A 2.4GHz SiGe Low Phase-Noise VCO Using On Chip Tapped inductor Ping Wing Lai*, Laszlo Dobosº, Stephen Long* *Dept. of Electrical and Computer Engineering, University of California Santa Barbara, CA93106 ºMaxim Integrated Products 7250 NW Evergreen Parkway Hillsboro, OR 97124 [email protected] Abstract: An on-chip tapped inductor technique is shown to be an efficient method for reducing oscillator phase noise. Higher signal amplitude can be achieved while avoiding breakdown and without penalty in area or tuning range. A commercial SiGe BJT process was used to fabricate the 2.4GHz VCO. The measured phase noise at 1MHz offset frequency is -128 dBc/Hz with 23% tuning range. The VCO dissipates 16.5mA at a 2.5V supply voltage. 1. Introduction The requirement of phase noise in integrated oscillators has been driven by wireless applications. In addition, cost concerns require that the chip area should be as small as possible. In order to achieve the phase noise requirement and smaller chip area, the signal amplitude has to be maximized. In a recent paper, it [1] states that Lesson’s hypothesized equation 2224)(=mormspmQVFkTRLωωω (1) holds and the oscillator’s noise factor F of a current-biased differential LC oscillator as shown in Fig. 1 is pbiasrmspbiasRgmVRIF94241γπγ++= (2) where Vrms is the rms oscillation amplitude, Ibias is the bias current, γ is the FET noise factor (2/3 for long channels), Rp is the equivalent parallel resistance of the resonator and gmbias is the gm of the FET current source. The first term in (2) is the noise contributed by Rp. The second term is the phase noise induced by differential pair thermal noise and is independent of the specifics of the transistors. The last term is the noise contributed by the current source. Equation (1) and (2) show that the relative contribution of the resonator loss is fixed. In the current-limited regime, where the FET current source remains in saturation, the oscillation amplitude Vrms is proportional to IbiasRp, so the differential pair contributes noise proportional to γ. The last term in equation (2) is proportional to gm of the current source. The same equation can be applied to a BJT LC oscillator with a different γ. Figure 1. Current-biased differential LC oscillator Suppose that the current source contribution is removed from equation (2), then the phase noise will be proportional to 221)(QRILpbiasm∝ω (3) and the oscillator can be designed for least phase noise by increasing Ibias, Rp and Q. However, the signal amplitude IbiasRp is constrained by breakdown mechanisms in the devices and the supply voltage. Equation (3) shows that phase noise can still be reduced if Ibias is increased while IbiasRp is kept constant. For example, if Rp is reduced by half, the maximum Ibias can be increased 2 times, so L(wm) will be reduced by 3dB according to equation (3) if Q does not change much. In 2.4GHz frequency range, the major contribution of the resonator loss is from the inductor. A smaller Rp means a smaller inductor. As far as area is concerned, a smaller inductor is preferable. But Rp cannot be too small, since Q will drop. An optimisation procedure is needed to find the optimum inductor for lowest phase noise under the requirement of chip area, voltage supply and tuning range.To varactor If a better phase noise is needed, several identical oscillators can be coupled to each other, the phase noise will be reduced by a factor of 1/(number of oscillators coupled)[3]. But the chip area will be increased. In this paper, a tapped inductor approach is presented which can further reduce the phase noise under the same chip area, breakdown and voltage supply constraint as the normal LC oscillator shown in Fig. 1. 2. Circuit Design In order to reduce the phase noise of the oscillator, a large output swing of the resonant tank is needed, but the large voltage swing will easily move the transistor into its breakdown region. Therefore, the Vce breakdown voltage will also limit the reduction of phase noise. In order to preserve the resonator voltage swing, an inductor tapping technique is used. (a) (b) To differential pair To differential pairVCC VCC L/2L/2C L/2 L/2 C L L Figure 2(a) LC resonator (b) a tapped inductor capacitor resonator Fig. 2 shows the LC resonator transformed into a tapped resonator. The same inductor and capacitor are used. Since the dominant loss of the resonator is the inductor, the Q of the resonator will not be changed but the Rp, which is the equivalent input impedance looking from the differential pair, is reduced by 1/(tapping ratio)2, which is ¼ in Fig. 2. Then four times more current can be drawn into the oscillator to maintain the same output amplitude at the differential pair. The amplitude of the resonator will be double which allow more energy to be stored in the tank, so it reduces phase noise. Moreover from equation (2), the 4 times larger gm noise contribution from the tail current source, if BJT is used, will be scaled 4 times smaller by the smaller (Rp/4). Therefore, the total noise contribution from the tail current source remains the same. The phase noise will be reduced by 6dB using 1 to 1 tapping ratio as estimated from equation (2) and (3). Besides, the inductor tapping can also increase the output impedance of the transistor, so the loading due to the transistor is reduced. Also the tuning range of the tapped inductor VCO is the same as a normal LC VCO, since four times larger current is drawn, four times larger parasitic capacitance is introduced by the four times larger differential pair, but only ¼ of the parasitic capacitor will load the resonator due to the inductor transformer. A comparison of coupled oscillator and tapped oscillator to a basic LC oscillator is given in Table 1. LC Couple Tapping Voltage swing at the differential pair 1x 1x 1x Tuning range 1x 1x 1x Supply voltage 1x 1x 1x Current bias 1x 4x 4x Chip Area 1x 4x 1x Phase noise 0dB -6dB -6dB Table 1. Comparison of coupling oscillator and tapping oscillator reference to LC oscillator From equation (2), the tail current source may have a large impact on the generation of phase noise, often being the largest contributor[4]. The effect of the tail current noise can be reduced only by reducing its gm. For mos, this implies an increase of transistor over-drive, and consequently an increase