Apparatus and method for frequency tuning an LC oscillator in a integrated clock circuit

A method and system in an integrated circuit for frequency tuning oscillator circuits. An oscillator circuit within an integrated circuit is formed on a substrate such that the oscillator circuit is coupled to an inductor component. The inductor component includes a first inductor positioned parallel to a second inductor such that a total effective inductance is associated with the first and second inductors. The first inductor is formed from a first spiral coil layer which is coupled to a second spiral coil layer on the substrate. The second inductor is formed of a single spiral coil layer on the substrate, wherein the single spiral coil layer provides an inductive feedback signal to the oscillator circuit when electrical current flows through the single spiral coil layer in response to current flowing through the first spiral coil layer and the second spiral coil layer. The feedback signal can be adjusted such that a change in a phase associated with the feedback signal results in an alteration of the total effective inductance, which in turn shifts a center frequency associated with the oscillator circuit thereby promoting frequency tuning of the oscillator circuit over a wide tuning range. In addition, a capacitor component can be connected to the oscillator circuit such that the capacitor component includes two series connected capacitors which have an associated effective capacitance. A change in the effective capacitance also results in a shift in the center frequency.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates in general to digital and analog oscillator 
circuits. In particular, the present invention relates to tunable clock 
oscillators for digital circuits. Still more particularly, the present 
invention relates to tunable LC oscillator devices utilized in digital 
clock circuits. 
2. Description of the Related Art 
In fabricating clock oscillator circuits utilized in digital circuits, it 
is often necessary to fabricate circuits which are tunable, such as 
circuits in which the center frequency may be altered. At high 
frequencies, oscillators based on LC circuits are preferred for reducing 
jitter and for scaling the power supply voltages. A tunable circuit 
contains some form of tuning so that the natural resonant frequency 
associated with the circuit may be varied. In such a circuit, the 
resonance condition of forced oscillations can be altered. Tuning may be 
carried out by adjusting the value of the capacitance or the inductance, 
or both. 
An LC oscillator is a type of harmonic oscillator. Harmonic oscillators 
generate waveforms that are sinusoidal in nature and contain one or more 
active circuit elements that function to continuously supply power to 
passive components associated with the LC oscillator. A simple harmonic 
oscillator is typically composed of a frequency determining device, such 
as a resonant circuit, and an active element that supplies direct power to 
the resonant circuit and which also compensates for damping which occurs 
as a result of resistive losses. The resonant circuit contains both 
inductance and capacitance arranged in a manner such that the circuit is 
capable of generating resonant frequencies, depending on the value of the 
circuit elements and their particular arrangement. 
In the case of a simple LC oscillator, application of a direct voltage 
causes free oscillations in the circuit which eventually decay because of 
the inevitable resistance in the circuit. Thus, an LC oscillator is 
essentially a tunable circuit that contains both inductance (L) and 
capacitance (C). The product, (LC), determines the center frequency of 
oscillation. The center frequency (.omega..sub.o) is represented by the 
following equation: 
##EQU1## 
These type of oscillator circuits are particularly important in 
synchronizing multiple processors such as those utilized in four-way or 
eight-way computer systems, well known in the art of digital and computer 
electronics. For a given computer system to operate properly, each 
processor must have an identical center frequency. Due to process 
conditions, one processor clock may differ from another processor clock. 
Even if such processor clocks are similar in structure and design, the 
oscillators upon which such clocks are based must be fine tuned in order 
to maintain an exact frequency match. 
Without an exact frequency match, phase slippage results over time. In 
configurations in which voltage controlled oscillators and LC oscillators 
are utilized, it is necessary to maintain continuous frequency tuning of 
the voltage controlled oscillator and the LC oscillator, in order for the 
configuration to be practical in operation. 
There are a number of methods which exist for tuning such circuits. For 
example, for one-time tuning, tuning can be accomplished utilizing wired 
fuses or focused ion beam tailoring, techniques well-known in the art. A 
particular tuning element also well known in the art is a reverse biased 
diode. When a diode is reverse biased, an associated capacitance is 
subsequently altered. This capacitance can be part of the C of an LC 
resonator. Such diodes have a very limited tuning range, usually a range 
of approximately 15%. When zero voltage occurs across the diode, the 
depletion capacitance is reduced approximately by an amount represented by 
the following equation: 
##EQU2## 
In this equation, .O slashed..sub.B represents bulk potential. The limited 
tuning range implies targeting a center frequency perfectly, which is a 
difficult task to accomplish. In a very narrow band system that does not 
include multiple processors, a limited tuning range may be adequate, but 
for most systems, such a limited tuning range is inadequate. 
Based on the foregoing, it can be appreciated that a need exists for a 
method and system which would allow a user to fine tune the frequency of 
an LC oscillator in a digital circuit clock. A need also exists to allow a 
wide range of tuning for such LC oscillators. Because a wide range of 
tuning is not currently feasible with current devices, applicability is 
limited to uniprocessor devices. A device, such as the one disclosed 
herein, not only solves processor problems associated with LC oscillators 
requiring a wide tuning range, but is also applicable to technological 
areas outside the processor arena. For example, such a device would also 
be advantageous in analog situations and technological areas such as 
wireless and communication networks. A device that allows fine tuning of 
clock oscillators would be welcomed by those in the industry currently 
limited in synchronizing circuits driven by current clock oscillator 
devices. 
SUMMARY OF THE INVENTION 
It is therefore one object of the present invention to provide improved 
digital and analog oscillator circuits. 
It is therefore another object of the present invention to provide improved 
tunable clock oscillators for digital circuits. 
It is yet another object of the present invention to provide an improved LC 
oscillator device utilized in digital circuit clocks. 
It is still another object of the present invention to provide a method and 
system for tuning an LC oscillator utilized in a digital circuit clock. 
The above and other objects are achieved as is now described. A method and 
system in an integrated circuit for frequency tuning oscillator circuits 
are disclosed. An oscillator circuit within an integrated circuit is 
formed on a substrate such that the oscillator circuit is coupled to an 
inductor component. The inductor component includes a first inductor 
positioned parallel to a second inductor such that a total effective 
inductance is associated with the first and second inductors. The first 
inductor is formed from a first spiral coil layer which is coupled to a 
second spiral coil layer on the substrate. The second inductor is formed 
of a single spiral coil layer on the substrate, wherein the single spiral 
coil layer provides an inductive feedback signal to the oscillator circuit 
when electrical current flows through the single spiral coil layer in 
response to current flowing through the first spiral coil layer and the 
second spiral coil layer. The feedback signal can be adjusted such that a 
change in a phase associated with the feedback signal results in an 
alteration of the effective inductance, which in turn shifts a center 
frequency associated with the oscillator circuit, thereby promoting 
frequency tuning of the oscillator circuit over a wide tuning range. In 
addition, a capacitor component can be connected to the oscillator circuit 
such that the capacitor component includes two series connected capacitors 
which have an associated total effective capacitance. A change in the 
total effective capacitance also results in a shift in the center 
frequency.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
With reference now to the figures and in particular with reference to FIG. 
1, there is depicted a schematic diagram depicting two inductors and an 
associated total effective inductance. In a configuration 10 composed of 
inductor 14 and inductor 12, the total effective inductance is given by 
equation 16 below. Inductor 14 and inductor 12 are inductors, which are 
circuit elements based on phenomena associated with magnetic fields. A 
time-varying magnetic field induces a voltage in any conductor that is 
linked to the field. The circuit parameter of inductance thus relates the 
induced voltage to the current. The effective inductance L.sub.eff is thus 
provided by the following equation, where L.sub.eff varies in a 
range(L.+-..alpha.M) by changing the phase .O slashed.: 
EQU L.sub.eff =L.sup.+.alpha. Me.sup.j.O slashed. (Equation 1) 
In equation 1 provided above, .alpha. represents an amplitude control 
factor and .O slashed. is a phase control factor. M represents mutual 
inductance. L is simply the self inductance associated with an inductor. 
Those skilled in the art will appreciate that an analogous configuration is 
available for capacitors. For example, FIG. 2 depicts a schematic diagram 
illustrating two capacitors and their associated effective capacitance. In 
FIG. 2, a two capacitor configuration 20 is presented. The two capacitor 
configuration 20 is composed of capacitor 22 coupled to capacitor 24, 
which results in an effective capacitance, as illustrated by equation 2. 
Capacitors are circuit elements based on phenomena associated with 
electric fields. The source of the electric field is separation of charge, 
or voltage. 
If the voltage varies with time, the electric field consequently varies 
with time. A time-varying electric field produces a displacement current 
in the space occupied by the field. The circuit parameter of capacitance 
relates the displacement current to the voltage, such that the 
displacement current is equivalent to the conduction current at the 
terminals of the capacitor. The effective capacitance C.sub.eff varies 
between (C.+-..alpha.k) by changing the phase .O slashed.. The effective 
capacitance C.sub.eff is represented by equation 2 below: 
EQU C.sub.eff =C.sup.-.alpha. ke.sup.j.O slashed. (Equation 2) 
In an LC oscillator circuit, a center frequency can thus be calculated 
based on the effective inductances and capacitances available for that 
particular oscillator circuit. Those skilled in the art will appreciate 
that the center frequency .omega..sub.o is represented by the following 
equation: 
##EQU3## 
FIG. 3(a) illustrates a cross-section view of an inductor circuit 30 which 
utilizes inductor feedback, in accordance with a preferred embodiment of 
the present invention. In FIG. 3(a), two inductors are presented, a first 
inductor 32 and a second inductor 34. First inductor 32 is composed of an 
additional inductor. Two layer spiral coils connected in parallel are 
utilized to form first inductor 32 in order to reduce the coil resistance 
of first inductor 32. These layers are indicated respectively by letter A 
and letter B. Second inductor 34 forms a feedback inductor constructed 
from a single spiral coil which is represented in FIG. 3(a) as letter C. 
Thus, two windings form first inductor 32, while a single winding forms 
second inductor 34. 
As the phase of the feedback signal provided by the configuration presented 
in FIG. 3(a) is altered, the effective inductance and center frequency are 
altered according to the aforementioned equations. FIG. 3(b) illustrates a 
side view of the inductor circuit 30 of FIG. 3(a), in accordance with a 
preferred embodiment of the present invention. In FIG. 3(a) and FIG. 3(b) 
like parts are indicated by like reference numerals. Those skilled in the 
art will appreciate that the configuration described in FIG. 3(b) depicts 
a layered view of inductors 32 and 34 that may be implemented as physical 
inductors on a silicon wafer. 
Those skilled in the art can also appreciate that based on the 
configuration of FIG. 3(b), flux generates a coupled field because the 
inductors are located physically one below the other, much in the same way 
as two or more coils positioned in parallel to one another. However, as 
will be explained momentarily, in order to implement the inductors in a 
preferred embodiment of the present invention and achieve multiple phases 
and a smooth interpolation, a phase interpolator and a four phase 
oscillator (0, 90, 180, 170) may be associated with these inductors. 
FIG. 4 depicts a schematic diagram illustrative of a quadrature oscillator 
circuit which forms a quadrature oscillator 40 in accordance with a 
preferred embodiment of the present invention. Quadrature oscillator 40 
comprises inductors 42, 44, 50, and 52 which are in turn connected to a 
bias network that includes transistors 80 and 82. The quadrature 
oscillator 40 follows the technique outlined in the text related to FIG. 
3. Inductor 42 is connected to inductor 44 which are each connected to a 
transistor 80 at node 45. Transistor 80 is in turn connected to V.sub.DD 
at node 47. An adjustable capacitor 58 is positioned between node 53 and 
node 51 such that adjustable capacitor 58 is connected to inductor 42 at 
node 53 and to inductor 44 at node 51. A V.sub.bias voltage is applied to 
the gate of transistor 80 and to the gate of transistor 82. Capacitors 58, 
48, 56 and 46 may be tunable capacitors. 
Transistor 62 is connected to adjustable capacitor 58 and inductor 42 at 
node 53. Transistor 64 is connected to inductor 44 and adjustable 
capacitor 58 at node 51. The gate of transistor 62 is also connected to 
node 51. Transistor 60 is connected in parallel to transistor 62, while 
transistor 66 is connected in parallel to transistor 64. Transistors 60, 
62, 64 and 66 are also connected to ground 68 at node 63. 
Inductor 50 is connected to inductor 52. Each of these inductors are in 
turn each connected to a transistor 82 at node 83. Transistor 82 is 
connected to voltage supply V.sub.DD at node 85. An adjustable capacitor 
56 is positioned between node 57 and node 59 such that adjustable 
capacitor 56 is connected to inductor 52 at node 59 and to inductor 50 at 
node 57. Transistor 74 is connected to adjustable capacitor 56 and 
inductor 50 at node 57. 
Transistor 76 is connected to inductor 52 and adjustable capacitor 56 at 
node 59. The gate of transistor 74 is also coupled to node 59. Transistor 
72 is connected in parallel to transistor 74, while transistor 78 is 
connected in parallel to transistor 76. Transistors 72, 74, 76 and 78 are 
also connected to ground 70 at node 71. Voltage phases (0, 90, 180, and 
270 degrees) are respectively generated at node 53, 57, 51, and 59. 
FIG. 5(a) depicts a block diagram of a circuit 110 that provides frequency 
tuning by phase interpolation, in accordance with a preferred embodiment 
of the present invention. Circuit 110 includes quadrature oscillator 40, 
which is an implementation of quadrature oscillator 40 depicted in FIG. 4. 
In FIG. 5(a), dashed lines respectively surround inductors 42, 44, 50 and 
52 which are analogous to inductors 42, 44, 50 and 52 of FIG. 4. Those 
skilled in the art will appreciate that each inductor actually is composed 
of two inductors, which are analogous to the inductors described in FIG. 
3(a) and FIG. 3(b). Each inductor provides inductive feedback as described 
herein. 
Inductors 42, 44, 50 and 52 are connected to bias network 106 and to 
quadrature oscillator 40 and phase interpolator 100. Phase interpolator 
102 is also tied to control voltage 102. Output 104 is provided from 
quadrature oscillator 40 to phase interpolator circuit 100. Output 104 
consists of four signal phases (0, 90, 180, and 270 degrees) from 
quadrature oscillator 40. Those skilled in the art will appreciate that 
based on the implementation depicted in FIG. 5(a), four phases are 
necessary to interpolate and generate a variable phase between 0 and 360 
degrees. Thus, phase interpolator circuit 100, which generates four phases 
0, 90, 180 and 270, can generate any phase in the range 0 to 270 degrees. 
Phase interpolator circuit 100 takes the four phases at a time and via a 
voltage control circuit, interpolates between these phases. 
FIG. 5(b) is a partial detailed illustration of the inductors depicted in 
FIG. 5(a), in accordance with a preferred embodiment of the present 
invention. Inductor 42 is composed of inductors 42a and 42b. Inductor 44 
is composed of inductors 44a and 44b, while inductor 50 is composed of 
inductors 50a and 50b. Inductor 52 is composed of inductors 52a and 52b. 
The inductors operate according to the principals related to the text 
describing FIG. 1 to FIG. 3(b) herein. Thus, for example, inductors 44a 
and 44b are respectively analogous to inductors 32 and 34 depicted in FIG. 
3(a) and FIG. 3(b). 
FIG. 6(a) depicts a performance chart 100 of quadrature oscillator 
performance, including supply voltage, in accordance with a preferred 
embodiment of the present invention. FIG. 6(b) illustrates a performance 
chart 102 of quadrature oscillator performance, including control voltage, 
in accordance with a preferred embodiment of the present invention. FIG. 
6(a) and FIG. 6(b) demonstrate the performance of an actual embodiment of 
the present invention where voltage is controlled from 0 to 1.6 volts in 
order to alter the center frequency from three gigahertz to about 1.3 
gigahertz. Those skilled in the art will appreciate, based on these 
performance charts, that a wide tuning range is indicated. Given a wide 
tuning range such as the one indicated herein, those skilled in the art 
can appreciate that a clock oscillator circuit based on the aforementioned 
LC oscillator circuit configuration provides an electronics user with the 
ability to fine tune digital circuits incorporating such LC oscillators. 
The embodiments and examples set forth herein are presented in order to 
best explain the present invention and its practical application and, 
thereby, to enable those skilled in the art to make and use the invention. 
However, those skilled in the art will recognize that the foregoing 
description and examples have been presented for the purposes of 
illustration and example only. The description as set forth is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed. Many modifications and variations are possible in light of the 
above teaching without departing from the spirit and scope of the 
following claims.