ABSTRACT
In modern wireless communication systems, frequency selectivity plays a very important role to extract the desired radio signals out of the increasingly crowded spectrum. A typical transmitter and a receiver along with an interfering transmitter are shown in Figure 20.1. With the tightly spaced communication channels, tunable
20.1 Introduction .................................................................................................. 515 20.2 DCO versus VCO.......................................................................................... 517 20.3 DCO System Design Considerations ............................................................ 518
20.3.1 DCO System Topology ..................................................................... 519 20.3.2 Oscillator Core Frequency ................................................................ 519 20.3.3 DCO Varactor Banks ........................................................................ 522 20.3.4 DCO Core ......................................................................................... 525
20.3.4.1 Design Considerations for Capacitor Arrays ..................... 526 20.3.4.2 MoM Varactors .................................................................. 527 20.3.4.3 Accumulation-Mode MOS Varactor .................................. 530 20.3.4.4 Inversion-Mode MOS Varactor .......................................... 531 20.3.4.5 Switchable Inductor ........................................................... 533 20.3.4.6 An Example DCO .............................................................. 535
20.3.5 Reliability ......................................................................................... 537 20.3.5.1 Gate Oxide Integrity .......................................................... 537 20.3.5.2 Channel Hot Carrier .......................................................... 538 20.3.5.3 Metal EM ........................................................................... 538
20.4 Integration with an All-Digital Phase-Locked Loop .................................... 539 20.4.1 Phase Noise due to ΣΔ Dithering ..................................................... 539 20.4.2 Phase Noise due to DCO Tuning Word Toggling .............................540 20.4.3 INL of KDCO ......................................................................................540
20.5 Summary ...................................................................................................... 542 References .............................................................................................................. 542
radiofrequency (RF) bandpass filters with sufficient out-of-channel and out-of-band attenuation are difficult to achieve. Therefore, a precise frequency translation of radio signals is needed to shift the desired RF signals down to a baseband frequency or an intermediate frequency (IF) to ease the suppression of interference.1
