# Why phase noise affects the constallation?

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#### Julian18

##### Full Member level 3
Hi there,
Can anybody here tell me or give a refereance on why phase noise affects the constellation of say PSK?

Thanks

#### Mazz

This is true for all modulation shemes.
The position of each symbol in the IQ space is defined by magnitude and angle. If, at a certain time, the signal is affected by a phase error, the phase of the symbol varies.
This is what happens when a phase noise is added to the correct signal. Each point makes a small circular movement in the constellation and the distance from the origin will be constant.
I hope it can help.
Mazz

### Julian18

Points: 2

#### t_tuaki

##### Member level 2
anyone can explain what is phase noise in a easier way? or any website explain it good? because i already read some books and i still cant fully understand what is phase noise

#### Soben

##### Member level 3
t_tuaki said:
anyone can explain what is phase noise in a easier way? or any website explain it good? because i already read some books and i still cant fully understand what is phase noise

Imagine you have an Oscillator loop consisting from a Noiseless amplifier a feedback network and noisy source of white or flicker spectra (very small amplitude and phase fluctuations with a specific spectrum distribution, constant (white) or 1/f Flicker). When you close loop your system and you have 360deg total feedback you have oscillation.The Noisy source then starts modulates your carrier by very small increments in amplitude and phase, the resulted signal therefore has amplitude and phase modulation by a very small random signal. That is your phase and amplitude noise. In reality in an oscillator Amplitude noise is suppressed enough due to your amplifier actually get clipping and the major source of modulation is from phase variations of the noisy source to your carrier, that is called Phase Noise in a simple manner.

I hope that helps,

#### springf2000

##### Full Member level 6
because the when demdulator, for example the syn demodulator, the phase noise shall turn to the Amplitude, this is called the PM/AM of the demodulator, y can from the formuation cos(wt+theta)*cos(wt+phase_noise) to see this effect

#### mickey_melomane

##### Member level 2
Signal Constellation is just a plot of complex numbers. Now when a phase noise is introduced, it means you are multiplying each of the complex numbers (constellation) by exp(i*phi), (phi is the phase noise). This results in rotation of the constellation space. This implies your decision boundaries also rotate...
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Now phase noise, to be simply stated is, noise in the phase term. The output of an oscillator if usually Ac*cos(2*pi*fc*t + phi), phi is an arbitrary constant initial phase. The spectrum of this signal is a delta function at ±fc.

Now a phase noise is included in this expression as
Ac*cos(2*pi*fc*t+phi+Nph(t)), where Nph(t) is the phase noise term. The spectrum of this signal is shown to have skirts around a delta function at fc and -fc. ie. you have frequency components at fc ± δf, and -fc ± δf.

The broader these 'skirts', the poorer your oscillator is. The jitter is also expressed to be the manifestation of phase noise.

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