US 5,511,126 AGranted

Method and apparatus for reducing jitter and improving testability of an oscillator

AssigneeMotorola, Inc.

InventorAlan L. Westwick

Life of this patent

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Abstract

An oscillator such as a pulled-crystal oscillator (50) provides low clock jitter by converting a sinusoidal voltage on a crystal's (51) terminals into a digital square wave with a comparator (56). The oscillating frequency of the crystal (51) is pulled by selectively switching in extra capacitance through capacitor digital-to-analog converters (CDACs) (57, 58). The oscillator (50) has built-in testability which allows individual capacitors in the CDACs (57, 58) to be quickly tested for opens. A scan path is connected to the inputs of the CDACs (57, 58) for selecting individual capacitors. A first input terminal of the comparator (56) is precharged before a capacitor under test (171) is connected. A comparison voltage is provided to the second input terminal. The capacitor under test (171) is determined to be functional if, after being connected to the first input terminal of the comparator (56), it discharges the first input terminal to a voltage below the comparison voltage, causing the comparator (56) to switch.

Claims23

Dependent claims

A method for reducing jitter and improving testability of an oscillator, comprising the steps of:

coupling a selected one of a first plurality of capacitors between a
first crystal terminal and a predetermined voltage terminal in response to a first digital input code;
coupling a selected one of a second plurality of capacitors between a second crystal terminal and said predetermined voltage terminal in response to a second digital input code;
coupling said first crystal terminal to a first input terminal of a comparator;
coupling said second crystal terminal to a second input terminal of said comparator;
providing a clock output signal at an output terminal of said comparator;
selecting one of said first plurality of capacitors as a capacitor under test;
coupling a first test capacitor between a first node and said predetermined voltage terminal;
precharging said first node and a second node to a precharge voltage at a beginning of a test period;
coupling said capacitor under test between said second node and said predetermined voltage terminal;
coupling a second test capacitor between said first node and said predetermined voltage terminal;
and
coupling said first and second nodes to said first and second input terminals of said comparator, respectively;
whereby said capacitor under test is determined to be functional if said output terminal of said comparator is in a first logic state at an end of said test period, and said capacitor under test is determined to be open if said output terminal of said comparator is in a second logic state at said end of said test period.

11 more elements

A method for testing a pulled-crystal oscillator, the pulled-crystal oscillator having a capacitor digital-to-analog converter (CDAC) coupled to a first crystal terminal, and a capacitor coupled to a second crystal terminal, comprising the steps of:

coupling a first terminal of a first test capacitor to a first terminal of a comparator;
providing a first predetermined voltage to a second terminal of said first test capacitor;
precharging a voltage at said first terminal of said first test capacitor to a second predetermined voltage during a first portion of a test period;
providing a comparison voltage to a second terminal of said comparator, said comparison voltage characterized as being between said first and second predetermined voltages;
coupling a first terminal of a selected capacitor of a plurality of capacitors of the CDAC to said first terminal of said comparator during a second portion of said test period;
coupling a second terminal of said selected capacitor to said first predetermined voltage during said second portion of said test period;
and
providing an output of said comparator as a test result of said selected capacitor at an end of said second portion of said test period;
whereby said selected capacitor provides a voltage to said first terminal of said comparator between said comparison voltage and said first predetermined voltage if said selected capacitor is functional, and said selected capacitor provides said voltage to said first terminal of said comparator between said second predetermined voltage and said comparison voltage if said selected capacitor is not functional.

6 more elements

An oscillator with improved testability, comprising:

a first node for being coupled to a first terminal of a crystal;
a second node for being coupled to a second terminal of said crystal;
a first capacitor digital-to-analog converter (CDAC) coupled to said first node, said first CDAC switched by a first digital input code;
a second CDAC switched by a second digital input code coupled to said second node;
a comparator having a first input terminal coupled to said first node, a second input terminal coupled to said second node, and an output terminal for providing a clock output signal;
test means coupled to a third node, for providing a first comparison voltage to said third node when said second CDAC is selected during a test mode;
first multiplexer means for coupling said first node to said first input terminal of said comparator during a normal operation mode, and for coupling said third node to said first input terminal of said comparator during said test mode;
and
second multiplexer means for coupling said second node to said second input terminal of said comparator during said normal operation mode, and for coupling a fourth node to said second input terminal of said comparator during said test mode;
said second CDAC further coupling a selected one of a first plurality of capacitors between said fourth node and a predetermined voltage terminal when selected during said test mode.

7 more elements

Dependencies are inferred from the claim text.

Facts

Application: 08/376,997Filed 23 Feb 1995 · division
Priority: 25 Aug 1993
Granted: 23 Apr 1996
Term ends: Not estimatedThe source records don’t support an estimate.

Classification7 codes

H03K 3/0231
By the use of differential amplifiers or comparators, with internal or external positive feedback: astable circuits
G01R 31/2824
Of microwave or radiofrequency circuits: testing of oscillators or resonators
H03K 3/0307
Astable circuits: stabilisation of output, e.g. using crystal
H03J 2200/10
Indexing scheme relating to tuning resonant circuits and selecting resonant circuits: tuning of a resonator by means of digitally controlled…