analyzers 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 analyzer displays the
summation of the amplitude and phase noise of the source under test
and the analyzer's own phase noise. This makes measuring low phase
noise oscillators difficult as the performance is often limited by the
spectrum analyzer. 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 sum to form the PLL's noise profile. The three main
sources are:

   1. Reference source
   2. PLL dividers
   3. VCO phase noise

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.

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, jitter and an approximate conversion
to EVM is possible. The integrated phase noise over an adjacent
channel will contribute to the adjacent channel power and adjacent
channel selectivity.

								
To top