Proposed Voltage controlled Oscillator Architecture

Figure 4.2 shows two typical LC tank oscillators. Figure 4.2(a) uses all-NMOS cross-coupled pair to provide negative-GM and Figure 4.2(b) employs all-PMOS cross-coupled pair. In both structures, MOS coupled pair is an active element to compensate for the losses of the inductor and the capacitor.

The phase noise of PMOS cross-coupled pair oscillator is lower than NMOS structure since the intrinsic noise of PMOS is lower than NMOS. Nevertheless, the output power of NMOS cross-coupled pair oscillator is larger than PMOS structure.

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

To sum up, we can use the NMOS and PMOS cross-coupled pairs that is called complementary cross-coupled pair) to provide negative GM. There are several reasons why the complementary structure is superior to the all-NMOS structure [35].

Figure 4.2 Two typical LC tank oscillator structures.

1. The complementary structure offers better rise- and fall-time symmetry. It makes less up-conversion of 1/ f noise and other lower frequency noise sources.

2. The complementary structure offers higher transconductance for a given current, which results in a better start-up behavior.

3. The complementary structure also exhibits better noise performance for all bias points illustrated in Figure 4.3

As long as the oscillator operates in the current-limited regime, the tank voltage swing is the same for both oscillators. However if we desire to operate in the voltage-limited region, the all-NMOS structure can offer a larger voltage swing.

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

Figure 4.3 Phase noise for the complementary and All-NMOS.

Figure 4.4 illustrates the schematic of the complementary cross-coupled LC-VCO without the tail current source, which is adopted in this work. From the phase noise point of view, this topology reveals better noise performance than the one in Figure 4.3. This is due to the fact that the 1/f ³ noise of the topology without the tail current can only originate from the flicker noise of the MOS transistor switches.

Figure 4.4 Complementary cross-coupled LC-VCO without the tail current source.

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

These switches are expected to feature lower flicker noise than the tail current source that dominates the 1/f ³ noise, for two main reasons. First, the switches operate in triode region for large portions of the oscillation period; hence, they exhibit lower current flicker noise than the tail transistor that continuously operates in saturation.

Second, switched MOS transistors are known to have lower flicker noise than transistors biased in the stationary condition [36]. Nevertheless, the main drawback of this topology is a higher sensitivity of the frequency to the voltage supply (frequency pushing). This effect can be alleviated by using a supply voltage regulator.

Here, we propose the current-reused LC-VCO that uses both NMOS and PMOS transistor in cross-coupled pair as a negative conductance generator to achieve low power consumption easily. As shown in Figure 4.5, the series stacking of NMOS and PMOS allows the supply current to be reduced by half compared to that of the conventional LC-VCO while providing the same negative conductance. This topology is not only low-power-consumption but also low-cost since it only used one inductor and two MOS transistors, but the conventional LC-VCO used two inductors and four MOS transistors.

Figure 4.5 The current-reused LC-VCO.

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

The conventional and current-reused LC-VCOs operate at 3.4GHz to 3.7GHz as shown in Figure 4.6 and the tuning sensitivity (KVCO) of both topologies are shown in Figure 4.7.

Figure 4.6 Simulated tuning range of the conventional and proposed LC-VCO at 3.5 GHz.

Figure 4.7 Simulated KVCO of the conventional and proposed LC-VCO at 3.5 GHz.

Figure 4.8 shows simulated phase noise for both the conventional and current-reused LC-VCOs which operate at 3.4GHz to 3.7GHz. The simulated values for the conventional LC-VCO and current-reused LC-VCO -119 dBc/Hz and -117 dBc/HZ, respectively, at 1MHz offset frequency. In addition, the power consumption of the

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

conventional topology and the current-reused topology are 1.973mW and 0.996mW, respectively. Figure 4.9 shows the output power of conventional and current-reused LC-VCO. It is found that the minimum values of output power are -1.13 dBm and -2.11 dBm, respectively, in the conventional topology and the current-reused topology.

Therefore, we know that the phase noise and the output power performances of conventional LC-VCO are better than current-reused LC-VCO, but the power consumption of current-reused topology is lower than the conventional topology.

Figure 4.8 Simulated phase noise of the conventional and proposed LC-VCO at 3.5 GHz.

Figure 4.9 Simulated output power of the conventional and proposed LC-VCO at 3.5 GHz.

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

Although the proposed topology can operate with only half amount of DC current compared to that of the conventional topology, the phase noise of current-reused topology is higher than conventional topology. In the modern wireless communication systems, the low power is an important concern, but the phase noise performance must be low enough since the most critical performance specification for an oscillator is phase noise. Hence, the current-reused LC-VCO combines with the Chapter 3 proposed that is adding an external and large resistor, which is located between the substrate node and the source nod of NMOS transistor. As shown in Figure 4.10, the current-reused LC-VCO combined with the external and large resistor Rbx is proposed.

Figure 4.10 Current-reused LC-VCO combined with the external resistor Rbx.

The conventional, current-reused, and proposed LC-VCOs operate at 3.4GHz to 3.7GHz as shown in Figure 4.11. Figure 4.12 shows the simulated tuning sensitivity (KVCO) for conventional, current-reused, and proposed LC-VCOs which operate at 3.5GHz.

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

Figure 4.11 Simulated tuning range of the conventional, current-reused, and proposed LC-VCO at 3.5 GHz.

Figure 4.12 Simulated KVCO of the conventional, current-reused, and proposed LC-VCO at 3.5 GHz.

Figure 4.13 shows simulated phase noise for the conventional current-reused and the proposed LC-VCOs which operate at 3.4GHz to 3.7GHz. The simulated values for the conventional, current-reused, and proposed LC-VCOs are -119 dBc/Hz, -117 dBc/HZ, and -121 dBc/Hz at 1MHz offset frequency. In addition, the power consumption of the conventional topology is 1.973mW, but the current-reused and the proposed topologies are both only 0.996mW.

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

Figure 4.13 Simulated phase noise of the conventional, current-reused, and proposed LC-VCO at 3.5 GHz.

Figure 4.14 Simulated output power of the conventional, current-reused, and proposed LC-VCO at 3.5 GHz.

Figure 4.14 shows the output power of current-reused and proposed LC-VCO. It is found that the minimum values of output power are -1.13 dBm, -2.11 dBm and -2.16 dBm in the conventional, current-reused, and proposed topologies. Therefore, we know that the phase noise and the output power performances of conventional LC-VCO are better than current-reused and proposed LC-VCOs, but the power

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

consumption of current-reused and proposed topologies is lower than the conventional topology.

From the simulated, the current-reused LC-VCO which combines with the Chapter 3 proposed that is adding an external and large resistor, which is located between the substrate node and the source nod of NMOS transistor can achieve low phase noise performance without increasing power consumption. Although the output power of proposed LC-VCO is the least less than the others, its still can be allow applying wireless communication systems. In order to achieve the low-power and low-phase-noise performance, we choose the current-reused LC-VCO combined with the external resistor to implement the dual-band LC-VCO. This proposed topology is not only to achieve low-power and low-phase-noise easily but also realizing to low-cost.