Crystal clock generator operating at third overtone of crystal's fundamental frequency
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United States of America Patent
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Feb 19, 2008
Grant Date -
Mar 29, 2007
app pub date -
Dec 21, 2005
filing date -
Jul 20, 2004
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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.times.(Xc1).times.(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.
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