INDUCTANCE ENHANCED ROTARY TRAVELING WAVE OSCILLATOR CIRCUIT AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority to and incorporates by reference U.S. Provisional Application 61/053,637, filed on May 15, 2008, titled "INDUCTANCE ENHANCED ROTARY TRAVELING WAVE OSCILLATOR CIRCUIT AND METHOD."

FIELD OF THE INVENTION [0002] The present invention relates generally to rotary traveling wave oscillators and more particularly to an inductance-enhanced version of such oscillators. Additionally, the invention relates to improvements to inductors.

DESCRIPTION OF THE RELATED ART [0003] Rotary traveling wave oscillators (RTWO) are described in U.S. Patent 6,556,089, which is hereby incorporated by reference in its entirety. FIG. 1 shows the general arrangement of the oscillator, which includes a pair of conductors acting as a transmission line, an odd number of phase-reversing elements, such as cross-overs, connected to the conductors, and a plurality of regeneration elements. In FIG. 1, the transmission line includes conductors 15a and 15b and one cross-over, 19, an odd number of cross-overs being needed to maintain oscillations on the transmission line. FIG. 1 also shows a plurality of regeneration elements 21, connected at spaced apart positions along the transmission line and between the conductors 15a and 15b of the line. The regeneration elements establish a traveling wave on the line and maintain the wave by supplying energy to the line to make up for small losses. [0004] FIG. 2 illustrates one embodiment of a regeneration element, a pair of cross-coupled CMOS inverters. The p-channel transistor of each inverter is connected between a first potential, VDD, and a conductor of the transmission line. The n-channel transistor is connected between that same conductor of the transmission line and a second potential, VSS. In each inverter, the input to the gates of both transistors is the other conductor of the transmission line, thereby cross- coupling the inverters. As a wave travels past them, the cross-coupled inverters switch, supplying energy to the wave to maintain its amplitude. As long as a regeneration element exhibits negative resistance, it can perform the function of starting and maintaining a traveling wave on the line. For example, the regeneration devices of attorney docket no. 221 CIP2 can perform the needed function. [0005] FIG. 3 shows one way to connect a regeneration element to the line. Such a connection biases the direction of rotation of the wave, because the wave arrives at the gates of the inverters before it arrives at their drains, as described in U.S. Patent No. 7,218, 180, which is

hereby incorporated by reference in its entirety. This gives the cross-coupled inverters the time they need to switch just as the wave arrives at their outputs. Because the switching is carefully timed, the regeneration element does not appreciably disturb the period of the oscillator, thereby resulting in low phase noise. [0006] FIG. 4 shows a folded rotary clock, which is described in the 7,218, 180 patent. The folded rotary clock has six folds and one cross-over. As described in the ' 180 patent, the folds have the advantage of providing a convenient way of making connections to the regeneration element to bias the wave in a particular direction. [0007] FIG. 5 shows in detail the connection of a regeneration element on a folded line so that the traveling wave is biased to travel in a particular direction. Note that the time for a wave to reach the drain of an inverter after it reaches the gate is determined by the length of the fold. In the inset 86 shown, the wave arrives at location 100, the gate of inverter 94, before it arrives at location 102, the drain of inverter 94. This arrangement thus allows for the length of the fold to be tailored the delay of the inverter.

BRIEF SUMMARY OF THE INVENTION

[0008] One embodiment of the present invention is a rotary traveling wave oscillator. The oscillator includes one or more conductive segments, an odd number of passive connection means, and a plurality of regeneration elements. Each of the conductive segments has a length of spaced apart first and second conductors between its ends, and a first characteristic inductance per unit length, where each length of conductor being electrically continuous. Each of the odd number of passive connection means has a length of spaced apart first and second conductors coupling to the ends of the one or more segments to form a closed loop of segments and the passive connection means and each has a second characteristic inductance per unit length. The plurality of regeneration devices are located at various spaced-apart positions on the loop and connected between the first and second conductors of a segment. The regeneration devices are operative to establish and maintain on the loop a wave traveling around the loop, where the traveling wave includes a voltage wave between the first and second conductors and a single lap of the wave around the loop defines a propagation time. Each of the passive connection means causes the voltage of the traveling wave between the first and second conductors to reverse polarity, so that, at any location on a segment, there is a pair of oppositely phased oscillations having a period equal to twice the propagation time. The first and second conductors of the passive connection means have a length that is substantially longer than the length of the segments and the second characteristic inductance is greater than the first characteristic inductance. [0009] In one embodiment, the impedance of the loop is substantially increased due to the increased inductance of the conductors of the passive connection means.

[00010] In one embodiment, the first and second conductors of a segment carry currents in opposite directions and the first and second conductors of the passive connection means carry currents in the same direction. [00011] In one embodiment, the first and second conductors of a passive connection means are routed so as to be disposed on two sides of an area, the other two sides of which are bounded by the first and second conductors of a segment.

[00012] One advantage of the present invention is that the power needed to operate the oscillator is decreased. [00013] Another advantage is that the phase noise is improved.

[00014] Yet another advantage is that the regeneration elements are conveniently connected to the oscillator so as to bias the traveling wave in a particular direction.

[00015] Yet another advantage is that the oscillator layout can take into account the delays of the regeneration elements. [00016] Yet another advantage is that the oscillator can be laid out in a smaller area.

[00017] Yet another advantage is that a large number of phases are conveniently available from the oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS [00018] These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:

[00019] FIG. 1 shows the general arrangement of a rotary traveling wave oscillator;

[00020] FIG. 2 illustrates one embodiment of a regeneration device, that of a pair of cross- coupled inverters;

[00021] FIG. 3 shows one way to connect a regeneration device to the line;

[00022] FIG. 4 shows a folded rotary clock as described in the US 7,218, 180 patent;

[00023] FIG. 5 shows in detail the connection of a regeneration element on a folded line to bias a wave for travel in a particular direction; [00024] FIG. 6 shows an embodiment of the present invention;

[00025] FIG. 7 shows a second embodiment of the present invention;

[00026] FIG. 8 shows a cross-section of the embodiment of FIG. 6;

[00027] FIG. 9 shows a cross-section of the embodiment in FIG. 7;

[00028] FIG. 10 shows the placement of representative regeneration elements for the embodiment of FIG. 6;

[00029] FIG. 11 shows another embodiment of the present invention in which the

transmission lines of the first or second embodiment form a closed loop;

[00030] FIG. 12 shows a perspective view of a portion of the transmission line; and

[00031] FIG. 13 shows a perspective view of a portion of the transmission line.

DETAILED DESCRIPTION OF THE INVENTION

[00032] FIG. 6 shows an embodiment 200 of the present invention. In this embodiment, a portion 202 of a pair of transmission line conductors of a rotary traveling wave oscillator carries the traveling wave. Current I _A flows in one conductor A 202 and current I _B flows in the other conductor B 204. A voltage V _AB 206 is present between the two conductors, where conductor A is assumed to be more positive on the upper section 208 of the line, where conductor B is more positive in the middle section 210 of the line, and where conductor A is more positive again on the bottom section 212. Therefore, the portion of transmission line depicted includes two crossovers. One cross-over 214 reverses the polarity of the wave between section 208 and section 210 and the other cross-over 216 reverses the polarity of the wave between section 210 and 212. Instead of minimizing the coupling between the conductors crossover each other, as suggested in the US 6,556,089 patent, the cross-overs in the present invention attempt to maximize the coupling between the conductors, so much so that the cross-overs constitute a major portion of the length of the transmission line. In fact, in the present invention the goal is to make the length of the horizontal runs W 220 depicted much longer than the vertical runs L 222. In one embodiment, the W/L ratio is approximately 3. However, this ratio is a design parameter that is selected to achieve a desired impedance as well as a gate-offset delay for the regeneration elements. An advantage of forming the loop with horizontal and vertical runs is that it is more compact, a significantly longer length of transmission line fitting into a smaller area. [00033] An important property of the embodiment illustrated in FIG. 6 is that the magnetic flux φ over the area 224 is four times that over the area 85 of the fold shown in FIG. 5. The reason is that the currents surrounding the area are twice as great, with approximately equal currents I _A and I _B adding in the adjacent horizontal conductors of sections 208 and 212. Therefore, the embodiment increases the inductance of the conductors by four times. [00034] Increasing the inductance of the transmission line leads to an increase in the impedance of the line according to the relationship Z — -J L I C ■ _> where L is the differential inductance per unit length and C is the differential capacitance per unit length of line. For example, increasing the inductance by a factor of four and holding the capacitance unaltered, increases the impedance by a factor of two. The higher impedance of the line has some very positive effects. One benefit is that the power of a wave on the line is reduced by a factor of two, because half as much current is needed for a given differential voltage between the conductors. Another effect is that the phase noise is improved by about 3 dB, which is equivalent to twofold

improvement. The phase noise improvement stems from an improvement in the Q factor

(Q = O)L I R )- Specifically, due to the inductance enhancements, the inductance per unit length increases by about a factor of four, but the series resistance of the line doubles. This causes a doubling in the Q, and thus a lowering of the phase noise. If a figure of merit for oscillators can be defined as the product of the power and phase noise, then the change in the figure of merit for the embodiment of FIG. 5 is about fourfold, about the same as the inductance increase. [00035] FIG. 8 shows a cross-section 400 of the embodiment of FIG. 6. The conductors A 402 and B 404 are shown as separated from a ground plane 406 by any convenient and suitable insulator 408, depending on whether the lines are implemented on a PC board or an integrated circuit.

[00036] FIG. 7 shows a second embodiment 300 of the present invention. In this embodiment, the horizontal runs of the A and B conductors stacked on top of each other and a cross-over is implemented by one or more feedthroughs or vias 324, 326 between the top metal and the bottom metal. Thus, the conductor with current I _A flows on the left and under the conductor with current I _B in section 308 and the conductor with I _A on the right flows on the top of the conductor with I _B in section 310.

[00037] FIG. 9 shows a cross-section of the embodiment in FIG. 7. In this embodiment, the A conductor on the left is disposed over an insulator, which is, in turn, disposed over the B conductor. The B conductor on the right is similarly disposed over the A conductor. In both cases, the conductors are insulated from the ground plane with a suitable insulator.

[00038] FIG. 1OA shows the placement of representative regeneration elements for the embodiment of FIG. 6. As is clear from the figure, the regeneration elements 602, 604 can be conveniently located between the A conductor 202 and the B conductor 204 such that a wave traveling on the line is predisposed to travel in a particular direction. One advantage of the present invention is that the length L of the horizontal conductors can be selected to match the propagation delays of various implementations of the regeneration elements. For example, it is well known that p-type transistors are slower than n-type transistors. Thus, if regeneration elements are implemented with n-type devices, such MOS or bipolar transistors, the length of the horizontal conductors may be smaller (L in FIG. 1OA < L' in FIG. 10B) than if p-type devices are used. FIG. 1OA shows the case where the regeneration elements are n-type devices, which need less time. FIG. 1OB shows the case where the regeneration devices 702, 704 are p-type devices, which need relatively more time.

[00039] In either embodiment, the vertical runs of a section have a different spacing compared to the horizontal runs. If the construction of the lines is the same, this makes the Z ₀ of a vertical run different from that of a horizontal run, causing reflections at the point of mismatch. Let the relationship between the inductances be ]_, = nL (where n is about 4, due to the enhancement)

and the relationship between capacitances be Q = mC (where m < \ due to the relative distances), where the "h" subscript refers to a horizontal run and the "v" subscript refers to a vertical run. Then, the relationship between impedances is Z = Z InIm _> indicating the presence of a significant mismatch when m is different from n. To correct the mismatch, the relationship between the capacitances must be altered so that Cy ₁ is about n times larger than C _v . One way to do this is to increase Cy ₁ by increasing the width of the horizontal run. Another way is to decrease the width and thus the C _v of the vertical run. Of course, both changes can be made as well. [00040] FIG. 11 shows another embodiment 800 of the present invention. In this embodiment, the top and bottom metal runs have a similar pattern to those in FIG. 6 or FIG. 7, but the vertical sides are altered so that the metal runs traverse a closed loop, such as the circle 802 shown. This arrangement permits circuitry located at or near the center of the closed loop to access more phase taps of the rotary oscillator with little or no skew, compared to other arrangements of the oscillator. Accessing more phase taps enables the circuitry to effectively operate at a higher speed compared to circuitry operating with, say, only two phases. For example, a clock operating a frequency /with N phase taps accessible permits circuitry to effectively operate at N */ [00041] FIG. 12 shows a perspective view of a portion of the transmission line in which one of the metal runs overlaps the other metal run. Importantly, the width of the overlapping portion is greater than the width of the portions at right angles to the overlapping portions. [00042] FIG. 13 shows a perspective view of a portion of the transmission line in which one or more feedthroughs 1024, 1026 connect the top metal run to the bottom metal run. Again, the widths of the overlapping portions are greater than those at right angles to the overlapping portions. [00043] Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. For example, at high frequencies it is important to avoid transmission imperfections that lead to reflections. One kind of imperfection occurs at right angle corners of a metal run that changes direction. At these corners the width of the metal run is greater byV2 ^~ , which changes the impedance of the metal run. A better way to change direction is to convert the right angle into a rounded corner or to make two 45-degree turns. Yet another way is to remove a portion of the metal run at the corner so that the distance is the same as the rest of the run. In FIGs. 11 and 12, a portion of the corner 906, 1006 is removed to maintain the impedance relative constant over the change in direction. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.