Browse Prior Art Database

IMPROVED FREQUENCY SYNTHESIZER

IP.com Disclosure Number: IPCOM000005589D
Original Publication Date: 1986-Oct-01
Included in the Prior Art Database: 2001-Oct-17
Document File: 3 page(s) / 87K

Publishing Venue

Motorola

Related People

William J. Ooms: AUTHOR

Abstract

A phase locked loop (PLL) frequency synthesizer is normally constructed using a stable crystal oscillator divided to a low reference fequency, and then using a phase locked loop with a programmable divider in the feedback loop to effectively multiply the reference frequency to some output frequency. One constraint of this method is that the output frequency is an integral multiple of the reference frequency.

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m MOmROLA Technical Developments October 1986

IMPROVED FREQUENCY SYNTHESIZER

by William J. Ooms

   A phase locked loop (PLL) frequency synthesizer is normally constructed using a stable crystal oscillator divided to a low reference fequency, and then using a phase locked loop with a programmable divider in the feedback loop to effectively multiply the reference frequency to some output frequency. One constraint of this method is that the output frequency is an integral multiple of the reference frequency.

   A new approach to the PLL frequency synthesizer is shown in Figure 1. The reference divider(R) is pro- grammed as well as the feedback divider(N). By programming both dividers, it is possible to come very close to most frequencies. By warping the crystal oscillator, an arbitrary output frequency can be realized. Most fre- quencies require very little warping of the crystal, but a few output frequencies will require a rather large warp as shown in Figure 2 which shows the required warp of a single crystal oscillator as a function of the syn- thesizer's output frequency. Observe that the spacing of those frequencies which require a large warp is a func- tion of the crystal oscillator frequency.

   The amount of warp required may be reduced by using two different crystal oscillators which have been chosen such that those output frequencies requiring a large warp of one of the crystals will require very little warp of the other crystal. Thus any arbitrary output frequency...