INTERLEAVED NON-LINEAR TRANSMISSION LINES FOR SIMULTANEOUS RISE AND FALL TIME COMPRESSION

Abstract

A non-linear waveguide comprises a transmission line including a first conductive line and a second conductive line; a first bias voltage supply connected with the transmission line; and one or more pairs of diodes connected between the first conductive line and the second conductive line, the one or more pairs of diodes including: a first diode having an anode connected with the first conductive line and a cathode connected with the second conductive line; a second diode having a cathode connected with the first conductive line and an anode connected with the second conductive line; and a second bias voltage supply connected between the anode of the second diode and the second conductive line.

Description

TECHNICAL FIELD

The present invention relates generally to generation of microwave and millimeter wave signals, clock waveforms and delayed signals.

BACKGROUND OF THE INVENTION

Non-linear transmission lines support shock waves and electrical solitons, as has been known theoretically and demonstrated experimentally. FIG. 1A shows a periodic structure 100 made up of a non-linear waveguide comprising a transmission line consisting of a pair of conductors 102,104 loaded with varactor diodes 106 implementable in gallium arsenide (GaAs) technology and demonstrated to compress the fall time of a sinusoidal microwave signal. The varactor diodes 106 are separated by plurality of transmission line segments 112 of approximately equal line lengths or period d. A DC power supply 116 provides reverse bias to the varactor diodes 106. A signal generator 108, one node of which is grounded, supplies a generally sinusoidal input voltage signal 190 with a typical waveform as shown in FIG. 1B to the non-linear transmission line. The signal generator 108 has source impedance represented by a resistor 114. A load 110 is connected to receive a resultant output signal 192 shaped by the varactor-loaded transmission line, a typical waveform of which is shown in FIG. 1B.

FIG. 2A shows a periodic structure 200 made up of a non-linear waveguide comprising a transmission line consisting of a pair of conductors 202,204 loaded with varactor diodes 206 implemented in gallium arsenide (GaAs) technology and demonstrated to compress the rise time of a sinusoidal microwave signal. The varactor diodes 206 are separated by plurality of transmission line segments 212 of approximately equal line lengths or period d. A DC power supply 218 provides reverse bias to the varactor diodes 206. An input signal 290 and resultant output signal 292 are shown in FIG. 2B.

U.S. Pat. No. 5,789,994 to Case et al. teaches a non-linear waveguide employing a pair of transmission lines loaded with anti-parallel varactor diodes used to simultaneously compress the rise and fall times of a sinusoidal signal. The conversion efficiency of the circuit is lower than that of a purely reactive non-linear transmission line because the anti-parallel arrangement of one set of varactor diodes is reverse biased (reactive non-linearity) while the other set of varactor diodes is forward biased (resistive non-linearity).

In another approach, heterostructure barrier varactor diodes (HBV) having a symmetric C-V characteristic are used to generate odd-frequency harmonics (i.e. a square wave). However, self heating caused by the conduction current through the bulk of the varactor diodes results in reduced conversion efficiency. In addition, the unconventional epitaxy of HBV diodes can make their integration with other common processes (e.g. PHEMT, HBT) difficult.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of embodiments of the present invention are explained with the help of the attached drawings in which:

FIG. 1A is a circuit diagram of a periodic non-linear transmission line for fall-time compression in accordance with the prior art.

FIG. 1B is an exemplary plot of an input signal passed through the non-linear transmission line of FIG. 1A, and a resulting output signal.

FIG. 2A is a circuit diagram of a periodic non-linear transmission line for rise time compression in accordance with the prior art.

FIG. 2B is an exemplary plot of an input signal passed through the non-linear transmission line of FIG. 2A, and a resulting output signal.

FIG. 3A is a circuit diagram of an embodiment of a periodic non-linear transmission line in accordance with the present invention for simultaneous rise and fall time compression.

FIG. 3B is a plot of an input signal passed through the periodic non-linear transmission line of FIG. 3A, and a resulting output signal.

FIG. 4A is a simplified circuit diagram of a portion of an embodiment of a non-linear transmission line as shown in FIG. 3A.

FIG. 4B is the simplified circuit diagram of FIG. 4A expressed using alternative structures.

FIG. 5 is an equivalent circuit expressing at least a portion of the circuit of FIG. 4B.

FIG. 6A is an embodiment of a co-planar-waveguide chip layout in accordance with the present invention for defining the non-linear transmission line of FIG. 3A.

FIG. 6B is an expanded view of the input and output of the co-planar-waveguide chip layout of FIG. 6A.

FIG. 7 is an embodiment of a test setup for measuring an output signal of an interleaved co-planar-waveguide type of non-linear transmission line in accordance with the present invention.

FIG. 8A is a plot of output power of second and third harmonics for a +24 dBm input signal measured using the test setup of FIG. 7.

FIG. 8B is a plot of output power of fourth and fifth harmonics for a +24 dBm input signal measured using the test setup of FIG. 7.

FIG. 9A is a plot of output power of second and third harmonics for a +28 dBm input signal measured using the test setup of FIG. 7.

FIG. 9B is a plot of output power of fourth and fifth harmonics for a +28 dBm input signal measured using the test setup of FIG. 7.

FIG. 10 is a plot of output voltage waveforms for different reverse-bias voltages measured using a sampling oscilloscope.

FIG. 11A is a plot of output voltage and a first normalized input waveform measured using a sampling oscilloscope.

FIG. 11B is a plot of output voltage and a second normalized input waveform measured using a sampling oscilloscope.

FIG. 11C is a plot of output voltage and a third normalized input waveform measured using a sampling oscilloscope.

DETAILED DESCRIPTION

Embodiments of nonlinear waveguides in accordance with the present invention can achieve simultaneous rise and fall time compression by interleaving a portion of a fall time compression circuit and a portion of a rise time compression circuit while providing a first reverse bias to rise compression varactor diodes 307 and a second reverse bias to fall compression varactor diodes 306 by way of two separate DC paths 316,318. The resulting interleaved structure is a reactive device that results in conversion efficiency improvements over typical non-linear waveguides. Referring to FIG. 3A, an embodiment of a non-linear waveguide in accordance with the present invention is shown comprising a transmission line including a pair of conductors 302,304 loaded with varactor diodes 306,307 arranged in an anti-parallel fashion. The varactor diodes 306,307 are separated by a plurality of transmission line segments 312 of approximately equal line lengths D. Non-linear transmission lines loaded periodically with varactor diodes at regular intervals d are said to be periodic.

The cell length d defines the pitch or periodicity of the interleaved nonlinear transmission line. A signal generator 308, one node of which is grounded, supplies a signal to the interleaved non-linear transmission line. The signal generator 308 has source impedance represented by a resistor 314. A load 310 is connected to receive a resultant output signal. When driven by a large input signal 390, for example as shown in FIG. 3B, the non-linear waveguide compresses simultaneously the rise and fall times of the generally sinusoidal input signal 390 to produce an output signal such as shown in FIG. 3B. The amount of compression is dependent on the amplitude of the input signal, the number of varactor-diode sections and spacing between the varactor-diode sections, attenuation along the interleaved non-linear transmission line, and DC-bias values. Embodiments of waveguides in accordance with the present invention can enable one or more of (for example) conversion of a sinusoidal signal into a clock signal or equivalently into odd harmonics, simultaneous enhancement of the rise and fall times of a clock or data signal, and simultaneous variation of the rise and fall times of a signal by varying the signal's amplitude and the DC bias.

In other embodiments, the plurality of transmission line segments can be separated by non-equal line lengths. For example, high-frequency performance of a nonlinear transmission line can be improved upon by progressively decreasing the pitch between varactor diodes from the input to the output, thereby increasing the output Bragg frequency (also referred to herein as cutoff frequency, F_c) of the periodic structure or circuit as taught in U.S. Pat. No. 5,014,018. However, decreasing spacing between varactor diodes changes the large-signal characteristic impedance of the nonlinear transmission line and must be compensated for by scaling the varactor-diode capacitance. The large-signal characteristic impedance is preferably constrained to approximately 50 ohms in a preferred embodiment (although in other embodiments in accordance with the present invention different characteristic impedances can be used to suit specific applications). In such a case, the cutoff frequency is limited mainly by spacing between adjacent varactor diodes and by lithographic constraints on the minimum junction area for the varactor. Adjacent varactor diodes are spaced far enough apart so as to result in low electromagnetic coupling between them. Non-linear transmission lines having transmission line segments of varying length can be said to be non-periodic.

In still other embodiments, the cutoff frequency of the nonlinear transmission line or waveguide can increase in blocks of sections. Thus, each block can contain a plurality of varactors. In the first block, the varactor diodes will have a first junction area and a first spacing. In the second block, the junctions will all be the same size but smaller than the size of junctions in the first section. Further, the spacing between the varactor diodes in the second section will be closer in proportion to the decrease in the junction area so as to maintain the characteristic impedance of that section at approximately 50 ohms. This pattern of ever-decreasing junction area and spacing between the varactor diodes in each block can be repeated, for example, until an appropriate length for the transmission line is achieved that results in a required electrical performance.

When driven by a small input signal, the non-linear waveguide can be used in an embodiment as a variable-delay line or phase shifter having a broad instantaneous bandwidth. Instantaneously broadband variable-delay lines can be used in myriad different circuits, for example as phase detectors for clock and data recovery, broadband wireless communications, phased arrays, etc.

FIG. 4A illustrates a simplified circuit diagram of a portion of an embodiment of a non-linear transmission line as shown in FIG. 3A for compressing the fall time and the rise time of a sinusoidal signal. The circuit of FIG. 4A can be expressed as shown in FIG. 4B, wherein the diode is expressed by way of relationships based on equations (1)-(3):

$\begin{array}{cc}{C}_j\left(V\right)=\frac{C_{j0}}{{\left(1-\frac{V}{\phi }\right)}^m}& \left(1\right)\\ i=i_s\left({}^{\alpha V}-1\right)& \left(2\right)\\ G\left(V\right)=\frac{i}{V}=\alpha \left(i-i_s\right)& \left(3\right)\end{array}$

An equivalent circuit for the circuit of FIG. 4B is illustrated in FIG. 5, and is derived by way of equations (4)-(7), below.

C _j1 =C _j(V_D1)=C_j(VR_F(t)−V_DC1)=C_j(V_RF(t)−V₀)  (4)

C _j2 =C _j(V_D2)=C_j(−V_RF(t)+(−V_DC2+V_DC1))=C_j(−V_RF(t)−V₀)  (5)

−V₀<V_RF(t)<V₀<V_BRi→i_sG(V)→0  (6)

C _eq =C _j1 ++C _j2 =C _j(V_RF)(t)−V₀)+C_j(−V_RF(t)−V₀)  (7)

An embodiment of a commensurate non-linear co-planar waveguide 400 employing the arrangement described above is illustrated in FIGS. 6A and 6B, and includes a sixteen-section GaAs co-planar-waveguide chip layout of the interleaved circuit in which each section is made up of two hyperabrupt Schottky varactor diodes 406,407 for a total of 32 varactors. The rise-time compression varactor diodes 407 are electrically connected with a reverse DC bias through a capacitor 430 by way of a pair of very-high-impedance traces 418. As shown, the traces 418 have a step geometry and a desired inductance. The traces 418 are isolated from the ground traces by an insulating under-layer, or some other barrier layer. DC bias for fall-time compression varactor diodes 406 is applied between the center conductor trace 402 and the ground trace 404 by way of a bias tee 442 (as shown in FIG. 7). The ground traces are connected by a plurality of bridges 440 spanning the center conductor 402. While the waveguide is shown having two ground traces and a center conductor trace between the ground traces, the structure is defined as such to comport with current standards for design of co-planar-waveguides. In other embodiments, the waveguide can have some other chip layout. For example, in an embodiment, the waveguide can have a single ground trace.

Conductor loss for a coplanar waveguide transmission line can be reduced by elevating the center conductor 402 above the substrate surface as taught in U. Bhattacharya et al., IEEE Microwave and Guided Wave Letters, vol. 5, No. 2, February 1995, pp. 50-52. Further, shock-wave coupling to surface-wave modes is reduced by elevating the center conductor 402 or all conductors 402,404 above the substrate surface as taught in U.S. Pat. No. 6,894,581. The elevated center conductor 402 is supported by means of conducting posts, or may be backed by a low-loss dielectric such as polyimide, BCB (benzo-cyclo-butene), or silicon nitride. Reduced coupling is further achieved by selecting properly the thickness of the semiconductor substrate. The reduced coupling enhances the high-frequency performance of nonlinear-transmission-line-based circuits. High-frequency harmonics can be generated in an efficient way by reducing the effect of conductor loss and loss to surface-wave modes.

FIG. 7 illustrates an experimental setup for measuring an output signal of an embodiment of an interleaved non-linear transmission line such as that shown in FIG. 6. As shown, a calibrated spectrum analyzer 414 is connected as a load to the non-linear waveguide 400 having rise compression (not shown) provided with a reverse bias from a first DC power supply 418 and fall compression varactor diodes provided with a reverse bias from a second DC power supply 416. As shown, a signal from a signal generator 408 has source impedance represented by a resistor 414. The input signal is amplified by a power amplifier 420 and passed through a low-pass filter 422 before being sent to the non-linear waveguide 400. An experimental setup as shown in FIG. 7 and another based on a sampling oscilloscope were employed to measure an ability of the interleaved non-linear waveguide 400 to perform one or more of converting a sinusoidal signal into a clock signal or equivalently into odd harmonics, enhancing simultaneously the rise and fall times of a clock or data signal, and varying simultaneously the rise and fall times of a signal by varying the signal's amplitude.

FIGS. 8A-11C are plots communicating measurements of an output signal given an input signal indicated on the plot provided to the test set-up of FIG. 7.

The foregoing descriptions of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.

Claims

1. A non-linear waveguide, comprising: a transmission line including a first conductive line and a second conductive line;a first bias voltage supply connected with the transmission line; andone or more pairs of diodes connected between the first conductive line and the second conductive line, the one or more pairs of diodes including: a first diode having an anode connected with the first conductive line and a cathode connected with the second conductive line;a second diode having a cathode connected with the first conductive line and an anode connected with the second conductive line; anda second bias voltage supply connected between the anode of the second diode and the second conductive line.

2. The waveguide of claim 1, further comprising: a signal generator connected to the transmission line to transmit an input voltage signal that varies with time;wherein the one or more pairs of diodes shortens a rise time and a fall time of the input voltage signal as the input voltage signal is transmitted through the transmission line to generate an output pulse signal.

3. The waveguide of claim 2, wherein: the first bias voltage supply provides a first DC bias voltage to the input signal; andthe second bias voltage supply provides a second DC bias voltage.

4. The waveguide of claim 2, wherein the input voltage signal is substantially sinusoidal.

5. The waveguide of claim 1, wherein the first diode is a first varactor diode and the second diode is a second varactor.

6. The waveguide of claim 1, wherein the first diode is a first Shottky diode and the second diode is a second Shottky diode.

7. The waveguide of claim 1, wherein the first diode is a p-n junction device and the second diode is a p-n junction device.

8. The waveguide of claim 1, wherein: the transmission line includes a first end and a second end;the signal generator is connected with the first end; andfurther comprising a load impedance coupled between the first conductive line and the second conductive line at the second end.

9. The waveguide of claim 1, wherein the transmission line is formed in gallium arsenide.

10. A method of shortening a rise time and a fall time of a time-varying input voltage signal as the input voltage signal is transmitted through a transmission line having a first conductive line and a second conductive line, the method comprising: employing one or more pairs of diodes connected between the first conductive line and the second conductive line, the one or more pairs of diodes including a first diode having an anode connected with the first conductive line and a cathode connected with the second conductive line and a second diode having a cathode connected with the first conductive line and an anode connected with the second conductive line;applying a first bias voltage to the transmission line;applying a second bias voltage between the anode of the second diode and the second conductive line.

11. The method of claim 10, wherein: the step of applying a first bias voltage includes applying a reverse bias voltage; andthe step of applying a second bias voltage includes applying a reverse bias voltage of substantially twice the first bias voltage.

12. A system for shortening a rise time and a fall time of a time-varying input voltage signal, comprising: a transmission line including a first conductive line and a second conductive line arranged generally parallel to one another and extending from an input end to an output end;a signal generator adapted to provide the time-varying input voltage signal to the input end;a first bias voltage supply connected with the transmission line and providing a first DC bias voltage to an input signal; andone or more pairs of diodes connected between the first conductive line and the second conductive line, the one or more pairs of diodes including: a first diode having an anode connected with the first conductive line and a cathode connected with the second conductive line;a second diode having a cathode connected with the first conductive line and an anode connected with the second conductive line; anda second bias voltage supply connected between the anode of the second diode and the second conductive line and providing a second DC bias voltage.

13. The waveguide of claim 12, wherein the input voltage signal is substantially sinusoidal.

14. The waveguide of claim 12, wherein the first diode is a first varactor diode and the second diode is a second varactor.

15. The waveguide of claim 12, wherein the first diode is a first Shottky diode and the second diode is a second Shottky diode.

16. The waveguide of claim 12, wherein the first diode is a p-n junction device and the second diode is a p-n junction device.

17. The waveguide of claim 12, wherein: the transmission line includes a first end and a second end;the signal generator is connected with the first end; andfurther comprising a load impedance coupled between the first conductive line and the second conductive line at the second end.

18. The waveguide of claim 12, wherein the transmission line is formed in gallium arsenide.

19. The waveguide of claim 12, wherein the input voltage signal is substantially a clock waveform.

20. The waveguide of claim 12, wherein the input voltage signal is substantially a digital pseudo random binary sequence waveform.

21. The waveguide of claim 12, wherein the input voltage signal is substantially a digital data waveform.

22. The waveguide of claim 12, wherein the lengths of the transmission-line segments are commensurate or non-commensurate.

23. The waveguide of claim 12, wherein the diode areas are commensurate or non-commensurate.

24. The waveguide of claim 12, wherein one or both of the transmission-line conductors is elevated above the semiconductor surface.

25. The waveguide of claim 12, wherein the unloaded waveguide is co-planar waveguide, coplanar strips, microstrip, or some other type of transmission line.

26. The waveguide of claim 12, wherein the input voltage is substantially a clock waveform, or a digital data waveform.

27. The waveguide of claim 12, wherein the DC power supply voltages are varied to adjust electrical characteristics of the output waveform.