6.5: Voltage-Controlled Oscillator (VCO)

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As its name implies, a controlling voltage sets the frequency of the output of a VCO. The block diagram of a VCO is shown in Figure 6.4.8(a) where a slowly varying input signal, \(v_{i}\), determines the frequency, \(f_{0}\), of the signal produced by an oscillator. This is usually achieved by varying the capacitance of a varactor diode in a resonator.

One of the important characteristics of a VCO is the tuning curve, which plots the output frequency \(f_{0}\) against the applied tuning voltage \(v_{i}\) as shown in Figure 6.4.8(b). Ideally the tuning curve is a straight line, but in practice it is shaped and the actual range over which the VCO is used is less than the full range supported. The tuning property is described by the tuning constant,

\(f_{0}\) \(\text{GHz}\)	\(f_{\text{BW}}\) \(\text{MHz}\)	\(P_{\text{RF}}\) \(\text{dBm}\)	\(P_{\text{DC}}\) \(\text{mW}\)	\(f_{m}\) \(\text{MHz}\)	\(\mathcal{L}(f_{m})\) \(\text{dBm/Hz}\)	\(\mathcal{L}(1\text{ MHz})\) \(\text{dBm/Hz}\)	\(\text{FOM}_{1}\) \(\text{dBm/Hz}\)	\(\text{FOM}_{2}\) \(\text{dBm/Hz}\)	Reference
\(4.92\)	\(770\)	\(0\)	\(150\)	\(1\)	\(-128\)		\(-106\)	\(-157\)	SiG HBT hybrid [13]
\(5.05\)	\(500\)	\(0\)	\(150\)	\(1\)	\(-130\)		\(-106\)	\(-155\)	SiGe HBT hybrid [13]
\(5.16\)	\(229\)	\(-0.43\)	\(24\)	\(1\)	\(-111\)		\(-98\)	\(-136\)	InGaP/GaAs HBT [14]
\(11.5\)	\(550\)	\(9\)		\(0.1\)	\(-91\)	\(-111\)		\(-138\)	GaAs MESFET [15]
\(9.33\)	\(440\)	\(3.3\)	\(30.5\)	\(1\)	\(-102\)		\(-87\)	\(-128\)	GaN HEMT [16]
\(6.40\)	\(150\)	\(5.5\)	\(173\)	\(0.1\)	\(-105\)	\(-125\)	\(-85\)	\(-127\)	SiGe HBT [17]
\(5.94\)	\(166\)	\(-4.0\)	\(8.1\)	\(1\)	\(-110\)		\(-94\)	\(-134\)	CMOS IC [18]
\(4.87\)	\(70\)	\(-4.0\)	\(4.8\)	\(1\)	\(-131\)		\(-124\)	\(-149\)	GaInP/GaAs HBT [19]
\(5.38\)	\(120\)	\(-4.0\)	\(12.8\)	\(1\)	\(-127\)		\(-108\)	\(-148\)	GaInP/GaAs HBT [20]
\(5.29\)	\(270\)	\(-5.5\)	\(14\)	\(1\)	\(-106\)		\(-94\)	\(-130\)	SiGe HBT [21]
\(2.17\)	\(385\)	\(11.2\)	\(1.9\)	\(0.6\)	\(-120\)	\(-125\)	\(-122\)	\(-150\)	CMOS IC [22]
\(1.72\)	\(262\)	\(-11.5\)	\(75\)	\(1\)	\(-129\)		\(-111\)	\(-153\)	InGaP/GaAs HBT [23]
\(4.80\)	\(1200\)	\(4.8\)	\(36\)	\(1\)	\(-111\)		\(-95\)	\(-141\)	SiGe BiCMOS IC [24]
\(9.35\)	\(2500\)	\(18.3\)	\(570\)	\(1\)	\(-110\)		\(-82\)	\(-144\)	GaN/SiC pHEMT [25]
\(1.72\)	\(261\)	\(-10.3\)	\(55\)	\(1\)	\(-120\)		\(-103\)	\(-144\)	InGaP/GaAs HBT [26]
\(4.17\)	\(70\)	\(-6.1\)	\(102\)	\(1\)	\(-116\)		\(-96\)	\(-134\)	GaInP/GaAs HBT [27]
\(2.09\)	\(360\)		\(20.8\)	\(3\)	\(-140\)	\(-130\)	\(-117\)		CMOS VCO [28]
\(1.53\)	\(330\)		\(21.2\)	\(0.6\)	\(-133.5\)	\(-138\)	\(-125\)		CMOS VCO [29]
\(4.89\)	\(650\)		\(22\)	\(1\)	\(-124\)		\(-111\)		CMOS VCO [30]
\(1.85\)	\(280\)		\(20\)	\(3\)	\(-143\)	\(-133\)	\(-120\)		CMOS VCO [30]

Table \(\PageIndex{1}\): Comparison of RF VCOs. Phase noise is worst case over tuning range; RF output power is the minimum. All oscillators are hybrids unless indicated by IC, denoting an integrated circuit. If \(f_{m}\) is not \(1\text{ MHz}\), then a \(1/f^{2}\) dependence is assumed for the phase noise to calculate the phase noise at \(1\text{ MHz}\). The CMOS VCOs are quadrature VCOs producing two outputs \(90^{\circ}\) apart. After [13] with corrected \(\text{FOM}_{1}\). (\(P_{\text{ref}} = 1\text{ mW},\: f_{\text{ref}} = 1\text{ MHz}\).)

which is also known as the tuning gain, \(K_{0}\). This is the change in oscillation frequency for a change in control voltage. For the VCO in Figure 6.4.2,

\[\label{eq:1}K_{0}=\frac{\Delta f_{0}}{\Delta v_{i}} \]

The performance of a microwave VCO is one of the most competitive aspects of RF design, as every decibel reduction in phase noise greatly increases overall system performance. A high-performance VCO also relaxes demands on other system components. While FOM\(_{1}\) (see Equation (6.4.2)) serves as a useful metric to compare VCOs, another FOM with bandwidth weighting provides a better comparison of the performance of different VCOs. This second figure of merit is [13]

\[\label{eq:2}\text{FOM}_{2}=\mathcal{L}(f_{m})-10\log\left(\frac{1\text{ MHz}}{f_{m}}\right)^{2}-10\log\left(\frac{f_{\text{BW}}}{f_{\text{ref}}}\right) \]

where \(f_{\text{BW}}\) is the tuning bandwidth and \(f_{\text{ref}}\) is the reference bandwidth, taken here as \(1\text{ MHz}\). Again the phase noise is referenced to \(1\text{ MHz}\). A number of high-performance microwave oscillators are compared in Table \(\PageIndex{1}\). The best phase noise that can typically be achieved by VCOs operating in the \(1– 10\text{ GHz}\) range is \(−130\text{ dBc/Hz}\) at \(1\text{ MHz}\). This compares to the phase noise component of white noise at standard temperature, which was shown in Section 4.2.2 to be \(−177\text{ dBc/Hz}\).