Publishing Venue
Motorola
Related People
Authors:
N. Todd Rollins
•
King F. Lee
Abstract
MOS IC's frequently contain a crystal oscillator to provide clocking. Typically, a linear amplifier (comprised of a digital inverter biased by the feedback resistance of an "on" transmission-gate connected between the inverter input and output) and clock buffers are integrated on the IC, while the crystal and loading capacitors are implemented discretely external to the IC. In addition, crystal vendors often recommend placing a damping resistor in series with the amplifier output to (1) decrease the current injected into the crystal to prevent crystal damage, (2) stabilize the resonant circuit by reducing coupling from the amplifier and (3) reduce the possibility of spurious oscillations at crystal overtones by decreasing the high frequency loop gain. Despite these advantages, the damping resistor is usually excluded to avoid the added cost and area of a descrete resistor.
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MOTOROLA Technical Developments Volume 11 October 1990
AN INTEGRATED TRANSMISSION-GATE DAMPING RESISTOR FOR MOS CRYSTAL OSCILLATORS
by N. Todd Rollins and King F. Lee
MOS It's frequently contain a crystal oscillator to provide clocking. Typically, a linear amplifier (comprised of a digital inverter biased by the feedback resistance of an "on" transmission-gate connected between the inverter input and output) and clock buffers are integrated on the IC, while the crystal and loading capacitors are implemented discretely external to the IC. In addition, crystal vendors often recommend placing a damping resistor in series with the amplifier output to (1) decrease the current injected into the crystal to prevent crystal damage, (2) stabilize the resonant circuit by reducing coupling from the amplifier and (3) reduce the possibility of spurious oscillations at crystal overtones by decreasing the high frequency loop gain. Despite these advantages, the damping resistor is usually excluded to avoid the added cost and area of a descrete resistor.
Another problem arises when IC's requiring an integrated oscillator for some applications must also be used in systems which provide an external clock to drive the amplifier gate. Current drain may be reduced by "turning-off" the feedback transmission-gate thereby removing bias from the inverter, but the inverter is still loaded by the output pin. Consequently, (1) current consumption is still large, (2) the output pin radiate...
