All-Digital Frequency Synthesizer in Deep-Submicron CMOS

CHAPTER 3

NORMALIZED DCO

The digitally controlled oscillator (DCO) of Chapter 2 provides only a raw bare minimum of functionality. In this chapter we introduce circuitry built around it for the purpose of adding the next hierarchical layer of arithmetic abstraction, which makes it easier to operate the DCO from the outside.

The oscillator frequency dependence on process spread and environmental factors such as voltage and temperature is tracked by a normalization circuit that belongs to this layer. Consequently, the oscillator described in this chapter is referred to as a normalized DCO (nDCO). The DCO normalization block includes circuitry to control the precise application of the tuning word, to reduce the spurious noise level. This is one of the advantages of operating the oscillator in the discrete-time domain that is not possible in conventional continuous-time designs.

As also mentioned in Chapter 2, the DCO is encapsulated at the top I/O level as a discrete-time system. Consequently, this view could be extended for a normalized DCO. This brings numerous benefits from being able to tap the rich body of knowledge from the digital signal processing (DSP) field.

3.1 OSCILLATOR TRANSFER FUNCTION AND GAIN

At the heart of the frequency synthesizer lies the digitally controlled oscillator. It generates an output with a frequency of oscillation f_V that is a physically inherent function of the digital oscillator tuning word (OTW) input. The f_V = f(OTW) mapping was defined by Eq. ...