US 7,002,423 B1Granted

Crystal clock generator operating at third overtone of crystal's fundamental frequency

AssigneePericom Semiconductor Corp.

InventorsBoris Drakhlis, Wing Faat Liu, Craig M. Taylor, Tony Yeung

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Abstract

A crystal oscillator operates at the third overtone of the crystal's fundamental frequency. A value of a shunt resistor between the two phase-shift leg nodes is chosen so that the absolute value of the product gm×(Xc1)×(Xc2) is greater than the effective reactance of the crystal, where gm is the gain of the amplifier attached to the phase-shift legs, and Xc1 and Xc2 are the effective capacitive reactances of phase-shift legs at nodes X1 and X2. The third overtone is doubled by a multiplier and the final output filtered to remove the third overtone and select a frequency six times the fundamental frequency. A pair of Colpitts or Pierce amplifier half circuits is attached to the phase-shift leg nodes. The leg nodes can be capacitively isolated from Pierce-amplifier circuit nodes to improve start-up. Frequency doubling can be performed by summing currents from the two oscillator half circuits.

Claims20

Dependent claims

An oscillator comprising:

an oscillating crystal having a fundamental frequency of oscillation but forced to oscillate at a third overtone of the fundamental frequency, the third overtone being a frequency of about three times the fundamental frequency;
the oscillating crystal coupled between a first input node and a second input node;
a first amplifier circuit having an input attached to the first input node and generating a first output, the first amplifier also having a first phase-shift node;
a second amplifier circuit having an input attached to the second input node and generating a second output, the second amplifier also having a second phase-shift node;
a tuning resistor coupled between the first input node of the first amplifier and the second input node of the second amplifier, the tuning resistor having a value such that an absolute value of a product (gm×Xc1×Xc2) is greater than an effective impedance of the oscillating crystal, wherein gm is a gain of the first amplifier circuit and gm is also the gain of the second amplifier circuit, and Xc1 is an effective reactance of an equivalent capacitance on the first input node of the first amplifier and Xc1 is also an effective reactance of an equivalent capacitance on the second input node of the second amplifier, and Xc2 is an effective reactance of an equivalent capacitance on the first phase-shift node of the first amplifier and Xc2 is also the effective reactance of the equivalent capacitance on the second phase-shift node of the second amplifier;
a multiplier, coupled to the first amplifier circuit and to the second amplifier circuit, for generating a doubled output from the first output and from the second output, the doubled output having an output frequency that is double the third overtone and about six times the fundamental frequency of the oscillating crystal;
and
an output filter, attached to the doubled output, for attenuating signals having a frequency of the third overtone, but for passing signals having the output frequency,
whereby the oscillating crystal is forced to oscillate at the third overtone, while the doubled output is six times the fundamental frequency of the oscillating crystal.

6 more elements

A clock generator comprising:

oscillating crystal means, having a fundamental frequency of oscillation and a third overtone, for oscillating at a third overtone of the fundamental frequency in response to a first crystal node and a second crystal node;
first amplifier means, having the first crystal node as an input, for generating a first output, the first amplifier means having a first node;
second amplifier means, having the second crystal node as an input, for generating a second output, the second amplifier means having a second node;
tuning resistor means, coupled between the first crystal node and the second crystal node, for tuning the clock generator and the oscillating crystal means to oscillate at the third overtone, the tuning resistor means having a value such that an absolute value of a product (gm×Xc1×Xc2) is greater than an effective impedance of the oscillating crystal means, wherein gm is a gain of the first amplifier means and the gain of the second amplifier means, and Xc1 is an effective reactance of an equivalent capacitance on the first crystal node or on the second crystal node, and Xc2 is an effective reactance of an equivalent capacitance on the first node or on the second node;
multiplier means, coupled to the first amplifier means and to the second amplifier means, for generating a doubled output from the first output and from the second output, the doubled output having an output frequency that is double the third overtone and about six times the fundamental frequency of the oscillating crystal means;
and
output filter means, attached to the doubled output, for attenuating signals having a frequency of the third overtone, but for passing signals having the output frequency,
whereby the oscillating crystal means is forced to oscillate at the third overtone, while the doubled output is six times the fundamental frequency of the oscillating crystal means.

5 more elements

Dependencies are inferred from the claim text.