Design Of VCO Applied To Direct Conversion Transmitter

The traditional transmitter mainly includes direct up-conversion and super-heterodyne structures.The frequency synthesizer is used as a stable frequency source as the local oscillator.Compared with the traditional structure,the direct conversion transmitter is based on the phase-locked loop frequency synthesizer,which has advantages in power consumption and system complexity.As one of the most difficult modules in the frequency synthesizer,the voltage-controlled oscillator(VCO)has the largest power consumption,it requires the VCO to have low power consumption and low phase noise.Based on the background of direct frequency conversion transmitter,a low-power VCO is designed in this thesis.The traditional class-C oscillator is improved to some extent and a capacitor divider is introduced to keep the cross-coupled pair in saturation region even when the output swing is large.In addition,the tail current transistor in the traditional structure is replaced by a resistor,which avoids the effect of the channel length modulation.It is clamped by an operational amplifier to define the bias current of the oscillator without additional reference voltage.The output of the operational amplifier is fed back to the gate of the cross-coupled pair to form a negative feedback loop.After the oscillator works stably,the bias voltage can be dynamically adjusted.At the same time,a divide-by-two circuit suitable for high frequency is designed in this thesis,which has lower power consumption than traditional structures.In addition,an automatic frequency calibration(AFC)module that can quickly select the appropriate sub-band is also designed.Based on TSMC28 nm CMOS process,the circuit schematic design and pre simulation,layout design and post simulation are completed.The simulation results show that under the condition of a power supply voltage of 0.8V,the oscillator can cover the target frequency range of 2.4?2.485 GHz under all process angles,and the phase noise is lower than-120 d Bc/Hz@1MHz when the power consumption is less than 1m W.The AFC module is verified by behavior level simulation,and the search time can be less than 10s.