Abstract:
A low noise oscillator constructed using a rotary traveling wave oscillator. The conductors of the rotary traveling wave oscillator provide at any tap position a pair of oppositely phased oscillations and these oscillations have slightly different phases at positions that are slightly different on the conductors. Regeneration devices establish and maintain oscillations on the conductors of the traveling wave oscillator. A regeneration device made from p-channel and n-channel transistors is connected to the conductors of the traveling wave oscillator in such a way that the gate connections of the transistors receive the traveling wavefront before the drains of the transistors receive the wavefront. By the time the regeneration device switches in response to the wavefront arriving at the gates of the transistors, the wavefront has arrived at the drains. This creates little or no disturbance to the wave on the conductors and results in low phase noise.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of pending U.S. application Ser. No. 10/275,461, filed Apr. 7, 2003, and titled “E LECTRONIC  P ULSE  G ENERATOR AND  O SCILLATOR ”, which application is incorporated by reference into the present application. 
   This application claims the benefit of application Ser. No. 10/275,461, which is a national stage application of PCT application, PCT/GB01/02069, publication number WO 01/89088, filed May 11, 2001, and this application and the PCT/GB01/02069 application claim priority to GB0011243.3, filed May 11, 2000, GB0024522.2, filed Oct. 6, 2000, and GB0102700.2, filed Feb. 3, 2001. The PCT/GB01/02069, GB0011243.3, GB0024522.2, and GB0102700.2 are incorporated by reference into the present application. 

   FIELD OF THE INVENTION 
   The present invention relates generally to traveling wave oscillators and more specifically to traveling wave oscillators that achieve low levels of phase noise. 
   DESCRIPTION OF THE RELATED ART 
   Phase noise (rms degrees or rms picoseconds), in an oscillator, is the unintentional, possibly rapid, modulation of the phase of a periodic signal usually from thermal noise, shot noise and/or flicker noise in active and passive devices that are part of the oscillator and surrounding system. Phase noise manifests itself, in time, as jitter and, in frequency, as a broadening of the generated waveform, or noise sidebands. The sideband level is generally highest close to the fundamental frequency of the oscillator. Phase noise is a serious concern if the periodic signal is used in data sampling applications, because such jitter leads to error in the sampled signal. A variety of sources can introduce phase noise into an oscillator. These sources include transistors involved in generating the periodic waveform, the power supply to which such transistors are connected, or other non-transistor sources. 
   Thus, there is a need for an apparatus and method for generating a periodic signal that has low phase noise to minimize errors introduced by phase noise. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed towards such a need. An embodiment of the present invention is a low noise oscillator that includes one or more transmission line segments, an odd number of passive connection means, and a plurality of regeneration devices. Each transmission line segment has between its ends a length of spaced apart first and second conductors and each length of conductor is electrically continuous. The passive connection means couple the ends of the segments to form a closed loop of segments and the passive connection means. The regeneration devices are located at various spaced-apart positions on the loop and are 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. 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. 
   In one embodiment, each of the regeneration devices includes first and second inverting amplifiers. The first amplifier has an input connected at a tap position to the first conductor and an output connected at an output position to the second conductor. The second amplifier has an input connected at a tap position to the second conductor and an output connected at an output position to the first conductor. Each amplifier has an associated delay time for responding to a change on its input. For each amplifier, there is a physical offset between its output position and its tap position so that there is a time difference between the traveling wave arriving at the tap position and arriving at the output position, the time difference being approximately equal to the amplifier&#39;s associated delay time. 
   A method in accordance with the present invention is a method for creating low phase noise oscillations. The method includes establishing a traveling wave on a loop of one or more transmission line segments, where each segment has between its ends a length of spaced apart and electrically continuous first and second conductors and where an odd number of passive connection means couple the ends of the one or more segments to form a closed loop. The traveling wave includes a voltage wave between the first and second conductors and the wave is maintained by a plurality of regeneration devices connected between the conductors of the transmission line segments. Each connection means reverses the polarity of the voltage wave between the conductors. Each regeneration device includes back-to-back inverting amplifiers, each of which has an input and an output and a delay time for responding to a change on the input. The method further includes causing a difference in time between the wave arriving at the input of each amplifier and the wave arriving at the output of each amplifier, where the time difference is approximately equal to the amplifier&#39;s delay time. 
   One advantage of the present invention is that a plurality of different phases are available from the oscillator, the different phases being available at different tap positions on the transmission line segments and each having low phase noise. 
   Another advantage is that the oscillator takes up very little physical space when the transmission line segments are folded. 
   Yet another advantage is that the oscillator consumes very little power. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIGS. 1A and 1B  show a rotary traveling wave oscillator; 
       FIGS. 2A and 2B  show the various phases of the rotating wave for each direction of rotation; 
       FIGS. 3A and 3B  show the detail of the regeneration devices connected between the transmission line conductors of the traveling wave oscillator; 
       FIG. 4  shows tap and output connections to bias the rotation direction of the traveling wave; 
       FIG. 5  shows a folded version of a traveling wave oscillator; 
       FIG. 6  shows the regeneration devices connected to the folded version of the traveling wave oscillator; 
       FIG. 7  shows a folded version of a traveling wave oscillator with each segment having twists to reduce coupling between adjacent portions; 
       FIG. 8A  shows a portion of a transmission line segment before folding; 
       FIG. 8B  shows a portion of a transmission line segment after folding; 
       FIG. 8C  shows a folded version of a traveling wave oscillator that increases inductance of the transmission line system; 
       FIG. 9  shows a plot of phase noise versus tap offset; and 
       FIG. 10  shows a plot of the sideband noise spectrum for various frequency offsets. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a transmission-line  15  as a structure that is physically as well as electromagnetically endless, specifically comprising a single continuous “originating” conductor formation  17  shown forming two appropriately spaced generally parallel traces as loops  15   a ,  15   b  with a crossover at  19  that does not involve any local electrical connection of the conductor  17 . The length of the originating conductor  17 , taken as S, corresponds to two ‘laps’ of the transmission-line  15  as defined between the spaced loop traces  15   a ,  15   b  and through the cross-over  19 . The crossover  19  produces a Moebius strip effect where edge traces of the loops  15   a ,  15   b  invert from lap to lap. Alternatively, the structure  15  can be viewed as one or more transmission line segments, where each segment has between its ends a length of spaced apart first and second conductors  15   a ,  15   b . Each conductor is electrically continuous, meaning that there are no breaks in the conductors and no active elements in series with the conductors. A passive connection means  19  connects the ends of each segment (the same segment, if there is only one segment) to form a closed loop. Only an odd number of connection means  19  is permitted. Each connection means  19  causes a reversal in the voltage polarity, without the use of active devices, between the first and second conductors. In one embodiment, the connection means is a crossover connection  19 , as shown in  FIG. 1A . 
   As a pulse generator or an oscillator, the transmission-line  15  has associated plural spaced active means  21  conveniently of bi-directional switching/amplifying nature shown as two inverters  23   a ,  23   b  connected back-to-back between the conductors  15   a ,  15   b , in  FIG. 1A . Alternative active regenerative means  21  could rely on negative resistance, negative capacitance or be otherwise suitably non-linear and regenerative (such as Gunn diodes).  FIG. 1  shows the respective input and output terminals of each circuit  21  connected to the transmission line  15  between the conductors  15   a ,  15   b  at substantially maximum spacing apart along the effectively single conductor  17 ; thus, each is located substantially halfway around the transmission-line  15  relative to the other. 
     FIG. 2  shows a convenient simplified/idealized representation that omits the active means  21 . These can be any odd number of crossovers  19 , and the transmission line loop  15  can be any shape, including geometrically irregular, as long as it has a length appropriate to the desired operating frequency. A signal leaving an amplifier  21  arrives back inverted after a full ‘lap’ of the transmission-line  15 , which is traversed in a propagation time Tp. This effectively defines a pulse width or half-cycle oscillation time of a full-cycle bipolar operating frequency. 
   Initially, random amplification of inherent noise within the amplifiers  21  quickly settles to an effective oscillation at a fundamental frequency F, where F=1/(2Tp), and this occurs typically within nanoseconds. 
   A small number of spaced-apart inverter pairs  23   a  and  23   b  connected between the bandwidth-limited conductors  15   a ,  15   b , leads to a substantially sinusoidal waveform at the fundamental frequency. A sufficient number of spaced-apart inverter pairs  23   a  and  23   b  connected between the bandwidth-limited conductors  15   a ,  15   b  and the fast switching times of the inverters  23   a ,  23   b  lead to a strongly square waveform, which contains odd harmonics of the fundamental frequency F effectively reinforced. At the fundamental oscillating frequency F, the terminals of the amplifiers  21  appear substantially unloaded, due to the transmission-line  15  being ‘closed-loop’ without any form of termination, which results very desirably in low power dissipation and low drive requirements. It can also be desirable and advantageous to alter, independently, the impedance and resistance of the conductors and the conductance and capacitance between the conductors of the transmission line  15 . Such alterations change the propagation constant, y, of the line, without changing its physical length. Changing the propagation constant without changing the physical length of the line is sometimes called changing the electrical length of the line. 
   The evident continuous DC path directly connecting all inputs and outputs of the inverters has no stable DC operating point, and this DC instability is compounded by the regenerative (+Ve feedback) action of the back-to-back inverters. For any inverter and its output signal path with reference to the ground plane, its output arrives back at its input after one lap of the transmission line  15 , in both the clockwise or anticlockwise direction, both waves being launched and arriving back together. Self-sustaining, reinforcing action occurs when the input arrives with a phase that differs with the output phase by 180 degrees and the additional 180-degree phase shift of the inverter contributes to such reinforcing. 
   Coherent pulse or oscillation operation occurs when the signal in the transmission line meets this requirement for all connected inverters. In such a case, all inverters are working in a coordinated manner resulting in known phase relationships between all points on the transmission line. A single rotating traveling wave, rotating either clockwise or anticlockwise, on the line, meets this criterion.  FIGS. 2A and 2B  show (i) the line current flow by arrow-heads, (ii) the polarity by circled plus and minus signs, (iii) the direction of rotation by full arrows, and (iv) the phase from an arbitrary 0/360 degree position, for a two-lap traverse of the path  15 . During rotation, the wavefront incident upon an inverter overrides its previous drive direction due to the low impedance of the wave compared to the input impedance of a single transistor. Once overridden, the inverters contribute to imposing the new wave polarity by connecting the transmission line terminal to the correct power source polarity. This maintains ‘top-up’ energy to give substantially constant amplitude in the presence of (mainly resistive) losses in the transmission line. Switching by the transistors also helps prevent the build-up of any counter-direction waves, effectively acting as wave gates. 
   Once the structure has established a rotation in one direction, the rotation can change only by removing and reversing the electromagnetic energy in the structure. To complete a full bipolar cycle of oscillation, a wave must make two ‘laps’ of the structure in order to complete a 360 degree phase shift, i.e., each complete lap is only 180 degrees of phase shift. Rapid rise and fall times are a consequence of the short transit-time of the MOSFETs, typically 1 ps to 5 ps range in VLSI CMOS, and a short length of transmission line between them. The transistors do not drive a capacitive load, as load and gate are switched by the incident wave, i.e., operation is transit time limited, and the waves are square with very good symmetry between phases, where there are a sufficient number of regeneration devices so that the length between the regeneration devices is short. 
     FIG. 3A  shows the connection of the regeneration devices between the conductors of the transmission line segment in more detail and  FIG. 3B  shows the internal transistor structure, in one embodiment, of the regeneration devices connected between the conductors of the transmission line segment. As is apparent from the figure, conductor A is connected to the gate inputs of transistors P 1  and N 1  and to the junction of the drains of transistors P 2  and N 2  at a first tap position  40 . Conductor B is connected to the gate inputs of P 2  and N 2  and to the junction of drains of P 1  and N 1  at a second tap position  42 . The sources of P 1  and P 2  are connected to VDD  44  and the sources of N 1  and N 2  are connected to ground  46 , and it is these power and ground connections that provide energy to the wave to make up for the losses. In another embodiment, the internal transistor structure of an inverter has a passive load device, such as a resistor, in place of either the p- or n-channel transistors. In yet another embodiment, the inverter is constructed from bipolar transistors. In yet another embodiment, the inverter is constructed from a combination of bipolar and FET transistors. 
   In operation, when a differential mode wave travels on the A and B conductors (say, with a positive voltage on A with respect to B) and reaches the first tap position  40 , the inverter comprising P 1  and N 1  begins to switch so as to make N 1  conductive, reinforcing the voltage on conductor B. At about the same time, the wave reaches the second tap position  42  and the inverter comprising P 2  and N 2  begins to switch to make P 2  conductive, reinforcing the voltage on conductor A. Thus, the switching tends to reinforce the wave traveling by the tap positions. 
     FIG. 4  shows the same regeneration device except with different tap positions. In particular, the output position A 1   50  on conductor A for the drains of M 3  and M 4  is offset from the tap position B 0   52  of the gates of M 3  and M 4  on conductor B. Similarly, the output position B 1   54  on conductor B is offset from the tap position of A 0   56  of the gates of M 1  and M 5 . This configuration promotes oscillation startup in the “Easy Direction,” as shown, because the drain outputs have a coherent delay in the Easy Direction. Another effect of this arrangement is that the waveforms at the taps have faster rise and fall times. 
     FIG. 5  shows a folded version of a traveling wave oscillator. The transmission line segment has a plurality of folds F 1 –F 6 , which permit a longer length segment to fit into a given area. Each fold is essentially an unclosed loop having a base  60 ,  62 ,  64 ,  66 ,  68 ,  70  at or near non-continuous ends of the loop. In the figure, a crossover  72  connects the ends of the transmission line segment  74 . 
     FIG. 6  shows regeneration devices connected to the folded version of the traveling wave oscillator. There are six regeneration devices  82 ,  84 ,  86 ,  88 ,  90 ,  92  distributed along the line  74 . A regeneration device, in one embodiment, comprises a pair of back-to-back inverters  94 ,  96 , one of which  86  is shown in detail in the inset. Each of the pair of back-to-back inverters is connected at or near the base of the fold, as shown. For each inverter  94 ,  96  of the pair, this connection effectively implements a lineal offset or displacement between the tap position  100 ,  104 , and its respective output position  102 ,  106 , the offset being approximately the length of the fold. 
   For lines without or without folds, when the tap position and output position are offset along the rotary traveling wave clock lines such that the wave arrives at the tap position before it arrives at the output position (and this time period approximates the time of gate-drain delay), current is delivered to the line at approximately the same time as the rotary edge arrives at the output position. Under these circumstances, it is believed that the amount of current delivered has little effect on the period of the oscillator. The result is that transistor noise currents and transistor current variation due to VDD changes have little effect on the period. 
   For lines with or without folds, the optimal displacement for low phase noise and low VDD dependency can be found through simulation. Using SPICE, it is straightforward to find the offset at which there is minimum VDD sensitivity to period (which is approximately the lowest phase noise position). A script that generates a Spice file contains an LCR approximation of the rotary clock. Inductors are created with nodes that represent the tap positions. Multiple Spice runs are executed, and in each run, the VDD is modulated to introduce a source of noise. The resulting period of the clock is monitored for variation.  FIG. 9 , discussed below, shows the results of one such simulation. 
     FIG. 7  shows a folded version of a traveling wave oscillator having a plurality of segments  110 ,  112 ,  114 ,  116 ,  118 ,  120 ,  122  and an odd number of crossovers (or twists)  124 ,  126 ,  128   130 ,  132 ,  134 ,  136 . In particular, the loop of segments and crossovers comprises, in sequence, segment  110 , crossover  124 , segment  112 , crossover  126 , segment  114 , crossover  128 , segment  116 , crossover  130 , segment  118 , crossover  132 , segment  120 , crossover  134 , segment  122 , and crossover  136 . The twists ensure that co-parallel differential pairs have minimal coupling to one another. Ideally, the placement of a twist is such that coupling to a conductor  152  on one side of a twist is mitigated by coupling to the conductor  154  on the other side of the twist. The arrows illustrate currents that might exist at one moment in time on the line, current flowing in one direction on conductor  152  and the other direction in conductor  154 , indicating that net differential inductive coupling is reduced by this arrangement. Rotary clock layouts featuring periodic twists also have a reduced response to external magnetic fields lines that cut the twisted wires, i.e., a positive inducement of a signal in one segment by an external field is substantially cancelled by a negative inducement of a signal caused by the same field in a sequential segment because of the twisting. 
     FIG. 8A  shows a portion of a transmission line segment with twists. As shown, areas A 1  and A 3  have a magnetic field that is opposite in orientation from area A 2 , thus creating a canceling effect among the areas. When the segment is folded as shown in  FIG. 8B , these areas maintain their orientation, but currents in conductors pairs (a 2 , b 2 ), and (a 3 , b 4 ) become co-parallel. This increases the mutual inductance between these conductor pairs and thus the overall inductance of the structure. High inductance increases transmission-line impedance and reduces power consumption but at the expense of possible coupling of the less well-contained magnetic fields with other circuitry, and at the expense of increased susceptibility to external fields. Rotary clock layouts have an inherent advantage compared to spiral inductor layouts in that the magnetic fields of the structure of  FIG. 8C  still have oppositely oriented areas, which preserves the canceling effect. 
   The choice of high, medium or low inductance lines depends on the tradeoff between the beneficial effects of high inductance and the negative effect of susceptibility for interference from external electromagnetic fields. 
     FIG. 9  shows a graph of phase noise (jitter) versus tap offset ratio for a particular oscillator. The horizontal axis represents the relative offset as a fraction of the distance between equally spaced regeneration devices. The vertical axis represents the factional change in the nominal period of the oscillator. According to the graph, there is a relative minimum at a particular relative offset of about 26% for the particular oscillator. This relative minimum may not be the same for any particular oscillator constructed in accordance with the present invention. Each oscillator is likely to have different offsets at which the noise is minimized. Even though the exact minimum is the most desirable point to achieve, relative offsets larger or smaller than offset at which the minimum noise is achieved still result in a large reduction in phase noise. In other words, some amount of relative offset, say less than 60%, achieves significant advantages. 
     FIG. 10  shows a graph of the sideband noise spectrum in dBc/Hz versus frequency offset from the carrier for a few relative tap offsets. A relative tap offset of approximately 26% reduces the noise power at the 10 Hz offset from the carrier (the nominal oscillator frequency) by about 20 dB, which is a factor of 100 reduction in power. 
   Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.