A Dual-Band LC-VCO for 2.5 GHz/3.5 GHz WiMAX

As wireless applications proliferate, demands for low-cost wireless communication which can support multiple bands and multiple standards with minimal hardware implementations are rapidly increasing [37]. In response to this, multiband terminals using multiple RF transceivers have been reported. This, however, increase die area or chip count in a radio, which, in turn, increases cost and complexity of radios. Another of the major issues in a dual-band transceiver is the implementation of a dual-band LC-VCO. The most popular implementation method of a dual-band LC-VCO is to use switching devices in the tank to change either capacitance [38] or inductance [39]. The resistance of the switching devices, however, is likely to cause the degradation of the tank quality factor (Q) and, consequently, the

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

oscillator’s phase-noise. A wide tuning range LC-VCO can be another choice to cover the dual bands [40], but the required varactors with wide tuning range are not usually available in a standard process and the relatively large LC-VCO gain (K_VCO) can easily lead to severe phase-noise degradation [41]. A set of multiple LC-VCOs can support multiple bands [42], however, this could be unaffordable in portable devices due to the overwhelming circuit overheads. In spite of these endeavors, the design of integrated dual band LC-VCOs still poses many challenges.

In general, the design of dual band LC-tank voltage controlled oscillator (VCO) still uses the complementary cross-coupled pair topology. Figure 4.15 shows the Simplified equivalent circuit of dual band complementary cross-coupled pair LC-VCO. This topology usually uses the switched resonator concept to realize the dual-band LC-VCO. In addition, the switched resonator concept also can reduce the degradation of phase noise.

Figure 4.15 Simplified equivalent circuit of dual band LC-VCO.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

Figure 4.16 shows the conventional complementary cross-coupled pair dual band LC-VCO [43-44]. The topology consists of two identical half circuits composed of switching transistors (M₁、M₂、M₃、M₄), band switching transistors (M₅、M₆), inductors (L1、L2、L3、L4), and varactors (C1、C2). A signal feeds back from the drain of M₁、M₃ (M₂、M₄) to the gate of M₂、M₄ (M₁、M₃) which acts as an active buffer, and vice versa.

In the design of radio frequency (RF) circuits, the demands for low-cost and low-power-consumption are important issues. Although this dual band LC-VCO has excellent phase noise performance, its power consumption is higher than NMOS or PMOS cross-coupled LC-VCO and it also uses over three inductors to achieve dual operation-frequency. This topology leads to the cost go up since the area of RF circuit is increased since the area of an inductor is large in the IC layout.

Figure 4.16 The conventional complementary cross-coupled pair dual band LC-VCO.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

At the last section, we choose the current-reused LC-VCO to implement the LC-VCO since this topology can achieve low-power and low-cost. If the dual-band LC-VCO can use current-reused topology, it also can achieve low-cost and low-power-consumption easily. Therefore, the Figure 4.16 can be simplified by Figure 4.17 which is called current-reused dual-band LC-VCO. The current-reused dual-band LC-VCO that uses both NMOS and PMOS transistor in cross-coupled pair as a negative conductance generator can achieve low-power-consumption and low-cost easily. Its theorem of circuit is as similar as last section, but the current-reused dual-band LC-VCO has the same issue that is the phase-noise performance must be worse than conventional topology.

Figure 4.17 The current-reused dual-band LC-VCO.

Therefore, we use the same method that is adding an external and large resistor, which is located between the substrate node and the source nod of NMOS transistor to reducing the phase noise of the current-reused dual-band LC-VCO. The proposed dual-band LC-VCO is shown in Figure 4.18. We choose the 2.5 GHz and 3.5 GHz

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

operating frequency to design the dual-band LC-VCO in order to conform to Taiwan giving fresh impetus to WiMAX (Worldwide Interoperability for Microwave Access).

In addition, we add the C₃ and C₄ which shunt between the drain node and the source node of NMOS and PMOS transistors to provide negative conductance [45]. The value of negative conductance is

2

(

)

N

Cgs C Cgd

G gm

ω +

=

Figure 4.18 The proposed dual-band LC-VCO.

The negative conductance can reduce the phase noise of LC-VCO since it not only reduces the loss but also increases the quality factor, Q, in the inductance. The phase noise is proportional to Q. The equivalent circuit of LC-Tank is shown in Figure 4.19 after shunting the C3 and C4.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

Figure 4.19 The equivalent circuit of LC-Tank after shunting the C3 and C4.

When the switch-transistors of conventional, current-reused, and proposed LC-VCOs are off, the LC-VCOs both operate at lower band frequency. The lower band frequency is 2.5GHz and tuning range is 2.5 GHz to 2.69 GHz as shown in Figure 4.20. We also can know that the tuning range of proposed dual-band LC-VCO is the maximum and it operates at 2.49 GHz to 2.72 GHz from Figure 4.20. In addition, Figure 4.21 shows the simulated tuning sensitivity (K_VCO) for the conventional, current-reused, and current-reused LC-VCOs. From Figure 4.21, we can see that the K_VCO of current-reused and proposed dual-band LC-VCOs are almost the same.

Figure 4.20 Simulated tuning range of the conventional and current-reused LC-VCO at 2.5 GHz.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

Figure 4.21 Simulated KVCO of the conventional and current-reused LC-VCO at 2.5 GHz.

In the Figure 4.22, the simulated result shows phase noise for the conventional, current-reused, and the proposed LC-VCOs which operate at 2.5 GHz to 2.69 GHz.

The simulated values for the conventional, current-reused, and proposed LC-VCOs are -118 dBc/Hz, -117 dBc/HZ, and -120 dBc/Hz at 1MHz offset frequency. We also can know that the phase noise of proposed dual-band LC-VCO reduction is about -3 dB compared to the current-reused dual-band LC-VCO. In addition, the power consumption of the conventional topology is 4.342mW, but current-reused and proposed topologies are only 1.456mW, respectively. Figure 4.23 shows the output power of conventional and current-reused LC-VCOs. It is found that the minimum values of output power are -0.52 dBm, -1.39 dBm, and -1.44 dBm in the conventional, current-reused, and the proposed topologies.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

Figure 4.22 Simulated phase noise of the conventional and current-reused LC-VCO at 2.5 GHz.

Figure 4.23 Simulated output power of the conventional and current-reused LC-VCO at 2.5 GHz.

In the other hand, when the switch-transistors of conventional, current-reused, and proposed LC-VCOs are on, the LC-VCOs both operate at higher frequency band.

The higher frequency band is 3.7 GHz and tuning range is 3.4 GHz to 3.7 GHz as shown in Figure 4.24. We also can know that the tuning range of the conventional

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

LC-VCO is 3.4 GHz to 3.7 GHz and the proposed and current-reused topologies are wider. The two topologies operate at 3.38 GHz to 3.71 GHz. In addition, Figure 4.25 shows the simulated tuning sensitivity (K_VCO) for the conventional, current-reused, and proposed LC-VCOs. From Figure 4.25, we can see that the KVCO of current-reused and proposed dual-band LC-VCOs are almost the same. The maximum value of KVCO is about 375 MHz/V in the current-reused and the proposed LC-VCOs.

Figure 4.24 Simulated tuning range of the conventional and current-reused LC-VCO at 3.5 GHz.

Figure 4.25 Simulated KVCO of the conventional and current-reused LC-VCO at 3.5 GHz.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

In the Figure 4.26, the simulated result shows phase noise for the conventional, current-reused, and the proposed LC-VCOs which operate at 3.4 GHz to 3.7 GHz.

The simulated values for the conventional, current-reused, and proposed LC-VCOs about are -115 dBc/Hz, -114 dBc/HZ, and -117 dBc/Hz at 1MHz offset frequency. We also can know that the phase noise of proposed dual-band LC-VCO reduction is about -3 dB compared to the current-reused dual-band LC-VCO. In addition, the power consumption of the conventional topology is 4.342mW, but current-reused and proposed topologies are only 1.456mW. We can know the result the same as the single-band which reduce the phase noise of current-reused VCO effectively without increase the power consumption. Figure 4.27 shows the output power of conventional and current-reused LC-VCOs. It is found that the minimum values of output power are -1.03 dBm, -2.06 dBm, and -2.07 dBm in the conventional, current-reused, and the proposed topologies.

Figure 4.26 Simulated phase noise of the conventional and current-reused LC-VCO at 3.5 GHz.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

Figure 4.27 Simulated output power noise of the conventional and current-reused LC-VCO at 3.5 GHz.

Therefore, we know that the output power performances of the conventional LC-VCO are better than current-reused and proposed topology, but the power consumption of proposed and current-reused topology is lower than the conventional topology. In addition, the phase noise of proposed LC-VCO is lower than the conventional LC-VCO. The current reused topology combines with an external and large resistor has lower phase noise power consumption, although its output power is lower than the conventional topology. Finally, we choose the proposed topology to implement a low-power low-phase-noise dual-band LC-VCO.

The power supply is added externally in the design the LC-VCO, so we have to bond wire to the print circuit board (PCB). Therefore, we must consider the pad-effect and bond-wire-effect which are provided by national chip implementation center (CIC) as shown in Figure 4.28and Figure 4.29, respectively, to avoid the frequency shifting and the higher phase noise issues. In addition, we also consider the layout effect to take the long layout line as shown in Figure 4.30. Running EM

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

simulation by advanced design system (ADS) Momentum and obtain the layout effect model.

Figure 4.28 The equivalent model of pad-effect.

Figure 4.29 The equivalent model of bond-wire-effect.

Figure 4.30 The equivalent circuit of parasitic effect of a wire.

When the LC-VCO is measured, the output of the circuit is terminated with 50 Ω, we also must design a buffer to connect with the output node of LC-VCO. The complete proposed dual-band LC-VCO circuit is shown as Figure 4.31.

Chapter 4 Design of a Dual Band VCO for 2.5 GHz and 3.5 GHz WiMAX

Figure 4.31 The complete circuits of proposed LC-VCO.