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AbstractThis paper investigates the impact of digital transformation on export reduction during COVID-19 by using unique data from Vietnamese manufacturing firms. Digital transformation reflects whether firms had already introduced digital technology before the COVID-19 outbreak or started using digital technology during the COVID-19 outbreak. Meanwhile, an export reduction is a partial de-internationalization in which firms decrease overseas sales. Estimation results from the instrumental variable probit method show that digital transformation has a negative effect on the probability of export reduction. This effect becomes particularly strong for micro, small and medium enterprises. Our findings have managerial implications for firm managers.

A novel multilayered vertically integrated inductor structure is developed for miniature CMOS RF integrated circuits, and its properties are investigated. The effect of mutual inductance both within and between adjacent multilayer inductors is also studied. A distributed low noise amplifier is designed by incorporating this novel inductor structure in a standard JAZZ 0.18-[micro]m RF/mixed signal CMOS process, demonstrating the significance of the proposed multilayered inductors in CMOS circuit miniaturization. The three-stage distributed amplifier occupies just 288 x 291 [micro]m or 0.08 [mm.sup.2] of die area, making it the smallest distributed amplifier reported to date. The circuit exhibits a relatively flat gain of 6 dB from 3.1 to 10.6 GHz with less than 0.5-dB ripple, with excellent input and output match of less than -12 and -25 dB, respectively. The noise figure is less than 5 dB to 14 GHz with only 2.7 dB across 8-10 GHz, while the power consumption is approximately 22 mW. Index Terms--CMOS RF integrated circuit (RFIC), distributed amplifiers, low-noise amplifiers (LNAs), multilayer inductors, ultra-wideband (UWB) circuits, UWB systems.

A novel millimeter-wave cavity resonator, completely compatible with commercial CMOS fabrication techniques, has been designed and fabricated in a 0.25-[micro]m CMOS process and tested. The resonator employs a capacitively loaded cavity topology effectively implemented using the CMOS multimetal-layer and via-hole structure. The CMOS capacitively loaded cavity resonator, including two coupling apertures and microstrip feed lines, occupies an area of 2 mm x 1.4 mm x 6.28 [micro]m, which is much smaller than the cavity resonator designed without a capacitive load at the same resonant frequency. It resonates at 30 GHz with a corresponding insertion loss of 2 dB and unloaded quality factor of 22.2. The resonant-frequency reduction is approximately 50%, as compared to that without the capacitive load. The design and performance of a new CMOS coupling aperture, which is useful as a coupling mechanism and a vertical interconnect for highly dense millimeter-wave CMOS RF integrated circuits (RFICs) are also presented. The developed low-loss and compact CMOS cavity resonator can be used to realize various millimeter-wave on-chip passive components such as wideband bandpass filters and diplexers and is suitable for direct integration with other CMOS circuits on a single chip. It is attractive for CMOS RFIC design at millimeter-wave frequencies. Index Terms--Aperture-coupled cavity resonators, CMOS RF integrated circuit (RFIC), coupling apertures, microwave and millimeter-wave integrated circuit (IC), microwave and millimeterwave resonators, passive components, RF resonators.

A fully integrated ultra-broadband transmit/receive (T/R) switch has been developed using nMOS transistors with a deep n-well in a standard 0.18-[micro]m CMOS process, and demonstrates unprecedented insertion loss, isolation, power handling, and linearity. The new CMOS T/R switch exploits patterned-ground-shield on-chip inductors together with MOSFET's parasitic capacitances to synthesize artificial transmission lines, which result in low insertion loss over an extremely wide bandwidth. Negative bias to the bulk or positive bias to the drain of the MOSFET devices with floating bulk is used to reduce effects of the parasitic diodes, leading to enhanced linearity and power handling for the switch. Within dc-10, 10-18, and 18-20 GHz, the developed CMOS T/R switch exhibits insertion loss of less than 0.7, 1.0, and 2.5 dB and isolation between 32-60, 25-32, and 25-27 dB, respectively. The measured 1-dB power compression point and input third-order intercept point reach as high as 26.2 and 41 dBm, respectively. The new CMOS T/R switch has a die area of only 230/zm x 250 [micro]m. The achieved ultra-broadband performance and high power-handling capability, approaching those achieved in GaAs-based T/R switches, along with the full-integration ability confirm the usefulness of switches in CMOS technology, and demonstrate their great potential for many broadband CMOS radar and communication applications. Index Terms--Broadband communications, broadband radar, CMOS switch, CMOS transmit/receive (T/R) switch, linearity, on-chip inductor, power handling, RF integrated circuit (RF IC), ultra-wideband (UWB) communications, UWB radar.

Multilayer design techniques are investigated for reducing the size and enhancing the performance of microwave and millimeter-wave distributed circuits in CMOS. Various distributed passive circuits are implemented in a standard 0.25-[micro]m RF/mixed-signal process, including a novel broadside-coupled Lange coupler, a microstrip ring hybrid, and a hairpin resonator incorporating a complementary slow-wave structure. The broadside-coupled Lange coupler exhibits -3.3 to -3.35 dB through, -3.3 to --3.7 dB coupling, more than 12-dB isolation, and 15-dB return loss across 25-35 GHz while occupying only 217 x 185 [micro]m of chip area. The multilayer ring hybrid has -3.1 to -3.18 dB through, -5.1 to -5.7 dB coupling, and more than 17-dB isolation and 10-dB return loss from 25-35 GHz while occupying 282 x 314 [micro]m. A slow-wave structure based on multilayer complementary design principle is implemented for hairpin resonators. The measured quality factor of the multilayer complementary slow-wave hairpin resonator increases to about 14.5 from 11.3 for a similar sized resonator with a single-layer slow-wave structure, while retaining similar size-reduction properties as the latter. Index Terms--CMOS radio-frequency integrated circuits (RFICs), couplers, hairpin resonator, Lange coupler, microwave and millimeter-wave circuits, multilayer circuits, resonators, ring hybrid, slow-wave structures.

A novel low-cost low-power fully integrated tunable transmit module composed of a tunable CMOS monocycle pulse generator and compact uniplanar antenna was designed, built, and tested for ultra-wideband (UWB) impulse systems. The CMOS tunable pulse generator integrates a tuning delay circuit, square-wave generator, impulse-forming circuit, and pulse-shaping circuit in a single chip using a standard low-cost 0.25-[micro]m CMOS process. It can generate a monocycle pulse and Gaussian-type impulse (without the pulse-shaping circuitry) signals with tunable pulse duration. A compact uniplanar UWB antenna was also developed and integrated directly with the CMOS pulse generator chip to form the complete integrated tunable UWB transmit module. Measured results show that the CMOS tunable pulse generator can produce a 0.3-0.6-V peak-to-peak monocycle pulse with 140-350-ps tunable pulse duration and a 0.5-1.3-V peak-to-peak impulse signal with 100-300-ps tunable pulse-duration, and the uniplanar antenna has less than a 18-dB return loss and is suitable for transmitting/receiving UWB time-domain impulse signals covering the entire UWB bandwidth of 3.1-10.6 GHz. Good agreement between measured and calculated performance is also achieved. The UWB transmit module was experimentally characterized and its performance is verified. This UWB module finds applications in various time-domain UWB systems including wireless communications and radar. Index Terms--CMOS RF integrated circuit (RFIC), impulse generator, monocycle pulse generator, ultra-wideband (UWB) communications, uniplanar antenna, UWB radar, UWB systems, UWB transmitter.

This paper presents a new carrier-based ultra-wideband (UWB) transmitter architecture. The new UWB transmitter implements a double-stage switching to enhance RF-power efficiency, reduce dc-power consumption, and increase switching speed and isolation, while reducing circuit complexity. In addition, this paper also demonstrates a new carrier-based UWB transmitting module implemented using a 0.18-[micro]m CMOS integrated pulse generator-switch chip. The design of a UWB sub-nanosecond-switching 0.18-[micro]m CMOS single-pole single-throw (SPST) switch, operating from 0.45 MHz to 15 GHz, is discussed. The design of a 0.18-[micro]m CMOS tunable impulse generator is also presented. The edge-compression phenomenon of the impulse signal controlling the SPST switch, which makes the generated UWB signal narrower than the impulse, is described. Measurement results show that the generated UWB signal can vary from 2 V peak-to-peak with 3-dB 4-ns pulsewidth to 1 V with 0.5 ns, covering 10-dB signal bandwidths from 0.5 to 4 GHz, respectively. The generated UWB signal can be tuned to cover the entire UWB frequency range of 3.1-10.6 GHz. The sidelobe suppression in the measured spectrums is more than 15 dB. The entire CMOS module works under a 1.8-V supply voltage and consumes less than 1 mA of dc current. The proposed carrier-based UWB transmitter and the demonstrated module provide an attractive means for UWB signal generation for both UWB communications and radar applications. Index Terms--CMOS RF integrated circuit (RFIC), pulse generator, single-pulse single-throw (SPST) switch, ultra-wideband (UWB) communications, UWB radar, UWB system, UWB transmitter.

A distributed amplifier with new cascade inductively coupled common-source gain-cell configuration is presented. Compared with other existing gain-cell configurations, the proposed cascade common-source gain cell can provide much higher transconductance and, hence, gain. The new distributed amplifier using the proposed gain-cell configuration, fabricated via a TSMC 0.18-[micro]m CMOS process, achieves an average power gain of around 10 dB, input match of less than -20 dB, and noise figure of 3.3-6.1 dB with a power consumption of only 19.6 mW over the entire ultra-wideband (UWB) band of 3.1-10.6 GHz. This is the lowest power consumption ever reported for fabricated CMOS distributed amplifiers operating over the whole UWB band. In the high-gain operating mode that consumes 100 mW, the new CMOS distributed amplifier provides an unprecedented power gain of 16 dB with 3.2-6-dB noise figure over the UWB range. Index Terms--CMOS RF integrated circuit (IC), distributed amplifiers, low-noise amplifiers (LNAs), low power-consumption amplifiers, RF IC, ultra-wideband (UWB) systems.

Development of a new sub-nanosecond monocycle pulse transmitter with tunable pulse duration for short-range low-power ultra-wideband radar and communication systems is presented along with detailed design and analysis. The developed pulse transmitter is simple, compact, and can be realized using planar or uniplanar integrated circuits. A novel RC coupling circuit along with a high driving current, provided by a high-speed amplifier and buffers, are used to obtain an increase in the output power. A decoupling circuit is implemented to reduce ringing on the monocycle pulse and provide necessary pulse clamping. Tuning of the output monocycle-pulse duration is achieved by using two distributed delay lines, coupled together by the decoupling network, each spatially loaded with antiparallel p-i-n diodes that are alternately switched on and off. Measurement results show tunable monocycle pulse durations in range of 0.4-1.2 ns, approximately corresponding to the operating frequency range of 0.15-3.7 GHz, and 200-400 mW of pulse peak power. The calculated and measured pulse durations also agree reasonably well. Index Terms--Pulse generator, pulse transmitter, ultra-wideband (UWB) radar and communication, UWB systems, UWB transmitter.

A new compact low-cost sampling mixer with internal local strobe-pulse generator for (carrierless) ultra-wideband (UWB) applications, especially for sub-sampling of UWB video pulse signals, is presented along with detailed design information. The sampler employs a double-sided planar structure to facilitate interface with the strobe-pulse generator and the baseband circuit without using wire connections or air bridges, enabling hybrid microwave integrated-circuit integration with significant reduction in assembly effort. A novel coupling structure for both coupling and termination of the RF and local-oscillator signals to the Schottky-diode sampling bridge was designed to suppress ringing in the strobe pulse signal, resulting in harmonic-distortion reduction in the baseband output signal. Development of strobe-pulse generators using step recovery diodes for good output power efficiency is also presented along with design information for relatively low clock frequencies. The developed sampler achieves an unprecedented conversion loss of 4.5-7.5 dB (without a baseband amplifier) and conversion gain from 6.5 to 9.5 dB (with an amplifier) over a 5.5-GHz RF bandwidth, a dynamic range of over 50 dB, and low harmonic distortion in the baseband output. A new intrinsic conversion-loss equation that allows the sampler's conversion loss and operating RF bandwidth to be predicted accurately was also derived. Index Terms--Pulse generator, sampling mixer, ultra-wideband (UWB) radar and communication.

A new multifunction millimeter-wave sensor operating at 35.6 GHz has been developed and demonstrated for measurement of displacement and low velocity. The sensor was realized using microwave integrate circuits and monolithic microwave integrated circuits. Measured displacement results show unprecedented resolution of only 10 [micro]m, which is approximately equivalent to [[lambda].sub.0]/840 in terms of free-space wavelength [[lambda].sub.0], and maximum error of only 27 [micro]m. A polynomial curve-fitting method was also developed for correcting the error. Results indicate that multiple reflections dominate the displacement measurement error. The sensor was able to measure speed as low as 27.7 mm/s, corresponding to 6.6 Hz in Doppler frequency, with an estimated velocity resolution of 2.7 mm/s. A digital quadrature mixer (DQM) was configured as a phase-detecting processor, employing a quadrature sampling signal-processing technique, to overcome the nonlinear phase response problem of a conventional analog quadrature mixer. The DQM also enables low Doppler frequency to be measured with high resolution. The Doppler frequency was determined by applying linear regression on the phase sampled within only fractions of the period of the Doppler frequency. Short-term stability of the microwave signal source was also considered to predict its effect on measurement accuracy. Index Terms--Displacement measurement, millimeter-wave sensor, radio interferometry, velocity measurement.

A displacement measurement sensor, capable of sub-millimeter resolution, using millimeter-wave interferometry has been developed. The sensor operates at 37.6 GHz and is realized using both microwave integrated circuits and monolithic microwave integrated circuits. It has been used to accurately measure the displacement of metal plate location and water level. A resolution of only 0.05 mm is achieved. A maximum error of 0.3 mm is also attained without correction for the nonlinearity of the phase-detection processor and agrees well with the theoretical calculation. Index Terms--Displacement measurement, microwave interferometer, millimeter-wave interferometer, sensor.

New types of planar transmission lines employing multilayer structures are proposed for possible applications in microwave and millimeter-wave integrated circuits. Detailed investigations are presented through numerical results calculated using the spectral domain technique. The newly proposed transmission lines have many attractive features such as a large impedance range, flexibility and ability to realize complicated, densely packed integrated circuits, as well as miniaturization through the use of thin dielectric layers. Additionally, they possess all of the inherent advantages of the CPW and microstrip line. Their use in microwave circuits is exemplified through a low-pass filter realized using the new slot-coplanar lines with less than 0.5-dB insertion loss and better than 20-dB return loss. The filter's measured and calculated performances also agree well.

Analytical expressions for the scattering (S) parameters for a planar multilayer Marchand balun, constructed of broadside-coupled microstriplines, are derived for the first time, based on the coupledline quasi-TEM normal mode parameters. New design equations and a new design procedure for the balun based on the derived S-parameters and a circuit synthesis technique are presented. Calculated results for a GaAs microwave monolithic integrated circuit balun designed by this procedure are compared to a general full-wave planar analysis, and good performance and agreement are obtained.

A quasi-static analysis and detailed investigation are performed on three coupled, symmetric suspended striplines and microstriplines. The self and mutual capacitances of the structure, the per-unit-length capacitance matrix elements, and the three quasi-TEM mode per-unit-length capacitances are derived explicitly for the first time in terms of capacitances corresponding to arbitrary excitations. Normal mode characteristic impedance and effective dielectric constant data are presented. The mode impedances of the lines are found to be unequal for lines of equal width unlike previously reported. Chain matrix parameters for a new bandpass filter employing three coupled lines are derived. A filter with passband centered at 3 GHz has been designed, built, and tested, and measured performance shows good agreement with theoretically predicted performance.

The dispersive properties for the even- and odd-modes of the broadside-coupled coplanar waveguide (CPW) are determined using the spectral domain method. Various numerical results of the even- and odd-mode effective dielectric constants as a function of frequency are presented and discussed. It is found that the structure has a very weak dispersion. This fact is further confirmed through a comparison between the calculated dynamic and quasi-static results of the even- and odd-mode effective dielectric constants. The low dispersion feature signifies the fact that the quasi-static analysis is adequate for designing practical microwave and millimeter-wave circuits employing broadside-coupled CPW.

Single- and multiple-step discontinuities for a shielded three-layer coplanar waveguide (CPW) are studied. The mode matching procedure is employed to obtain the scattering (S) parameters of the discontinuities. The analysis is validated through a comparison of the calculated S-parameters of a single step discontinuity for a shielded single-layer CPW and those published previously. Calculated S-parameters for various single, double, and triple step discontinuities are presented. Effect of the modal orthogonality criterion on the discontinuity S-parameters is given. Extensive investigation of the numerical convergence of the S-parameters is also described.

Modified broadside-coupled microstrip lines, suitable for microwave and millimeter-wave integrated and monolithic integrated circuit (MIC and MMIC) applications requiring wide bandwidths and tight couplings, are presented. Their analysis, based on the quasi-static spectral domain technique, is described. Using these broadside structures, a new class of broadside-coupled band-pass filters has been developed at X-band (8-12 GHz) with about 1-dB insertion loss. Fair agreement between the measured and calculated results has been observed even though a major approximation is used.

A new type of completely uniplanar broad-band singly balanced diode mixer, which utilizes a coplanar waveguide (CPW) and slot line as the main transmission lines, is presented. With the radio frequency (RF) swept from 7.1 to 10.5 GHz and the local oscillator (LO) of 3.5 dBm at 7 GHz, the mixer exhibits a conversion loss from 6 to 10 dB. The LO-to-RF isolation is better than 20 dB, and the LO-to-intermediate frequency (IF) and RF-to-IF isolations are more than 36 dB. The mixer also exhibits good return loses at all three ports. It has several desirable features such as simplicity, wide bandwidth, good interport isolations, easy mounting of solid-state devices, and no via-hole ground connections. The uniplanar nature of this mixer makes it very suitable for low-cost microwave and millimeter-wave integrated circuits. Index Terms - Microwave integrated circuits, microwave monolithic integrated circuit, mixer, uniplanar circuit.

We report the development of completely uniplanar broad-band balanced push-pull FET amplifiers using slot line and coplanar waveguide. The amplifiers employ broad-band uniplanar baluns to achieve the push-pull function over a wide bandwidth. One amplifier, designed in the unconditionally stable region, exhibits a gain of 3.5-5 dB over the frequency range of 5.4-10 GHz and an output 1-dB compression point of 19 dBm at 10 GHz. The other amplifier was designed in the potentially unstable range and achieves a high gain between 10-11 dB from 2 to 4 GHz and an output 1-dB compression point of 17 dBm at 4 GHz. These results show the feasibility of the push-pull FET amplifier configuration using uniplanar technology for microwave and millimeter-wave integrated circuits and systems. Index Terms - Amplifiers, MIC's, MMIC's, push-pull amplifiers, uniplanar circuits.

This paper presents the development of a new monocycle pulse generator and pulse-to-monocycle-pulse transformer operating in the subnanosecond regime. These circuits employ Schottky diodes, step recovery diodes, and simple charging and discharging circuitry, and are completely fabricated using coplanar waveguides. Simple transient analysis and design of the circuits are presented along with their operating principles. The pulse-to-monocycle-pulse transformer converts a 1-V 300-ps pulse into a 0.7.V 350-ps monocycle pulse. The monocycle pulse generator produces a monocycle pulse having 333-ps pulsewidth and more than 2 V from an input square wave of 10-MHz repetition rate. The generated monocycle pulses have very symmetrical positive and negative portions and low ringing level. Index Terms--MIC, pulse generator, transmitter, ultra-wide-band radar, uniplanar circuits.

In this paper, we report on the development of a new integrated-circuit sampling down-converter having its own pulse generator, local oscillator (LO), and IF amplifier. The internal pulse generator uses a step recovery diode together with a unique ultra-wide-band hybrid junction to generate sub-nanosecond balanced pulses for gating the sampling diodes. The down-converter exhibits a conversion gain from 12 to 15.5 dB over an RF frequency of 0.01-3 GHz with 10-MHz LO and sampling pulses of about 100 ps. Return loss at the RF port is better than 15 dB over this RF bandwidth. The down-converter exhibits a good linearity and low harmonic levels. This down-converter employs a coplanar waveguide and slot line to make the entire circuit uniplanar and is, thus, suitable for low-cost production. In addition, it has an internal pulse generator, LO, and IF amplifier, making it a compact receiver subsystem, which can readily be used in many microwave systems. Index Terms--MIC, mixer, receiver, sampler, uniplanar circuit.