Browse Prior Art Database

FULLY DIFFERENTIAL CRYSTAL CLOCK GENERATOR

IP.com Disclosure Number: IPCOM000008540D
Original Publication Date: 1998-Mar-01
Included in the Prior Art Database: 2002-Jun-21
Document File: 2 page(s) / 105K

Publishing Venue

Motorola

Related People

Michael Cave: AUTHOR [+2]

Abstract

Current implementations to squaring up a sine- wave from a crystal oscillator may be viewed in Figure I below (referred to as the prior art). Figure I, Prior Art I, being a single inverter, requires that the output swing of the crystal oscillator be large enough to trigger the inverter. In applications using Prior Art I to square up the crystal signal, the output swing of the crystal is generally rail-to-rail. This large voltage swing requires a large power dissipa- tion by the crystal which reduces the lifetime of the crystal or requires the crystal to cost more due to specifying a higher power dissipation. Therefore, a smaller voltage swing is desirable from the crystal oscillator, which leads to the Prior Art II shown in Figure I.

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Technical Developments

FULLY DIFFERENTIAL CRYSTAL CLOCK GENERATOR

by Michael Cave and Michael May

  Current implementations to squaring up a sine- wave from a crystal oscillator may be viewed in Figure I below (referred to as the prior art). Figure I, Prior Art I, being a single inverter, requires that the output swing of the crystal oscillator be large enough to trigger the inverter. In applications using Prior Art I to square up the crystal signal, the output swing of the crystal is generally rail-to-rail. This large voltage swing requires a large power dissipa- tion by the crystal which reduces the lifetime of the crystal or requires the crystal to cost more due to specifying a higher power dissipation. Therefore, a smaller voltage swing is desirable from the crystal oscillator, which leads to the Prior Art II shown in Figure I.

  Figure I, Prior Art II, shows an inverter biased at its trip point. In this case, due to the ac coupling to the crystal oscillator while being D.C. biased at its trip point, a squared up version of the crystal's sine-wave is observed at the output of the inverter. This implementation does allow for lower power to be dissipated by the crystal itself by reducing the required voltage swing level of the oscillator. However, the inverter is susceptible to false triggering due to the poor power supply rejection of this type of circuit.

  Figure I, Prior Art III, shows a fully differential architecture which is capable of squaring up a lower voltage swing from the oscillator and has good power supply rejection. However, in some circum- stances, there is a D.C. offset due to the crystal oscillator between the positive input and negative input (vinn and vinp) of the fully differential comparator. This D.C. offset may be enough, in conjunction with a small sine-wave output of the crystal oscillator, that the comparator does not trigger. This problem is catastrophic but is solved with the

current invention observed in Figure 2.

  The present invention (Figure 2) provides a method and apparatus for producing an improved...