Oscillators – Ring oscillators
Reexamination Certificate
1998-12-09
2001-09-25
Kinkead, Arnold (Department: 2817)
Oscillators
Ring oscillators
C331S176000, C331S111000, C331S17700V, C331S034000, C331S03600C
Reexamination Certificate
active
06294962
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to ocillators generally and, more particularly, to an oscillator that provides a stable output over a range of temperatures without using an external reference such as a crystal.
BACKGROUND OF THE INVENTION
One conventional approach used to implement an oscillator without a crystal is to use a simple resistor/capacitor (RC) network to implement a timer. The original 555 timer chip design used an RC network. However, RC networks are susceptible to process variations and temperature variations. A typical mainline CMOS process does not control resistors or capacitors to tolerances of better than 5%. In some processes, the tolerance is even lower. Laser trimming and other techniques can be used to achieve higher tolerances, but may add to the overall cost of the device.
A second conventional approach used to implement temperature insensitive current sources is described by R. A. Blauschild in his paper entitled AN INTEGRATED TIME REFERENCE, which is hereby incorporated by reference. Such an approach develops a temperature invariant current by using a bias generator that sums currents with different temperature coefficients and combines them with a threshold cancellation circuit. The technique allowed a current that was proportional to oxide thickness. This method was applied to time interval measurement and to filtering, but not to oscillator design.
A third conventional approach used to implement an oscillator is to use a ring oscillator that is stable across process and temperature variations. This is often used in timing recovery PLL circuits. The ring oscillator approach appears to be able to achieve frequency stability on the order of 5%, which is not good enough for a target of 2% or less.
Referring to
FIG. 1
, a portion of a ring oscillator
10
is shown. The ring oscillator
10
comprises a number of devices
12
a
-
12
n
.
FIG. 2
generally illustrates the temperature dependence of the frequency of oscillation of the devices of the ring oscillator
10
. The temperature dependence of the ring oscillator
10
adversely affects the frequency of oscillation.
Referring to
FIG. 3
, a circuit
20
is shown illustrating a biasing circuit for a delay cell that may be used with a conventional ring oscillator. A delay cell
22
generally presents a signal VDD, a signal PBIAS, a signal BIASA, a signal BIASB and a signal VSS to a biasing circuit
24
. The biasing circuit
24
may include a current source
26
that responds to the signal PBIAS. The biasing circuit
24
may provide biasing to a voltage reference circuit
28
that is used as a VCO input. Additionally, a bandgap current bias circuit
30
provides additional biasing to the voltage reference
28
. However, while the circuit
20
may be roughly temperature independent, it does not generally provide a high precision frequency of oscillation (i.e., less than 2%).
SUMMARY OF THE INVENTION
The present invention concerns a circuit comprising an oscillator circuit, a current source and a load circuit. The oscillator circuit may be configured to present an output signal having a frequency in response to (i) a current, (ii) a load, and (iii) an input signal. The current source may be configured to generate the current in response to one or more first control signals. The load circuit may be configured to generate the load in response to one or more second control signals.
The objects, features and advantages of the present invention include providing a circuit and method that implements a precision on-chip current controlled oscillator without using an external reference such as a crystal. The present invention provides such an oscillator that (i) provides accurate frequencies (e.g., in the order of 2% or less), (ii) may be used with a ring oscillator to eliminate the need for a resonator or a crystal, (iii) may be used with a microcontroller to provide a single-chip clocking solution for a entire system and (iv) may reduce process sensitivity.
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A 30-MHz Hybrid Analog/Digital Clock Recovery Circuit in 2&mgr;m CMOS, By: Beomsup Kim, David N. Helman and Paul R. Gray, Journal of Solid-State Circuits, vol. 25, No. 6, Dec. 1990, pp. 1385-1394.
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Cypress Semiconductor Corp.
Kinkead Arnold
Maiorana P.C. Christopher P.
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