2. FUNDAMENTALS OF PLL-BASED FREQUENCY SYNTHESIZERS
2.2 PLL M AIN B UILDING B LOCKS
2.2.3 Voltage-Controlled Oscillators (VCOs)
The purpose of the VCO is to generate a frequency, which is controlled by the control voltage provided by the loop filter. In wireline and wireless communication systems, two types of the VCO are widely used; a ring VCO and an LC VCO. The simplified structure of the LC VCO and the ring VCO are shown in Figure 27(a) and (b), respectively. Basically, the LC VCO generates a frequency based on the resonance of the LC tank, thus, the output frequency of LC VCO is mainly defined by the value of the inductor and the capacitor. In the case of ring VCO, the output frequency is defined by the unit delay of the unit delay, τ, cells and the number of stages. Since one of the factors that determine the output frequency of the ring VCO is the unit delay, the ring VCO is more sensitive to the voltage and temperature (VT) variations. Table 4 shows the characteristics of the two VCOs.
τ τ τ
L C
LC VCO Ring VCO
# of stage = 3
f
VCOLC
= 1 f
VCO2·(# of stage)·τ
= 1
Unit delay cell
τ
= Unit delay(a) (b)
Figure 27. Simplified structure of: (a) LC VCO and (b) ring VCO with the defined frequency Table 4. Characteristics of the ring and LC VCO
Ring VCO LC VCO
Phase noise Moderate or bad Good
Freq. tuning range Wide Moderate or narrow
Multiple phases Depends on the number of stage Differential
Size (or area) Small Large
Scalability Good Not easy
Sensitivity to VT Vulnerable Robust
As shown in the above table, even though the ring VCO has many advantages over LC VCO such as wide frequency tuning range, multiple phases, scalability, and small chip size, the usage of ring VCO
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in modern wireless communication systems is limited due to the moderate or bad phase noise performance. Thus, in this thesis, only the LC VCO will be further detailed in Chapter 3, since the LC VCOs are extensively used for high-performance frequency synthesizers.
Since the understanding of the characteristics of the VCO is important for the proper design of the PLL, in the following, each characteristic will be explained.
29 Gain of the VCO
The gain of the VCO is usually denoted as KVCO and the unit is Hz/V. Figure 28 shows graphically the gain of the VCO with an assumption that the gain is constant. However, in the practical design of the VCO, KVCO cannot be a constant value since fVCO will be controlled by the VTUNE connected to a varactor, which is a non-linear device. Thus, the variations of KVCO could degrade the overall performance of PLL in terms of phase noise, stability, and settling time because the open loop gain of the PLL changes.
K
VCOV
TUNEf
VCOV1 V2
f1 f2
f0
VCO
f
VCOV
TUNEf
VCO= f
0+ K
VCO·V
TUNEFigure 28. Graphical representation of the gain of the VCO
Frequency Tuning Range (FTR)
Literally, the frequency tuning range (FTR) means the range that the VCO can cover. As mentioned above, an LC VCO has narrower FTR than a ring VCO. For example, even though the ring VCO can easily cover octave frequency range, but the LC VCO is not. When we denote the minimum and the maximum frequency of the VCO as fVCO,min and fVCO,max, the widely used definition of the FTR is
FTRሺ%ሻ= VCO's frequency range
VCO's center frequencyή100 (%)= ቀfVCO,minെfVCO,maxቁ
ቀfVCO,minfVCO,maxቁή2ή100 (%) (20)
Overtly, having wide FTR is always desired in the design of the VCO. However, wide FTR has a trade- off, which is generally phase noise. This is because if there is a target frequency, for the wide FTR, the inductance should be small for the large variation in the capacitance, which implies that the Q-factor of the LC tank decreases. Therefore, the target FTR should be carefully designed not to sacrifice the phase noise.
30 Frequency Pushing
If the VCO is ideal, the output frequency of the VCO, fVCO, only depends on the input voltage, i.e., VTUNE. Unfortunately, in practice, fVCO is also sensitive to the supply voltage of the VCO. Thus, as the supply voltage changes, fVCO also changes. This phenomenon is called as the frequency pushing.
Frequency pushing is expressed as Hz/V and it could be a positive or negative value. If the supply voltage is noisy, the VCO would have more frequency drift, which leads to the degraded phase noise.
In addition, if the supply voltage has a glitch, the PLL could lose the locked status and starts again the settling process. To minimize the frequency pushing, first, the supply voltage should be well regulated.
Second, an LC tank must be designed to have a high Q-factor to be robust to the environmental variations.
Frequency Pulling
When fVCO varies due to the attached load at the output of the VCO, this phenomenon is called as the frequency pulling. Therefore, the frequency pulling can be minimized by isolating the VCO from the load. One of the popular methods is to insert a buffer or an amplifier since those have large reverse isolation.
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