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Phase Noise in PLLs Impact Communication Systems

Key Takeaways

  • Random noises in the building blocks of PLLs—such as the phase-frequency detector, loop filter, voltage-controlled oscillator(VCO), and spurious side-bands—are some of the causes of phase noise in PLLs.

  • As a VCO is the key component of PLL, phase noise is common in PLLs. It degrades the most critical specification of the oscillator, which is spectral purity. 

  • The effects of phase noise in communication systems include random rotations of received signals, amplitude variation of received signals, spectral regrowth, and channel interference.

Mobile phone

Phase noise in PLLs is a major problem in wireless communication systems

The demand for noiseless, high speed, high performance wideband communication systems has increased. In the context of modern digital communication, using phase modulation such as Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), or Phase-Locked Loops (PLLs) is becoming more important as this demand increases. PLLs are a crucial part of many communication circuits, such as frequency synthesizers and demodulators, and they play an important role in controlling the overall performance of a system. 

Phase noise and jitter are major concerns when analyzing the performance of a PLL. Random noises in the building blocks of PLLs—such as the phase-frequency detector, loop filter, voltage-controlled oscillator (VCO), and spurious side-bands—are some of the causes of phase noise in PLLs. Phase noise is detrimental to spectral purity and it is the root cause of reciprocal mixing, bit error rates, and channel interferences in communication systems. 

Phase Noise in PLLs

Any system consisting of oscillators and signal sources are susceptible to phase noise. As VCOs are the key component of PLLs, phase noise is common. It degrades the most critical specification of the oscillator, which is spectral purity.  Ideal oscillators generate output signals at one desired frequency. In an ideal oscillator, there will not be any other frequency components in the output signal. However, in practical cases, the oscillator generates the output signal of the desired frequency along with harmonics. The power spectrum distribution of a practical oscillator plotted contains the center frequency or desired frequency (⍵0) with power distributed to harmonic frequencies such as 2⍵0 , 3⍵0, and 4⍵0. The instantaneous output voltage of a practical VCO can be described in the equation:

V(t)=V0[1+A0(t)]sin(0t+(t))+higher order harmonics

where V0 is the ideal oscillator voltage amplitude, and A0(t)and  are the amplitude and phase fluctuation in the voltage signal. In the output spectrum of the practical oscillator, there are two-phase terms. The one that appears as a distinct component is called a spurious tone, as shown in the figure below. External noise sources such as noises in the power supply, control voltages, bias currents, and clock signals produce spurious tones.  The random phase fluctuations present in the spectrum correspond to phase noise which is the second phase term in oscillators. The phase noise is mainly caused by internal noises such as Johnson-Nyquist noise, flicker noise, or shot noise. The spurious tones are deterministic, whereas the phase noise is random in nature. The phase noises form the single-sideband noises which are quantified as the ratio of the phase noise power in 1 Hz bandwidth at a certain frequency, ⍵0 + ∆⍵ , offset to the center frequency,0, and expressed in unit dBc/Hz.

The output spectrum of oscillators

The output spectrum of (1) an ideal oscillator and (2) a practical oscillator

Effects of Phase Noise on Communication Systems

Any noise in PLL components or spurious sidebands generates phase noise and radiates it into the communication system. This noise mixes with desired signals and pollutes or degrades the system performance. Some of the effects of phase noise in communication systems are:

Random rotation of received signals: Phase noise creates random rotation of the received signal constellation which results in symbol detection errors in phase modulated transmission of signals. The random variation of the phase of the received signal leads to constellation rotation. The phase noise is the main factor causing decision errors in received signals. 

Amplitude variation of received signals: In multiple-antenna systems, phase noise varies the amplitude of the received signal randomly. The combination of out-of-phase signals at the receiving end produces this random amplitude variation in received signals. Phase noise in PLLs is the source  creating out-of phase signals at the receiving end.   

Spectral regrowth and channel interference : In Orthogonal Frequency-Division Multiplexing (OFDM) systems, phase noise is responsible for spectral regrowth and  neighboring channel interferences. In the presence of phase noise, the oscillator fails to generate the desired frequency and radiates the wideband noise and induces neighboring channel interferences.

The phase noise in PLLs can be catastrophic to the performance of RF receivers and transmitters. Along with effects such as spectral regrowth, amplitude variation, and rotation of the received signals, phase noise radiates noise to nearby channels and spreads interference throughout the communication system. The modeling of phase noise is critical when designing RF receivers and transmitters. Communication circuits should be designed in such a way that the phase noise in PLLs always remains under acceptable limits. 

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