Good Practice Guide to Phase Noise Measurement ... - Sequanux

Measurement Good Practice Guide No. 68
1.6.1 Analog communication systems
For analog audio communication systems the most important offset frequencies are those
around 1 kHz since this strongly influences the residual FM, and therefore the ultimate signal
to noise ratio 7 , of the transmitted or received signal. The noise can be heard as an audio “hiss”
in the audio bandwidth of the systems. The phase noise, whether generated in the
transmission systems or the receiver electronics and local oscillators, can be measured as a
residual modulation (phase modulation or frequency modulation) in a specified bandwidth.
At the higher frequency offsets phase noise affects transmitted power into other RF channels,
while the local oscillator noise in the receiver may affect the amount of power unintentionally
mixed back into the receiver bandwidth from other sources.
1.6.2 Digital communications
Phase noise characteristics are important for digital, as well as analog, communication
systems. The 1 kHz phase noise characteristics of oscillators in transmitters using Time
Domain Multiple Access (TDMA) or Time Domain Duplex (TDD) techniques often strongly
influences the residual phase or frequency jitter within a single burst of the carrier frequency.
As wider bandwidth systems are adopted, phase noise at larger offsets will become
increasingly specified, but in general, the toughest target is likely to remain the 100 Hz to 3
kHz offset performance.
The sensitivity to the noise in the 1 kHz offset region in digital modulation systems arises
because the signal is split into blocks of information, typically with a duration of 1 ms to 20
ms, for the purpose of encoding speech or adding error correction. The details of this are
beyond the scope of this guide. The blocks of information usually have within them, a
sequence of digital bits that are used to extrapolate the phase and frequency reference of the
transmitted signal over the entire block. Having obtained this phase reference, the digital data
can be derived. This phase or frequency estimation process means that phase noise at low
carrier frequency offsets is removed, whereas noise at frequencies corresponding to times
shorter than the data block length can directly lead to an increase in measured modulation
error. The longer the length of the data block used, the more susceptible the system is to lower
frequency noise.
The varying amplitude of many digital communication systems can lead to the generation of
noise like signals; in fact spread spectrum systems are designed to look like noise when
viewed on a spectrum analyzer. Any non-linear behavior in the source with this sort of
modulation will lead to spectral spreading of the signal. This type of signal should not be
confused with phase noise.
Irrespective of whether a communication system is digital or analog, the measurement of
phase noise is likely to continue to be an important activity in the design of communication
systems, whether they are analog or digital.
7 More information on signal to noise and distortion ratio can be found in an application note “All you need to know about SINAD
measurements using the 2023” by David Owen which is available on the Aeroflex web site www.aeroflex.com
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