Oscillators – Ring oscillators
Reexamination Certificate
1999-10-08
2001-10-02
Kinkead, Arnold (Department: 2817)
Oscillators
Ring oscillators
C331S135000, C331S10800D, C331S17700V
Reexamination Certificate
active
06297706
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to a ring oscillator circuit.
BACKGROUND
In many types of electronic circuits, it is necessary to generate an internal clock signal. A simple circuit for generating such a signal is a conventional ring oscillator circuit, such as shown in FIG.
1
. Such a circuit
10
includes a plurality of series-coupled inverters
12
a,
12
b,
and a cross-coupled feedback path
14
connecting the output of the last inverter
12
b
to the input of the first inverter
12
a.
In order for such a circuit to oscillate, the overall gain in the loop must be greater than one. In addition, the overall phase shift must be 180 degrees, and thus at least two stages arc required to introduce the two poles required for this phase shift. The frequency of oscillation of a conventional ring oscillator is related to the delay between each of the stages, which delay is a result of the type of inverter circuit used as well as the magnitude of parasitic capacitances, Cp, which are present in the circuit. If each stage introduces a delay of &tgr;, the loop delay, corresponding to the time to complete one-half a cycle, is 2&tgr;. Thus, the nominal frequency of operation for a two-stage circuit, such as in
FIG. 1
, is 1/4&tgr;.
Although easy to construct, ring oscillators arc not particularly effective at high frequencies relative to the limits of technology at issue. For example, in a particular technology, a latch can be clocked at 40 GHz. In that technology, a two-stage ring oscillator typically cannot generate an output signal which has a frequency greater than 20 GHz due to the switching delay of each stage. Delay of inverters which are used in conventional ring oscillators can be reduced by decreasing their load resistance. Although this increases the operating frequency of the ring oscillator, it also reduces the small signal gain at the operating frequency. Thus, such an oscillator generally has low power at the output and is therefore subject to unacceptable levels of noise.
An alternative oscillator, suitable for high frequency applications, makes use of LC resonator circuits. A wide variety of LC oscillators are known to those of skill in the art. A drawback to this class of circuits is that they can take up a large amount of area on an integrated circuit. A further drawback is that they have a very limited range of tunability and thus require high precision during fabrication, increasing overall cost.
Accordingly, it would be advantageous to provide a modified ring oscillator configuration which has reduced loop delay and a higher gain at the desired frequency of oscillation than conventional ring oscillator circuits to thereby provide increased oscillation frequencies.
SUMMARY OF THE INVENTION
A single stage voltage controlled ring oscillator design according to the invention includes a trans-admittance (TAS) voltage-to-current circuit coupled to a trans-impedance current-to-voltage (TIS) circuit. The TAS circuit may be connected to the TIS circuit by a pair of transmission lines, the length of which can be selected according to a desired (nominal) oscillation frequency. The voltage output of the TAS is connected to the input of the TIS circuit. In addition, the circuit components are preferably conjugately matched such that the parasitic output capacitance of the TAS circuit is canceled by the input inductance of the TIS circuit to provide increased gain at the oscillation frequency. Preferably, the TIS circuit includes a tunable current driver which can be adjusted to vary the oscillation frequency. Tunable components in the TAS circuit may also be provided. The combined TAS/TIS circuits are configured to have two or more poles and therefore to introduce a sufficient phase shift to promote oscillation.
In a particular embodiment, a limiting amplifier is used for the TIS circuit. Unlike conventional limiting amplifiers, which are configured to have a relatively broad-band response so that the gain does not peak at specific frequencies, the components in the limiting amplifier are selected to provide a narrow-band response where the gain peaks at the desired oscillation frequency.
Advantageously, a TAS/TIS ring oscillator circuit according to the invention provides a higher oscillation frequency and higher power than can be realized with conventional multi-stage ring oscillator designs. Further, these increases are achieved without sacrificing the relatively small size and simplicity of a voltage controlled ring oscillator design. The circuit achieves high frequency oscillation without the use of passive inductors or varactors, thus providing a wider tuning range than that available with high-frequency LC oscillators are used.
REFERENCES:
patent: 4818952 (1989-04-01), Kawano et al.
patent: 5581215 (1996-12-01), Ogasawara
Kenichi Ohhata et al., “Design of a 32.7-Ghz Bandwidth AGC Amplifier IC with Wide Dynamic Range Implemented in SIGe HBT”,IEEE Journal of Solid-State Circuits, vol. 34, No. 9, Sep. 1999.
Darby & Darby
Kinkead Arnold
Lucent Technologies - Inc.
LandOfFree
Single stage voltage controlled ring oscillator does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Single stage voltage controlled ring oscillator, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Single stage voltage controlled ring oscillator will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2586656