16 Issues in RFIC Design: Noise, Linearity, and Signals optc ccuR BBRR-=+ (2.30)2.2.6 Phase NoiseRadios use reference tones to perform frequency conversion. Ideally, these tones would be perfect and have energy at only the desired frequency. Unfortunately, any real signal source will have energy at other frequencies. Local oscillator noise performance is usually classified in terms of phase noise, which is a measure of how much the output diverges from an ideal impulse function in the frequency domain. We are primarily concerned with noise that causes fluctuations in the phase of the output rather than noise that causes amplitude fluctuations in the tone, since the output typically has a fixed, limited amplitude. The output signal of a reference tone can be described as outLO( )cos(( ))onvtVttωϕ=+ (2.31)Here, wLOt is the desired phase of the output and fn(t) are random fluctuations in the phase of the output due to any one of a number of sources. Phase noise is often quoted in units of dBc/Hz or rad2/Hz. The phase fluctuation term fn(t) may be random phase noise or discrete spuri-ous tones, as shown in Figure 2.5. The discrete spurs at a synthesizer output are most likely due to the fractional-N mechanism (discussed in detail in Chapter 10) and the phase noise in an oscillator is mainly due to thermal, flicker, or 1/f noise and the finite Q of the oscillator tank.Figure 2.5 An example of phase noise and spurs observed using a spectrum analyzer.