Crystal Based Oscillator Design with the LMK04000 Family

This application note for the LMK04000 family of clock conditioners covers the design of an external oscillator circuit using a crystal resonator. The LMK04000 family supports either an external, self-contained VCXO module or a discretely implemented voltage controlled crystal oscillator based upon a crystal resonator. The LMK04000 family features an internal loop amplifier that enables the implementation of the crystal oscillator. In many applications, the crystal resonator option will offer a cost effective method for cleaning the jitter of the incoming reference clock. In this application note, we will first review basic oscillator circuit fundamentals, and along the way highlight certain specifications for crystals which are critical to the design of the oscillator circuit. It also examines some important aspects of circuit operation. A reference design is presented along with examples of measured performance. A suggested design procedure is presented along with specific LMK040xx programming required for crystal os-cillator implementations. 1.1 LMK04000 CLOCK CONDITIONER FAMILY The LMK04000 family of precision clock cleaners employs a cascaded phase locked loop (PLL) architecture to achieve its highly effective jitter cleaning and frequency synthesis. Figure 1 illustrates the dual PLL architecture of the LMK04000.