Patent Application
20100117744
The present invention relates to electronic oscillators. More specifically, the present invention relates to correcting for phase errors in rotary traveling wave oscillators (RTWOs).
An electronic oscillator is a type of electronic circuit that produces a periodic signal. Electronic oscillators are used in a wide variety of applications including digital sampling circuits and quadrature oscillator generators in communications transceivers, for example.
Many modern electronic systems require electronic oscillators capable of generating signals at microwave and millimeter-wave frequencies. Conventional electronic oscillators (e.g., those using lumped element tank circuits) are limited in their ability to generate signals at these frequencies while also maintaining low phase noise. For this reason, alternative oscillator mechanisms have been sought. One category of electronic oscillators that has gained recent interest as a possible alternative is the category of oscillators known as “wave-based” oscillators. Wave-based oscillators dispense with the need for lumped element tank circuits and, instead, rely on the distributed inductance and capacitance of a transmission line to achieve oscillation. Recent developments in wave-based oscillator design have demonstrated the ability of wave-based oscillators to operate at high frequencies, low power, and low phase noise. These characteristics have made wave-based oscillators attractive candidates for microwave and millimeter-wave applications.
The regenerative circuits 21 have input/output terminals connected to the signal trace loops 15a and 15b and are evenly distributed along the closed-loop of the transmission line 15. During start-up, when power is first applied to the regenerative circuits 21, a traveling wave is generated from inherent noise within the regenerative circuits 21. The regenerative circuits 21 reinforce the traveling wave as it is created, forcing it to travel in either a clockwise or counterclockwise direction around the closed-loop differential transmission line 15, the direction of rotation depending on the start-up conditions. Once the traveling wave is fully established, the regenerative circuits 21 continue to reinforce (i.e., amplify) the traveling wave, to counter losses the traveling wave experiences as it travels along the transmission line 15. Typically, the regenerative circuits 21 are implemented as pairs of cross-coupled inverters, like the pair of cross-coupled inverters 23a and 23b in
The null point of the component oscillation waveforms Φ1 and Φ2 sweeps around the closed-loop differential transmission line 15 at a rate of 1/(2 Tp), where Tp is equal to the half period of the component oscillation waveforms Φ1 and Φ2. The sweep rate 1/(2 Tp) defines the fundamental oscillating frequency fosc of the RTWO 10, and relates to the physical properties of the RTWO 10 as follows: fosc=1/(2 Tp)=vp/(2 l), where vp=(LoCo)−1/2 is the phase velocity of the component oscillation waveforms Φ1 and Φ2 traveling in the transmission line 15, Lo and Co are the inductance and capacitance per unit length of the transmission line 15, and l is the length of the transmission line 15.
In addition to having the ability to oscillate at high frequencies and with low phase noise, the RTWO 10 has excellent power dissipation characteristics, even at high frequencies. In fact, once a traveling wave is generated in the RTWO 10, little power is required to sustain it. The energy used to switch the regenerative circuits 21 is part of the wave energy that circulates around the transmission line 15. When the regenerative circuits 21 are formed using CMOS technology, i.e., using CMOS inverters 25 like those in
Another attractive feature of the RTWO 10 is that it provides a multi-phase output. Various applications require or use multiple signal phases. For example, quadrature modulators and demodulators in communications transceivers require the generation of in-phase and quadrature phase local oscillator signals which are ninety degrees out of phase with respect to each other. Digital sampling circuits also use multi-phase clocks to increase effective sampling rates and data transmission speeds. For example, data transmission speeds can be increased beyond the fundamental switching speed limits of the underlying logic of a digital sampling circuit by serializing data sampled by multiple phases of a multi-phase clock. Conventional multi-phase clock generators employ phased-locked loops and delay-locked loops to generate the multi-phase clock. However those approaches are complex, do not generate square waves, exhibit high levels of jitter, and suffer from large area penalties. The RTWO 10, at least in theory, avoids these problems, naturally generating and providing high-frequency multi-phase square wave signals at different physical positions along the transmission line 15.
While the RTWO 40 can be used to implement an oscillator having a multi-phase output, the phase accuracy among the multiple phases is not always as accurate as needed or desired, particularly when the RTWO 40 is configured to operate at microwave and millimeter-wave frequencies. Phase accuracy is adversely influenced by a number of factors, including device mismatches among the regenerative circuits 21 (e.g., caused by processing variations), asymmetry of the physical layout of the RTWO 40, lack of uniformity in signal trace widths and other dimensions of the closed-loop differential transmission line 15, and the difficulty in forming the tap positions along the transmission line 15 with the physical precision necessary to achieve a constant phase separation among phases of the multi-phase output.
The lack of phase accuracy in the RTWO 40 detracts from its use in various applications. For example, in quadrature oscillator applications, sub-degree phase accuracy is often required. At microwave and millimeter wave frequencies, this level of phase accuracy may be difficult or even impossible to achieve with currently available RTWOs, such as those described above. Further, in high-frequency multi-phase clock generator applications, the phase accuracy of the RTWO is often so poor that the skew among output phases is greater than can be tolerated. It would be desirable, therefore, to have an RTWO capable of providing a more phase accurate output than can be realized in currently available RTWOs.
Rotary traveling wave oscillator (RTWO) apparatuses and methods are disclosed. An exemplary method for correcting phase inaccuracy among output phases of a multi-phase RTWO includes detecting a phase error between first and second output phases of the RTWO and controlling the phase velocities of a traveling wave traveling in first and second transmission line segments of a closed-loop transmission line to reduce the detected phase error. According to one aspect of the invention, first and second voltage controlled capacitors having substantially the same capacitance versus voltage characteristics are coupled to the first and second transmission line segments, and first and second control voltages for controlling the first and second voltage controlled capacitors are generated based on the detected phase error. Applying the control voltages causes the capacitance of the first transmission line segment to increase by a capacitance differential +ΔC and the capacitance of the second transmission line segment to decrease by a capacitance differential −ΔC. Controlling the voltage controlled capacitors in this manner decreases the phase velocity of a traveling wave in the first transmission line segment compared to the phase velocity of the traveling wave in the second transmission line section. This allows the phase error between the first and second output phase of the RTWO to be reduced while the total capacitance of the closed-loop transmission line remains at a constant level.
The RTWO methods and apparatus of the present invention are extensible to multi-phase RTWOs having more than two phases. An exemplary N-phase RTWO, where N is a positive integer greater than or equal to two, includes a closed-loop transmission line formed as a Moebius strip. The closed-loop transmission line includes N transmission line segments, to which N voltage controlled capacitors are coupled. The N transmission line segments provide N output phases. A phase correction circuit operates to detect phase errors between output phases, and, depending on the detected phase errors, generates N control voltages for controlling the capacitances of the N voltage controlled capacitors. Controlling the capacitances of the N voltage controlled capacitors in this coordinated manner reduces the phase errors among the N output phases, thereby providing a phase accurate multi-phase RTWO output.
Further features and advantages of the present invention, including a description of the structure and operation of the above-summarized and other exemplary embodiments of the invention, are described in detail below with respect to accompanying drawings, in which like reference numbers are used to indicate identical or functionally similar elements.
Referring to
The transmission line 604 includes a physically and electromagnetically endless conductive signal trace of length l having generally parallel first and second signal trace loops 604a and 604b that merge at a half-twist 609 so that the signal trace forms a Moebius strip. The first and second signal trace loops 604a and 604b are formed on or within a dielectric or semiconductor substrate, and may be disposed either in a single plane using a planar transformer to complete the half-twist 609, or in separate metal layers with a via to close the loop and connect the first signal trace loop 604a to the second signal trace loop 604b. In one embodiment, the RTWO 602 is formed with other circuitry in an integrated circuit, and manufactured according to a semiconductor manufacturing process, such as the complementary metal oxide semiconductor (CMOS) fabrication process. The RTWO 602 in this exemplary embodiment is formed in the shape of a square. However, it can be formed in other shapes, so long as the length l of the signal trace is of the appropriate length to achieve the desired oscillation frequency.
The regenerative circuits 606 comprise pairs of cross-coupled inverters (or other negative resistance, negative capacitance or nonlinear regenerative means, such as Gunn diodes) having input/output terminals connected to the first and second signal trace loops 604a and 604b. Tuning capacitors may be optionally connected in parallel with the regenerative circuits 606 to provide the ability to frequency tune the RTWO 602, as indicated in
The RTWO 602 of the exemplary RTWO apparatus 600 is configured to provide a two-phase differential output, making it suitable for use in a quadrature modulator or demodulator of a communications transceiver, for example. The first phase of the two-phase differential output is provided by a first differential amplifier 614 having a differential input coupled between the first and second signal trace loops 604a and 604b at a first location of the transmission line 604. The second phase is provided by a second differential amplifier 616 having a differential input coupled between the first and second signal trace loops 604a and 604b at a second location of the transmission line 604. The first and second locations are spaced one-half ‘lap’ apart, where a lap corresponds to half the length l of the transmission line 604. Accordingly, the first differential amplifier 614 provides a first differential output signal having a nominal phase of 0°/180° and the second differential amplifier 616 provides a second differential output signal having a nominal phase of 90°/270°.
The first and second voltage controlled capacitors 608 and 610, which may be comprise, for example, first and second varactors, have substantially identical capacitance versus voltage characteristics. They are coupled between the first and second signal trace loops 604a and 604b of the transmission line 604 so that they alternate with the locations at which the first and second differential amplifiers 614 and 616 are coupled to the first and second signal trace loops 604a and 604b. Viewed in another way, the first and second voltage controlled capacitors 608 and 610 are coupled to first and second transmission line segments 618 and 620, respectively of the transmission line 604. As shown in
The capacitances of the first and second voltage controlled capacitors 608 and 610 are controlled by first and second control voltages V1 and V2, respectively, provided by the phase correction circuit 612. Applying the first and second control voltages V1 and V2 across the first and second voltage controlled capacitors 608 and 610 increases the capacitance of the first transmission line segment 618 by a capacitance differential +ΔC and decreases the capacitance of the second transmission line segment 620 by a capacitance differential −ΔC. As explained in further detail below, the phase correction circuit 612 controls the capacitances of the first and second transmission line segments 618 and 620 in this manner, to correct for phase inaccuracies between the two output phases of the first and second differential amplifiers 614 and 616.
Changing the capacitances of the first and second voltage controlled capacitors 608 and 610 alters the phase velocities of the traveling wave in the first and second transmission line segments 618 and 620 of the RTWO 602. The phase velocity describes the rate at which the phase of a traveling wave propagates along a transmission line (i.e., the propagation speed of the traveling wave), and is defined by vp=(LoCo)−1/2, where Lo and Co are the inductance and capacitance per unit length of the transmission line. Accordingly, as illustrated in
The ability to control the propagation speeds of the traveling wave in the first and second transmission line segments 618 and 620 provides the ability to correct for any phase error Δφ that may be present between the output phases of the first and second differential amplifiers 614 and 616. Ideally, the phase error Δφ is zero. However, as was explained above, various factors, such as device mismatches among the regenerative circuits 606 and other electrical components, and asymmetry of the physical layout of the transmission line 604 of the RTWO 602, can cause the phase error to be nonzero. The phase correction circuit 612, which is connected in a feedback arrangement with the RTWO 602, operates to counter these negative influences and force the phase error Δφ to zero. Specifically, the phase correction circuit 612 determines the phase error Δφ between the first and second differential output signals of the first and second differential amplifiers 614 and 616 (i.e., the phase error between the two output phases of the RTWO 602), and, in response, generates the first and control voltages V1 and V2 that set the +ΔC and −ΔC capacitance differentials of the first and second voltage controlled capacitors 608 and 610. The capacitance differential +ΔC of the first transmission line segment 618 and the capacitance differential −ΔC of the second transmission line segment 620 alter the phase velocities of the first and second transmission line segments 618 and 620. Consequently, the phase separation between the two output phases of the first and second differential amplifiers 614 and 616 is also altered.
With the first and second control voltages applied V1 and V2 to the first and second voltage controlled capacitors 608 and 610, the phase correction circuit 612 determines a new phase error between the two output phases of the RTWO 602, and based on the new phase error generates new first and second control voltages V1 and V2 that produce new capacitance differentials +ΔC and −ΔC in the first and second transmission line segments 618 and 620. The RTWO 602 and phase correction circuit 612 operate in this coordinated feedback manner, forcing the phase error Δφ between the two output phases of the RTWO 602 to zero.
In the RTWO apparatus 600 shown and described above, the first and second first and second voltage controlled capacitors 608 and 610 coupled between the first and second signal trace loops 604a and 604b are used to correct for phase inaccuracies between the two output phases of the first and second differential amplifiers 614 and 616. In an alternative embodiment, shown in
The phase correction circuit 612 of the two-phase RTWO apparatus 600 in
When the phase correction circuit 900 in
The first and second time-to-phase converters 1010 and 1012 operate to convert the first and second digital time signals at the outputs of the first and second TDCs 1006 and 1008 to first and second digital phase signals representing the phases of the first and second differential output signals cos(ωt) and sin(ωt+Δφ). The subtractor 1014 forms the difference between the first and second digital phase signals, to produce a digital phase error signal Δφ (z), where z=1, 2, 3, . . . is the sample index.
The digital phase error signal Δφ (z) provides a digital representation of the phase error Δφ detected between the first and second differential output signals cos(ωt) and sin(ωt+Δφ). The decision circuit 1004 operates to add a voltage representation of the digital phase error signal, i.e. μΔφ (z), where μ is a step size parameter (e.g., have a value between 0 and 1), to a voltage differential ΔV(z) used to generate the first and second control voltages V1 and V2 in a previous sample, to generate a new voltage differential ΔV(z+1) having a magnitude dependent upon the phase error Δφ represented in the digital phase error signal Δφ (z). New values for the first and second control voltages V1 and V2 are then computed, i.e., V1=Voffset+ΔV(z+1) and V2=Voffset−ΔV(z+1).
In the phase correction circuits 900 and 1000 described in
The phase error correction methods and apparatus described above have been described in the context of a two-phase (N=2) RTWO 602. However, the methods and apparatus are extensible to RTWOs having any number of phases.
Similar to the transmission line 604 of the RTWO 602 in
The first, second, third and fourth regenerative circuits 1206-1, 1206-2, 1206-3 and 1206-4 comprise pairs of cross-coupled inverters (or other suitable regenerative means) having input/output terminals connected to the first and second signal trace loops 1204a and 1204b. Although not shown in
The four phases of the four-phase RTWO 1202 are provided by four differential amplifiers 1210-1, 1210-2, 1210-3 and 1210-4, each coupled between the first and second signal trace loops 1204a and 1204b at four different locations of the transmission line 1204. The four differential amplifiers 1210 provide differential outputs having nominal phases of 0°/180°, 45°/225°, 90°/270° and 135°/315°.
The first, second, third and fourth voltage controlled capacitors 1208-1, 1208-2, 1208-3 and 1208-4 have capacitances that are controlled by first, second, third and fourth control voltages V1, V2, V3 and V4 provided by the phase correction circuit 1216, as is explained in more detail below, and are coupled between the first and second signal trace loops 1204a and 1204b so that they alternate with the locations at which the four differential amplifiers 1210-1, 1210-2, 1210-3 and 1210-4 are coupled to the first and second signal trace loops 1204a and 1204b. Viewed in another way, the first, second, third and fourth voltage controlled capacitors 1208-1, 1208-2, 1208-3 and 1208-4 are coupled to first, second, third and fourth transmission line segments 1212-1, 1212-2, 1212-3 and 1212-4 of the transmission line 1204.
The phase correction circuit 1216 is configured in a feedback arrangement between the differential outputs of the first, second, third and fourth differential amplifiers 1210-1, 1210-2, 1210-3 and 1210-4 and the voltage control inputs of the first, second, third and fourth voltage controlled capacitors 1208-1, 1208-2, 1208-3 and 1208-4. As shown in
The first, second and third LPFs 1224-1, 1224-2 and 1224-3 operate to filter out the high-frequency components at the outputs of the first, second and third mixers 1222-1, 1222-2 and 1222-3, thereby leaving first, second and third low-frequency component signals (½)*sin(Δφ1,3), (½)*sin(Δφ2,4) and (½)*sin(Δφ1,2), which include a first phase error Δφ1,3 between the differential outputs of the first and third differential amplifiers 1210-1 and 1210-3, a second phase error Δφ2,4 between the differential outputs of the second and fourth differential amplifiers 1210-2 and 1210-4, and a third phase error Δφ1,2 between the differential outputs of the first and second differential amplifiers 1210-1 and 1210-2. The decision circuit 1220, generates the first, second, third and fourth control voltages V1, V2, V3 and V4 based on the detected first, second and third phase errors Δφ1,3, Δφ2,4 and Δφ1,2. In one embodiment, the decision circuit 1220 is configured to perform this control voltage generation process using a least mean squares algorithm. However, any other suitable algorithm may be used. Finally, the first, second, third and fourth control voltages V1, V2, V3 and V4 are used to alter the capacitances of the first, second, third and fourth transmission line segments 1212-1, 1212-2, 1212-3 and 1212-4, to affect the relative phase velocities of the traveling wave propagating in the first, second, third and fourth transmission line segments 1212-1, 1212-2, 1212-3 and 1212-4 so that the first, second and third phase errors Δφ1,3, Δφ2,4 and Δφ1,2 are reduced. The reduced phase errors are then again detected by the phase detection circuit 1218, and based on the reduced phase error values the decision circuit 1220 generates new first, second, third and fourth control voltages V1, V2, V3 and V4 to further reduce the first, second and third phase errors Δφ1,3, Δφ2,4 and Δφ1,2. The RTWO 1202 and phase correction circuit 1216 operate in this coordinated feedback manner, forcing the first, second and third phase errors Δφ1,3, Δφ2,4 and Δφ1,2 to zero.
Although the present invention has been described with reference to specific embodiments, those embodiments are merely illustrative and not restrictive of the present invention. Further, various modifications or changes to the specifically disclosed exemplary embodiments will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.
