[0001 ] PASSIVE PHASED INJECTION LOCKED CIRCUIT

CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of priority to US Provisional

Application No. 62/ 1 07,409 filed on January 24, 201 5, the content of which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0003] N/A

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT [0004] N/A

BACKGROUND OF THE INVENTION Field of the Invention

[0005] The present invention relates to passive phased injection locked circuit and ring-based voltage controlled oscillators.

Description of Related Art

[0006] Ring oscillators (or ROs) based on digital logic building blocks are a popular choice for multi-protocol phase-locked loops (or PLLs) operating in the 0.5- 1 2GHz range due to their minimal area, wide-tuning range, low power consumption, scalability to and between sub-μ ηΊ technologies, and general lack of required analog process extensions. Compared to tuned, high-Q (or quality factor) Inductor-capacitor resonant (or LC) oscillators which target specific higher frequencies at the expense of an increased power and area trade-off, ROs have inferior phase noise performance which restricts their use to only non-critical applications . Specifically, the "resonator" quality factor (or Q) of a ring oscillator is particularly low because the energy stored in every cycle at each output node capacitance is immediately discarded, then restored at the worst possible time at the resonator edges instead of at the ideal peak voltage as in an LC oscillator. In general, from a broad perspective, this lack of energy efficiency accounts for the well-known overall poor phase noise performance exhibited by state of the art ROs.

[0007] Other factors such as flicker (i.e. 1 /f), s hot, thermal, and white noise, wh ich affect phase noise in both s ingle-ended and differential ring oscillators, have been extens ively studied over the last 20 years while integrated circuit implementation has been dedicated to applying these principles and developing ci rcuitry to improve the performance of ROs in PLLs which operate in the mu lti-GHz range. The importance of doing so lies in the inherent non-feas ibility of fabricating LC oscillators at smaller feature sizes due to large area and cost as well as the lack of necessary analog extensions being read ily available for deep sub- τη CMOS processes .

[0008] Among the various practices utilized to lower the phase noise of a ring oscillator operating i n a phase-locked loop, two techniques wh ich have been proven successful at smaller feature sizes stand out: including,

1 ) us ing additional i njection locking (or IL) circu itry and 2) exploiting creative, yet strict symmetry in the ring desig n and physical layout. For instance, in J. Chien et al., "A pulse-position-modulation phase-noise- reduction technique for a 2-to~1 6GHz injection-locked ring oscillator in 20nm CMOS," ISSCC Dig. Tech. Papers, pp. 52-53, Feb. 2014, it uses precisely timed IL which yields extremely low phase noise results at frequencies up to 1 6GHz; while, in M. Chen et al., "A calibration-Tree 800MHz fractional-N digital PLL with embedded TDC," ISSCC Dig. Tech. Papers, pp. 472-473, Feb. 201 0, it presents a unique symmetrical differential O which can loosely be classified as IL though the use of passive resistors. In W. Deng et al., "A 0.0066mm2 780pW fully synthesizabie PLL with a current-output DAC and an interpolative phase- coupled oscillator using edge-injection technique," ISSCC Dig. Tech. Papers, pp. 266-267, Feb. 201 4, IL techniques are applied to an innovative, highly symmetric ring oscillator structure composed of 3 single-ended logic-based rings. In these examples, IL techniques require extra circuitry which may increase the power and/or area of an integrated circuit. Additionally, symmetry may require extra design time and area therefor.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention utilizes phase injection locking via a network of symmetrically placed passive metal interconnect coupling capacitors to reduce the phase noise of an inverter-based ring VCO. The result of the proposed RO design is a more energy efficient circuit which evenly distributes charge between the various nodes during oscillation. Furthermore, the fundamental basic building blocks of the proposed ring oscillator are discussed in order to provide a straightforward methodology for expanding the design to work for multiple phases and a variety of frequencies in the 0.5~to~75.6GHz operating range. Using the aforementioned procedure, a variety of configurations of the VCO have been fabricated and tested in an all-digital 40nm TS C CMOS process. Also, a 0.8-to-28.2 GHz quadrature ring VCO was designed, fabricated, and physically tested with a PLL in the same process.

[001 03 According to one aspect of the present invention, it provides a passive phased injected locked circuit.

[001 1 ] According to another aspect of the present invention, it provides a voltage controlled oscillator, including first and second oscillators.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[001 2] Embodiments of the present invention described are described by way of example with reference to the accompanying drawings.

[001 3] Figure 1 A is a block diagram of a capacitively phase-coupled circuit of a preferred embodiment of the present invention;

[001 4] Figure T B is a block diagram of a capacitively phase-coupled ring voltage controlled oscillator of a preferred embodiment of the present invention; [001 5] Figure 2 is a block diagram of an expandable ring voltage controlled oscillator of another preferred embodiment of the current invention;

[0016] Figure 3 is a block diagram of a single-stage unit for voltage controlied oscillator or gyrator;

[001 7] Figure 4 is a 4 x 3 quadrature ring voltage controlled oscillator of yet another preferred embodiment of the present invention;

[001 8] Figure 5 shows a graph plot of Vc versus VCO output frequency for the C _eq turning bank;

[0019] Figures 6 shows experimental results of the ring voltage control oscillator structures;

[0020] Figure 7 shows comparison of ring voltage controlled oscillator based phase lock loop results in comparison with a state of the art examples;

[0021 ] Figure 8 shows an example of phase noise and output spectrum characteristics of the quadrature ring voltage controlled oscillator shown in Figure 4; and

[0022] Figure 9 shows a an example of a die micrograph of the quadrature ring voltage controlied oscillator in a phase lock loop (or PLL)

[0023] DETAILED DESCRIPTION OF THE INVENTION

[0024] Figure I B shows a block diag ram of an inverter-based ring voltage controlled osci llator of the present invention , wh ich uses phase injection locking via a network of symmetrically placed passive metal intercon nect cou pli ng capacitors C _EQ to reduce the phase noise thereof.

[0025] Two or more chains (or transmission lines) rl and r2 of ci rcuit elements provide progressive s ignal delays ΘΑΙ and ΘΒΙ along the chains (or transmission l ines) when two out of phase AC signals are applied. When two chains are used they are 1 80 deg rees out of phase, and the output of inverter Al shepherds the input of inverter Bl to the same phase angle by virtue of AC coupling th rough its C _EQ capacitance l ink, thus d irectly AC connecting Al output to Bl i nput, in return , at the same phase angle, inverter Bl output return s hepherds inverter Al input to the exactly opposite phase ang le.

[0026] Note that, in phase lock, there is no current through these coupling capacitors except the current that is needed to correct for parasitics. Thus as a resu lt, ideally there is no change in voltage across the capacitors through a cycle: while the output of one inverter is at its peak, the input of the other inverter is also at its peak, etc. Here the value and matching of these coupling capacitors C _e q has insignificant effect in maintai ning phase lock. No change in voltage means that there is no power being wasted, making the circuit ultra-high Q (Quality factor). In effect, the capacitor/ inverter or i nverting amplifier gai n) masquerade as the dual of an inductor. A capacitor in the feedback becomes the dual of an inductor, which replaces radio frequency (RF) inductors with capacitors resulting in high Q. In addition, these "inductors" are evenly distributed. An additional insight is that the resistance loss of a capacitor is low, unlike that of an inductor.

[0027] This results in an extremely high degree of spectral purity sine wave, arguably exceeding the spectral purity of an inductor based circuit. That is to say that odd harmonics are almost non-existent, spawning a spectrally pure highly engineered design parameter in RF circuits. Using capacitors instead of inductors also evades inductive coupling unwanted external fields into these circuits.

[0028] As additional stages are added in Figure 1 B (inverters AN and BN), the Ceq capacitor network couples the two chains of elements together at inter-element cross connection points shepherding the phase angles into place, to injection lock the individual chains together.

[0029] The capacitors couple the two (or more) signal chains (or transmission lines) together insuring that the phases of the chains advance in concert along the elements in the chains, thus tightly coupling the signals together to provide a precise phase relationship.

[0030] This also works for coupling multiple phase angles together in that additive super-position sums to control the phase displacement as is used in Figure 4 below. Here there is a transfer capacitor current, but the stored charge is only moved around between directly connected capacitors and not dissipated through any transistor, maintaining a high distributed Q.

[003 13 Because the phases of both signals are moving together along the capacitiveiy coupled chains, the capacitors are not being externally recharged or discharged as they cou ple the chains together. This leads to a phase lock circuit with wide ranging tunabil ity, inductor-like quality and stabil ity without using inductors.

[0032] Furthermore, opening the coupled loops of the r x s circuit, where r and s could be eve n or odd integers, wi l l result in a discrete lumped transmission line circuit with inverti ng stages to ensu re the propagation of signals through each row are phase locked together. This is shown in Figure 1 A.

[0033] The number of circuit elements is arbitrary and the limit becomes infinite as the case with a wire. I n the minimum extreme, the Differential Cyrator example of Figure 3, where the number of rows is r= 2 and the nu mber of stages is s = l , produces acceptable complementary phases with reasonable spectral pu rity and can run at frequencies approaching the cutoff frequencies of the inverti ng stages of Figure 3 Al and Bl inverting stages .

[0034] In order to make a ring voltage controlled oscillator the number of stages must be an odd positive integer whi le the number of rings may be any positive integer. The result of the RO des ig n shown in Figure I B is a more energy efficient circu it wh ich evenly d istributes charge between the various nodes during oscillation, when comparing it with LC oscillators. Furthermore, the fundamental basic building blocks of the ring oscillator of the present invention are subsequently considered in order to provide a straightforward methodology for expanding the design to work for multiple phases and a variety of frequencies in the 0.5-to-75.6GHz operating range when implemented in an all-digital 40nm TSMC CMOS process as a baseline for translating to other IC process nodes. Using the aforementioned procedure, a 0.8-to-28.2 GHz quadrature ring VCO was designed, fabricated, and physically tested with a PLL in an all-digital 40nm TSMC CMOS process. The ring VCO could also be implemented in any CMOS or other semiconductor technology such as GaAs, GaN, or SiGe, to further increase the frequency range as desired.

[0035] The ring VCO circuit design disclosed herein is designed using an inverter-based ring oscillator structure rl or r2 of Figure I B. One advantage of using this type of O is its simplicity. More importantly, rings of this nature can be built using basic circuit elements readily available in any given IC process. In fact, multiple-staged inverter-based ring oscillators are used extensively on practically all silicon dies for process monitoring. However, traditional ROs suffer from two major disadvantages which have limited their usefulness in PLL designs: 1 ) poor jitter (noise) characteristics and 2) lack of spectral purity (distortion).

[0036] A design approach of the present invention is presented in

Figure 1 B, which takes two or more identical inverter-based staged-ROs rl and r2 and uses phase injection-locking via capacitive coupling Ceq to provide a VCO with improved phase noise performance and spectral purity properties than state of the art RO designs, making the proposed ring VCO design more comparable to those of LC-based ones. Additionally, the application of the proposed ring VCO offers many other desirable properties beyond low noise attributes including: ability to have precise quadrature with many additional phase outputs available, wide range tunability, inductor-like quality and stability without using inductors, full scalability to and between deep sub-^m iC process nodes, compact physical size with minimal sized inverters, and the ability to work at supply voltages at I V and below, with extremely low power operation due to the capacitors not dumping their energy on a cycle by cycle basis as in a ring oscillator.

[0037] Figure 2 shows a block diagram of another embodiment of the present invention, where the ring VCO includes: 1 ) current-starved inverters IAT , IA2, IA3, IBI , IBZ, and IB3 or 1 00 for control voltage (or control signal), either Vs or Vc, tuning, 2) two or more rings, n , Γ2 , made up of a number of odd current-starved inverter stages, si ... SN, and 3) relatively small interconnect symmetrically laid-out capacitors, Ceq or CO, to couple the phases of the neighboring input and output nodes of the rings, and optionally, 4) a logic-controlled bank of interconnect capacitors, CI , C2, for wider frequency range tuning using transmission gate switches Sci , Sc2.

[0038] The simplest unit form of the proposed ring VCO is the single-staged, double-ring differential oscillator as shown in Figure 3. If the input is connected to the output of its respective ring with an appropriate impedance, ZA, Zg, the 2x1 ring will possess a behavior likened to that of a gyrator in that the capacitive circuit acts inductively due to its structure. This is due to a capacitor in the amplifier loop creating a "gyrator" that can masquerade as an inductor which functions as its "dual" circuit element. These capacitors phase-couple the input and output nodes of the stages together forming a distributed spiral virtual inductor as is shown in Figure 3. Figure 3 shows an example of a gyrator, which with row (or r) equal to 2, inverter stages (or s) equal to 1, which is not to be confused with a simple latch where r=l, s=2. In the latter case, the inverters act in series and do not oscillate due to oscii!ation conditions not being satisfied. The differential gyrator must be strictly cross-coupled in the layout of the circuit and additionally, ZA and ZB must be set appropriately to bias the inverting amplifiers in their active region. Additionally, the requirements for oscillation can be expedited via sufficient delay through the layout wire parasitics, which are readily found on any chip due to imperfect isolation and slight process variation, and therefore should be used to an advantage in this circuit. Although exploratory examples of this gyrator point to very high frequencies being obtainable up to 75.6GHz in 40mn CMOS, the circuit suffers from poorer phase noise performance as compared to multiple stages of s = 3 and higher. This is due to the noise being correlated to a minimum number of nodes. Increasing the number of nodes to 3 or 5 significantly improves the performance of the proposed ring VCO. Silicon measurements showing this can be found in Figures 6 and 7. [0039] The single-stage unit of Figure 3 may be easily expanded to a more useful ring VCO which provides multiple phases. The output phases available for the r x s tuned ring VCO may be found at every &.

Q _ 360° __ _ 3_6_0_°

# of phases available r*s where s is an odd, positive integer representing the number of inverter stages in a single ring; r is a positive integer greater than 1 representing the number of rows.

[0040] For the ring VCO in Figure 2, there are s=3 ring inverter stages and r=2 rows connected by neighboring node capacitances. Θ for this example is then calculated to be 60°; therefore, there are 6 output phases available at 0°, 60°, 120°, 180°, 240°, and 300° in this ring.

[0041] The conventional implementation uses current-starved inverters, but any inverter-type of implementation may be used. In this case, the frequency of a general r x s ring VCO is governed by the propagation delay of the s current-starved inverters in a single ring. The finely-tuned VCO output frequency, fvco, is controlled by means of V _c , by starving current through either the top (PMOS) 101 or bottom (NMOS) 102 transistors shown Figure 2; in the present invention, the bottom NMOS transistors 102 were used as the inverters' current control. The inverters

101 and 102 symmetrically self-bias around their midpoint. Optionally, four is also affected by the intentional loading by the tuning capacitor(s) and any switch and wiring path resistance at each node; for instance, increasing the capacitance and/or resistance lowers r.

£0042] The general output frequency of an r x s VCO may be found by the following equation:

f - ¹ - ¹

total ¹ ring ^{T 1} interconnect

1

~ (2sT _pd ) + (2(r ~ l)R _eq C _eq ) where T _P( J is the propagation delay of a single inverter in the ring;

2(r-l) is the number of node connections to the neighboring row(s);

Ceq is the parallel combination of the coupling capacitors C0-2 that are in- use; and

R _eq is the equivalent parallel resistance of the wired path and any switch resistance connected to the coupling capacitors in use.

[00433 Parasitic capacitances should be factored into this equation for accuracy. This basic r x s ring VCO structure is reconfigurable to allow for a variety of phases (e.g. by adjusting r and s) and frequencies (e.g. by varying the Vc for fine tuning and C _eq for course), an example of this will be presented in the next section for the quadrature configuration. Also, for the VCO to produce the desired phases, at least one stage in each row must be cross coupled to the other stage(s) in the other row(s). [0044] Figure 4 shows a block diag ram of a tu ned ring 4x3 VCO, which further shows such expansion of the r x s ring oscillator. In this case, 4 of the 1 2 phases have been used to produce the quadratu re outputs for the PLL.

[0045] The 3D cross section of VCO in the u pper left of Figu re 4 provides an il lustration of how charge is differentially cross-cou pled within the ring through relatively small yet symmetrically laid-out, spiral ly-lin ked neigh boring interconnect capacitances at every node. The charge coupling path creates a continuous, vi rtual inductor, adding to the resonance purity of the ring VCO though passive, balanced IL via capacitive charge cou pling. A capacitor i n a feedback path of the oscillator acts much l ike an inductor allowing the VCO to operate in a linear (i.e. sine-wave) to produce the quadrature outputs for the PLL.

[0046] The 3D cross section of VCO in the upper left of Figure 4 further provides an illustration of how charge is differentially cross- coupled within the ring through relatively small yet symmetrically laid-out, spirally-linked neigh bori ng i nterconnect capacitances at every node. The charge coupling path creates a continuous, virtual inductor, add ing to the resonance purity of the ri ng VCO. A capacitor i n a feedback path of the oscillator acts much like an inductor allowing the VCO to operate i n a linear (i .e. high-qual ity sine-wave) mode, s imilar to an LC oscil lator as opposed to a O which operates in a switching mode. Th is provides low distortion which can be seen in the Experimental Results section including

Figu res 6 to 8. All of the inverters are operating in concert to produce a single sine wave cycle in precisely equal incremental phase steps. This distributed pseudo-inductor causes the energy lost during a cycle to be restored at the phase angle that adds mini mal noise (i.e. jitter), which is the exact opposite of a conventional ri ng oscillator where energy is added at the most jitter sensitive phase ang le. Lastly, the wide operating range of the proposed ri ng VCO is due to the digital logic-controlled bank composed of 3 symmetrically laid-out i nterconnect cou pling capacitors, allowi ng for coarse tuning over 4 overlapping frequency ranges s hown in Figure 5.

[0047] Figu res 6 shows overviews of the silicon measurements of a variety of expansions of the ring VCO structure and the proposed quadrature ring VCO implemented inside a charge pump PLL, al l of which were fabricated in a 40nm all-digital CMOS process and tested. A micrograph of the proposed 4x3 quadrature VCO in the PLL is shown in Figu re 9 and the phase noise and output spectrum are shown in Figure 8. Figu re 7 compares the proposed 4x3 ring VCO resu lts to state of the art examples.

[0048] This work has i ntroduced an expandable structure for a tunable wide-operating range capacitively phase-coupled low noise, low power ring-based VCO for use in multi-GHz PLLs. Using this tech nique, a quadrature ring-based VCO was i mplemented in an all-digital 40n m TSMC

CMOS process. Most notably, the proposed 4x3 ring VCO occupies an area of 0.0024mm ² , consumes a power of 0.88mW at a 1 .0V supply voltage, and possesses a phase noise of - 1 24.5dBc/Hz at the 1 0MHz offset for a carrier frequency of 28.0GHz. Furthermore, this work has the widest reported operating frequency range of any published VCO from 0.8-to- 28.2 GHz. The VCO FOM is also the best reported for ring-based VCOs and is comparable to that of LC oscillators due to the passively-phase coupled IL symmetric ring topology and inherent low power operation.