Chapter 2 Basics of Voltage Controlled Oscillator (VCO)
2.2 Performance Parameters
2.2.1 Phase Noise
An ideal output spectrum of oscillator has only one impulse at the fundamental frequency as shown in Figure 2.5(a). A realistic oscillator has a spectrum with spurious signals due to its harmonics or intermodulation products. In addition, the frequency spectrum of the realistic oscillator exhibits skirts around the carrier frequency. The realistic oscillator output spectrum and the definition of phase noise is shown in Figure 2.5(b). The phase noise due to random fluctuations caused by thermal and other noise sources, and appears as a broad continuous distribution localized about the output signal. The phase noise is defined as the ration of power in one phase modulation sideband to the total signal power per unit bandwidth at a particular offset, fm, from the signal frequency, and is denoted as L(fm). It is usually expressed in describes relative to the carrier power per Hertz of bandwidth (dBc/Hz).
Phase noise is typical expressed as [10]
( )
m 10log SSB Hz,1( )
m( )
WherePSSB Hz,1( )
fm represents the single sideband power at a frequency offset of f from the carrier with a measurement bandwidth of 1Hz, Pm C is the carrier power.From (2-1), the output power must be maximized in order to reduce to phase noise, but it will suffer from high DC power consumption.
Lesson has proposed an equation of phase noise by analyzing a feed back oscillator and is written as [11]
( )
10log 2 1 0 2 1 1 3Chapter 2 Basics of Voltage Controlled Oscillator (VCO)
(a) (b)
Figure 2.5 Output spectrum of ideal and realistic oscillators.
where F is the excess noise factor, K is the Boltamann’s constant, T is the standard noise temperature, ω0 is the oscillator frequency, Q is the loaded Q, and
1 3
f
ω
Δ
isthe corner frequency, where the slope of the phase noise spectral density changes from -30 dB/dec to -20 dB/dec. Figure 2.6 shows the Lesson’s phase noise model. The equation is from the cure fitting after measured results of VCO. Therefore,
1 3
f
ω
Δ
isfrom measured results. If the output wave form is odd-symmetry, it can suppress 1f noise effectively. This will be lower
1 3
f
ω
Δ
. From equation (2-2),increase Q factor of LC-tank and output power can improve phase noise.
Low phase noise can be achieved by using low noise figure or low flicker noise active device and high-Q resonator. Moreover, using a low noise power supply or filtering out the noise of power supply are good approaches to reduce phase noise.
Chapter 2 Basics of Voltage Controlled Oscillator (VCO)
Figure 2.6 Lesson’s phase noise model.
2.2.2 Tuning Range
For the LC-VCO design, Choosing suitable inductors and varactors to cover all the frequency band of the specified application is important. A CMOS oscillator must be designed with a large tuning range to overcome process variations. The output frequency of LC-VCO is tuned by varactors, which is like as diode varactors or MOS varactors, so the capacitance versus tuning voltage characteristic is important.
In general, the NMOS cross-coupled pair LC-VCO has higher tuning range than double cross-coupled LC-VCO topology for equal effective tank transconductance.
When control voltage change, the bias voltage of transistor will also change. S parameter and Γ will change according to DC current variation. This phenomenon in will cause output frequency shift. This is called pushing effect. To avoid pushing effect, using high quality resonator can reduce the pushing effect. In addition, using regulator also can overcome pushing effect such as band gap circuits. Loading effect is another important issue. When loading change, its impedance is also change. This will cause output frequency shift, too. This called load pulling effect. To avoid this
Chapter 2 Basics of Voltage Controlled Oscillator (VCO) issue, we can use buffer circuit to overcome load pulling effect.
2.2.3 Tuning Sensitivity (K
VCO)
The varactor is basically a reverse biased pn junction diode whose depletion capacitance is a function of voltage [12]. Even though popular, this type of tuning does not have a wide tuning range and is also sensitive to temperature variation. We now quantify the KVCO of such an arrangement. We definition of KVCO with units in Hz/s： KVCO in general should be designed to be as small as possible. However, it must be large enough that f0 can span the whole frequency range with a tuning voltage, Vtune, that is within the power supply. KVCO should be made small because the varactor is connected to the LC-tank via a small fixed capacitor. If KVCO is large, then this coupling capacitor is large and the varactor has a large influence on the resonant frequency of the LC-tank. In addition, the varactor itself has a low Q factor, in particular when compared with the inductor or capacitor in the oscillator. This is due to the resistance in the varactor itself and also due to packing. A large varactor influence and a low Q varactor mean that the varactor resistance is translated across to the tank circuit, and this would reduce the Q of the tank significantly.
The gain, KVCO, of idea LC-VCO must keep constant in the whole tuning range. In fact, the KVCO is not a constant, the tuning characteristic (oscillation frequency versus control voltage) is nonlinearity. Thus, the minimum variation of KVCO across the tuning range is needed for phase-locked loops (PLLs) performance like setting time.
Chapter 2 Basics of Voltage Controlled Oscillator (VCO)
2.2.4 Output Power
In general, it is not easy to predict the output power of the realistic VCO, but we can know that the maximum output power of VCO is not larger than the output power of the transistor in the VCO through large-signal analysis. The output power must be maximized in order to make the waveform less sensitive to noise or to lower phase noise. It trades with power consumption, supply voltage, and tuning range. The designer can choose the active devices whose parameter is known. Therefore, when the VCO is designed, we also can predict the output power of the VCO.
2.2.5 Harmonic Rejection
The VCO has a good harmonic rejection performance that means it is closed to a sinusoidal output waveform. In wireless communication systems, harmonic rejection is specified how much smaller the harmonics of the output signal are compared with the fundamental output power.
2.2.6 Power Consumption
With fast growth in the radio-frequency (RF) wireless communications market, the demand for low-power and high-performance but low-cost RF solutions is rising.
Low–power operation can extend the lifetime of the battery and save money for consumers.
Chapter 2 Basics of Voltage Controlled Oscillator (VCO)