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Low Output Jitter Monolithic Digitally Controlled Oscillator

IP.com Disclosure Number: IPCOM000121096D
Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 96K

Publishing Venue

IBM

Related People

Koskowich, GN: AUTHOR

Abstract

Digitally Controlled Oscillators (DCOs), advantageously used in Digital Phase-Locked Loops, are designed to convert an input digital code to a clock voltage waveform with a particular period. An important characteristic of a DCO is its output "jitter," also known as cycle-to- cycle variations of the output waveform period due to device noise and power supply variations.

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Low Output Jitter Monolithic Digitally Controlled Oscillator

      Digitally Controlled Oscillators (DCOs), advantageously
used in Digital Phase-Locked Loops, are designed to convert an input
digital code to a clock voltage waveform with a particular period.
An important characteristic of a DCO is its output "jitter," also
known as cycle-to- cycle variations of the output waveform period due
to device noise and power supply variations.

      Monolithic implementations of relaxation oscillators routinely
use capacitors with a constant value as the timing element and are
currently realized by means of discrete devices.

      The period of oscillation T is determined by the time it takes
for the capacitor voltage to decrease from Vref to Vth.  The rate of
change (or slope) of the capacitor voltage is given by:
dV/dt = I/C,
which is constant for a particular period.  Since the difference
between the Vref and Vth is usually determined by technology
considerations and T by system requirements, the slope S cannot be
adjusted arbitrarily.

      The object of this circuit is to increase the slope of the
voltage waveform at threshold by using a non-linear capacitor,
thereby decreasing the output jitter (see Fig. 1).  Shown is the
voltage waveform resulting from two distinct slopes S1 and S2, that
correspond to the high and low capacitor values, respectively.

      The circuit was implemented in GaAs Metal Semiconductor Field-
Effect Transistor (MESFET) tech...