Don’t let the phase noise of a signal source destroy your RF measurements What is spectral purity of an oscillator Spectral purity is the inherent stability of a signal It varies in frequency and time The major measurements of spectral purity are phase noise harmonics and spurs Phase noise is the most important figure of merit of a signal generator In this post you will learn what phase noise is and how it impacts your RF measurements How Big Is the Gap Between Ideal Reality Let's look at ideal and real signals first As shown in Figure 1 below an ideal signal is a perfect sinusoidal waveform in the time domain In the frequency domain it s a single spectral line However in the real world there are always unwanted amplitude and phase fluctuations on the signal Both random amplitude fluctuations and phase fluctuations are added to the ideal sinusoidal waveform equation The waveform has a phase shift and amplitude shift in the time domain In the frequency domain the signal has both amplitude and frequency modulation Figure 1 Equations and waveforms of ideal and real continuous waves Phase noise is a frequency domain view of the noise spectrum around the oscillator signal It describes frequency stability of an oscillator Frequency stability can be broken into two components long term stability and short term stability shown in Figure 2 below Figure 2 Long term frequency stability and short term frequency stability Long term stability is characterized in terms of hours days months or even years Short term stability refers to frequency changes that occur over a period of a few seconds or less There are 2 categories of short term stability Random noise The variation is random and is commonly called phase noise The sources of random noise in a signal include thermal noise amplifier noise and flicker noise in active and passive components Deterministic signals Signals appear as distinct components on the ideal spectrum These signals commonly called spurious signals result from power line frequency or mixers
These short cycle variations have a much greater effect on systems that rely on extreme processing to extract more information from a signal This discussion will focus on short term stability Short term stability can be described in many ways but the most common way is the single sideband SSB phase noise The definition of SSB phase noise is the ratio of the power density at a specific frequency offset from the carrier and the total power of the carrier signal The unit of phase noise is dBc Hz Figure 3 below shows single sideband SSB phase noise and deterministic signals Figure 3 Single sideband SSB phase noise and deterministic signals Why is the phase noise of a signal generator important for RF measurements Any phase noise on the local oscillator LO signal is translated directly to the output of the mixers If the desired signal is the smaller of the two mixed signals the translated noise in the mixer output may completely mask the smaller signal Figure 4 shows how the LO phase noise affects receiver sensitivity Figure 4 LO phase noise affects receiver sensitivity Radar systems require excellent phase noise performance A radar transmits pulses at a specific frequency and measures the change of each returning pulse s frequency Each returning pulse s change in frequency is related to the velocity of the moving object based on the Doppler effect If the object moves very slowly the frequency shift of the returning pulse is small Using Figure 4 above the returning pulse of a moving object is the signal of interest and the returning pulse of a fixed object e g ground is the interfering signal
The radar receiver can t identify the moving object if the downconverted signal of interest is masked by the phase noise Let's look at digital modulation Figure 5 below is a simplified QPSK digital receiver block diagram The phase noise of the LO signal is translated into the output of the mixers The direct effect of phase noise on the constellation diagram is the radial smearing of the symbols For high order modulation scheme e g 256 QAM the symbols are closer The symbols smearing results in bad receiver sensitivity Figure 5 A simplified digital receiver block diagram The phase noise of a LO results in the symbols smearing Another good example is Orthogonal Frequency Division Multiplexing OFDM OFDM is a popular modulation scheme for wideband digital communication OFDM uses many closely spaced orthogonal sub carrier signals to transmit data in parallel shown in Figure 6 below The sub carrier with phase noise spreads into other sub carriers as an interference The phase noise degrades the modulation quality of the OFDM signal Figure 6 The phase noise causes the OFDM subcarrier to lose orthogonality Phase noise performance is often the key factor of a signal generator for a demanding application It can be a limiting factor for specific applications in aerospace and defense as well as in digital communications
