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On-Chip THz Spectroscopy SystemXue Wu, Student Member, IEEE and Kaushik Sengupta, Member, IEEE
Abstract—The research report presents the chip-scale solutionsfor THz spectroscopy applications. On the transmitter side,the architecture achieves dynamic waveform shaping of THzperiodic waveform in free space by allowing interference ofradiated electromagnetic-fields with rich harmonic componentsand proper delays at far field. On the receiver side, theelectromagnetic interface between the on-chip receiver and theincoming THz wave creates an opportunity to perform spectralanalysis of incident signal, without requirement of traditionaldown-conversion architecture where requirement of a largebank of frequency synthesizers is necessary. The electromagneticscattering of the incident signal onto the on-chip radiator containsinformation about the incident spectrum. The research reportincludes implementation of the proposed architectures for bothtransmitter and receiver, as well as the measurement results.
Index Terms—Terahertz, CMOS, on-chip antenna, scattering,spectroscopy.
I. INTRODUCTION
TErahertz frequency range between 0.3-3.0 THz hasrich applications ranging from high-resolution radar and
imaging to spectroscopic sensing in chemical and biomedicalsciences. However, lack of low-cost, integrated THz technol-ogy at room temperature has affected the progress in thisfield. Classical technology to perform THz spectroscopy relieson optical equipment including femtosecond laser, photocon-ductive substrates, nonlinear optical elements and mechanicalcomponents making the system expensive, bulky and unableto be integrated. On the other hand, solid-state technology per-forms spectroscopy using the narrowband frequency sourcesand classical down-conversion architecture which requires alarge bank of frequency synthesizers and multipliers coveringthe entire THz range making it unsuitable for integration. Theresearch report presents silicon-based chip-scale solutions forTHz spectroscopy applications.
On the transmitter side, the report presents a scalablearchitecture which allows programmable periodic waveformgeneration by controlling the amplitudes and phases of mul-tiple harmonic frequencies beyond fmax. As an example, if asignal at f0 and its n harmonics with equal amplitudes andproportional phases (equal delays) combine, then sharp pulsesare generated with time-widths (δT ) comparable to half thetime-period of the highest harmonic i.e. δT ∼ 1/(2nf0). Thisprinciple is proposed as the method of generating picosecondtime signatures. An optical analogy is a mode-locked laserwhere the harmonic frequencies similarly align in amplitudesand phases [1].
On the receiver side, the key concept is to exploit theinteraction of the front-end antennas and the incident signalto extract spectral information without the down-conversion
X.Wu and K.Sengupta are with the Department of Electrical Engineering,Princeton University, Princeton, NJ, 08544, USA.
architecture. When a broadband THz signal incident on anon-chip radiator, a corresponding surface current distributionis excited. If the current distributions for multiple singlefrequency excitations are known, when an incident electromag-netic signal with a combination of these frequencies impingeson the radiator, the current distribution is a scaled summationof the current distributions excited by those single frequencies.By knowing all the information, the spectrum of the incidentwave can be analyzed [2].
The rest of the research report is organized as follows:Section II presents the implementation of the architectures.Section III demonstrates the measurement results. Section IVconcludes the whole research report.
II. ARCHITECTURE IMPLEMENTATIONS
On the transmitter side, a reconfigurable radiated periodicsignal generator with picosecond time-widths is realized. Theimplementation of the architecture is shown in Fig.1(a). Thedifferential fundamental signal at 108 GHz is generated bya central VCO which is locked to a reference signal. Thedifferential signal is amplified and the quadrature differentialsignals are generated by a λ/4 network and distributed intofour channels. In each channel, the signal is phase rotated withan IQ phase shifter and then amplified to drive two pseudo-differential harmonic generators connected with an integratedloop antenna with a modified ground aperture. The antennaand its matching network is designed to co-optimize the gainand power delivered at the two harmonic frequencies of 108GHz and 216 GHz. At far field, the delayed signals fromthe four channels are combined quasi-optically to producedesirable waveform, as shown by the example of the pulsetrain in Fig.1(a). The ability to program the amplitudes andphases of the harmonic frequencies of the radiated fieldsenables us to reconfigure the waveform and compensate for thedispersion in the radiation path, process variations, mismatchesand modeling inaccuracies.
On the receiver side, a 40-to-330GHz synthesizer-free THzspectroscope is implemented. The architecture of the imple-mented spectroscope is presented in Fig.1(b). The impressedsurface currents are converted locally into detectable voltageswings with local ground rings, which can be measured bysquare-law detectors. The detector consists of an HBT, biasedin nonlinear region and the rectified chopped output signal iscompared to a reference and amplified by a chain of basebandstages with controllable gain of 0 to 70 dB. There is no passivematching networks between the scatterer and the individualdetectors. The design is optimized so that the collectivepower captured by the detectors results in an aperture areacomparable to its physical area over the frequency range. Intotal, 84 detectors are distributed over the scatterer and theoutputs are multiplexed into 6 groups, as shown in Fig.1(b).
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Fig. 1. (a)4-element array with integrated antennas which can generate and radiate reconfigurable periodic waveforms in free-space with picosecond time-widths. The array can also electronically beam-steer the radiated fields in one direction. (b)Structure of the on-chip antenna, distributed sensors and thearchitecture of the THz spectroscope.
Fig. 2. (a)Measured power spectra at 107.5 and 215 GHz. (b)Measured radiation patterns at 107.5 and 215 GHz.(c)Waveform reconfigurability demonstration.(d)Time-domain waveforms away from the pulse radiating direction showing slight pulse degradation effect. (e) Spectral estimations of c.w., multi-tones andwideband excitations. (f) Relative phase offset estimation exploiting variable nonlinearity of the detectors (g) Incident angle estimation. (h) Linearity test ofthe THz spectroscope and noise profile of a detector.
III. MEASUREMENT RESULTS
The transmitter chip is fabricated in a 65nm LP-CMOSprocess with fmax ∼ 190GHz (σbulk = 13.5Ω • cm) andoccupies an area of 2.7 mm × 3.1 mm. Fig.2(a) shows thecalibrated spectra of the fundamental frqeuency at 107.5GHzand second harmonic at 215GHz with EIRPs of 4.6dBm and5.0dBm, respectively. The radiation patterns are presented inFig.2(b). The maximum directivities are 7.6dB and 9.0dB at107.5GHz and 215GHz. Fig.2(c) demonstrates several radiatedwaveforms which can be generated by the chip. As an exampleapulse train is radiated with a pulse width of 2.6ps and -3.2 dBm EIRP. Fig.2(d) shows the slight pulse degradationmeasured away from the broadside direction by 5 and −5.
The 40-to-330GHz synthesizer-free THz spectroscope isfabricated in 0.13µm SiGe BiCMOS technology with anarea of 2.6 mm × 1.9 mm. The estimation examples withc.w., multi-tones and wideband excitations are demonstratedin Fig.2(e). Fig.2(f) demonstrates the ability to extract thetime-domain information of the incident signal by exploitingthe variable nonlinearity of the detectors. The capability of
estimating incident angle is shown in Fig.2(g). In Fig.2(h),the linearity test of the THz spectroscope and noise profile ofa detector are demonstrated.
IV. CONCLUSION
In the report, a reconfigurable radiated periodic signalgenerator with picosecond time-widths and a 40-to-330GHzsynthesizer-free THz spectroscope are presented. The radiatoris demonstrated to radiate pulse trains of 2.6ps, pure tones at107.5GHz, 215GHz and any combination of amplitudes anddelays of these two harmonic frequencies. Successful estima-tions for c.w. frequencies, multi-tones and wideband signalsbetween 40-330GHz are presented for the spectroscope. Botharchitectures together lead to chip-scale solutions for THzspectroscopy systems in silicon-based integrated technology.
REFERENCES
[1] X. Wu, and K. Sengupta, ”Dynamic waveform shaping with picosecondtime-widths,” IEEE J. Solid-State Circuits, vol. 52, no. 2, Feb. 2017.
[2] X. Wu, and K. Sengupta, ”On-chip THz spectroscope exploiting electro-magnetic scattering with multi-port antenna,” IEEE J. Solid-State Circuits,vol. 51, no. 12, Dec. 2016.
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