Abstract
A high power 490~530 GHz monolithic integrated frequency tripler is demonstrated based on Gallium Arsenide material. Based on the proposed symmetrical and balanced configuration, the tripler could not only achieve good amplitude and phase balances for efficient power synthesis, but also provide a DC bias path without any bypass capacitor to ensure efficient frequency doubling efficiency. Tolerance simulations are also carried out to analyze the effects of key electrical and structural parameters of the diode on the frequency doubling performance in order to maximize the frequency doubling performance. Finally, the developed 510 GHz triplet, driven by approximately 80-200 mW input power, has an output power of 4-16 mW in the frequency range of 490~530 GHz, where the peak frequency doubling efficiency is 11%. At the 522 GHz frequency point, the triplex produces a maximum output power of 16 mW driven by an input power of 218 mW. The triplexer will later be used as the local oscillator source of a 1 THz solid-state external super outlier mixer.
Over the past years, compact and robust local oscillators based on Schottky-based frequency multipliers for heterodyne receivers and spectrograph have provided enormous astronomy observations in the range of 0.1-10 THz, including stellar origins and evolution as well as molecular cloud
Generally, terahertz multiplier sources are typically composed of a series of doublers and triplers to achieve the desired output frequenc

Fig. 1 Diagram of the classical balanced tripler with on-chip MIM capacitor
图1 集成MIM电容的经典平衡式三倍频器示意图
However, the traditional design of Schottky-diode balanced tripler has two inherent limits. On the one hand, small asymmetries are inevitable and introduced by the physical structure of series diode and cause imbalance of input and output coupling per diode, which would influence the performance of this quasi-symmetrical triple
This present approach is based on the concept of our novel frequency tripler demonstrated as shown in

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Fig. 2 Diagram of the proposed and novel 510 GHz tripler structure (a) tripler overall principle structure, (b) 3D view of the bottom part of the waveguide block
图2 提出来的新型510 GHz 三倍频器示意图 (a) 三倍频器整体原理结构,(b) 下部分波导腔体的三维结构图

Fig. 3 The schematic cross-section and physical image of the fabricated varactors array on 15 μm thick GaAs substrate
图3 在15 μm厚的GaAs衬底上制造的二极管阵列的截面示意图和实物图像
The tripler is a split-block waveguide module that features eight Schottky planar varactor diodes and monolithically fabricated on a 15 μm-thick GaAs-based substrate with a dimension of 1938 μm×170 μm, which is sufficiently thick for thermal dissipation. Meanwhile, each tripler cell comprises four varactor diodes which are connected in anti-series at dc (shown in
In addition to the electrical parameter of the Schottky diode, the three-dimension structure would influence significantly the simulated tripler performance. The 3D geometrical structure of the diode is modeled accurately in electric-magnetic simulation software. This needs to be stressed that the channel width Lslot and air-bridge width are the crucial parameters to preserve the coupling balance between the anodes. Hence, compared with the quartz-based tripler circuit based on commercial diode
The diode series resistance could be reduced by minimizing the epilayer thickness and increasing doped N+ layer
The design and optimization objective in the tripler circuit is to maximize tripler efficiency with a pumping power over 200 mW available in our laboratory. It could be divided into two critical steps-1) determining details of the varactor array, such as the anode and air bridge dimensions, zero-junction capacitance as mentioned and 2) determining the proper embedding impedances to present to each junction at the internal coaxial wave ports driven under the operating frequencies (f0=170 GHz and 3 f0=510 GHz) based on the methodology presented in Ref. [
In this circuit, the input signals in WR5.1 waveguide would be divided into two parts with equal amplitude and opposite phase over the bandwidth through an E-plane Y-junction waveguide power splitter, which provides the critical differential inputs to the tripler pairs in mirror symmetry. The phase relations of this Y-junction structure are indicated by different arrows in
Inside each tripler cell, an E-plane probe (integrated with dc path) located in the input waveguide couples the fundamental signals in TE10 mode to the suspended microstrip circuit which operating in quasi-TEM mode. This suspended network features several sections of high and low impedance which could match the varactors and prevent the high order harmonics including 2f0 and 3f0 from leaking into the input waveguide. Distributions of the electrical fields and related harmonic currents are represented by the arrows in red, as shown in
The impedances and length of the suspended lines in
The crucial 180-degree combining structure in
After the global optimization based on the above three symmetrical structures, the balance between the diodes should be analyzed to avoid the power overload in one diode. The simulated balance of input coupling presented in

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Fig. 4 Diagram and simulated result of critical sections of the tripler Note:(a) input E-plane Y-junction structure with related simulation results, (b) tripler cells in mirror symmetry with matching and filtering networks, (c) output combing structure based on E-probes with related amplitude and phase properties, (d) simulated input coupling per diode
图4 三倍频器关键结构和仿真结果 注:(a)输入E平面Y型的仿真结构,(b)镜像对称的倍频单元,(c)合成输出E面探针幅相特性,(d)每个二极管的耦合系数
Finally,

Fig. 5 Final processing structure and physical diagram of the 510 GHz tripler (a) layout of the full monolithic integrated tripler chip with a dimension of 170 μm×1938 μm×15 μm, (b) the circuit mounted on the half split-block, (c) the entire inner close-up view of tripler, (d) dimension and waveguide parameter
图5 510 GHz 三倍频器最终加工结构与实物图 (a)尺寸为170 μm×1 938 μm×15 μm的整体三倍频器电路结构,(b)安装在下腔体的电路细节图,(c)倍频器完整的内部结构图,(d)三倍频器的尺寸与波导参数

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Fig. 6 Tolerance analysis of mounting positions and diode parameters Note: (a) horizontal X offset of installation position, (b) vertical Y offset of installation position, (c) +/- 20% change in DC resistance Rs, (d) +/- 15% change in zero-bias junction capacitor Cj0
图6 对安装位置和二极管参数进行公差分析 注:(a)安装位置的水平X偏移,(b)安装位置的水平Y偏移,(c)串联电阻的±20%以内的变化,(d)零偏结电容±15%以内的变化
As shown in
The driver chain of the 510 GHz novel balanced frequency triper is contributed by a W-band quadrupler followed by a high power amplifier with 27 dBm saturation power, a W band isolator and a 170 GHz frequency doubler presented in


Fig. 7 The measured result of 510 GHz tripler (a) test setup for the 490-530 GHz tripler, (b) available 170 GHz doubler output power, (c) the measured output and conversion efficiency as a function of output frequency
图7 510 GHz三倍频器测试结果 (a)490~530 GHz三倍频器测试平台,(b)可用的170 GHz倍频器输出功率,(c)实测输出功率与倍频效率随输出频率的变化
The outpower of 510 GHz tripler is measured using Erickson PM5 power meter and a WR1.9 to WR10 taper. The bias voltage is adjusted between -2 V to -6 V for maximum output power at each frequency.

Fig. 8 The output power of the tripler at 505 GHz varies with bias voltage, input power
图8 三倍频器的输出功率在505 GHz处随偏置电压和输入功率的变化
The spectral purity of the 510 GHz frequency tripler is measured with Rohde & Schwarz FSW spectrum analyzer and Farran WR2.2 325~500 GHz frequency extension module. Due to RF power limits of extension module (-10 dBm) and lack of WR2.2 attenuator, the measured multiplier and extension module is connected with a pair of WR 2.2 diagonal horn antenna. The input frequency F0 of measured tripler is set to 165 GHz, hence, the second harmonic 2*F0 and third harmonic 3*F0 would be detected by WR2.2 frequency extension module.

Fig. 9 The spectrum result of the 495 GHz signal produced by tripler (a) the conventional ACST tripler, (b) the proposed tripler
图9 由三倍频器产生的495 GHz频率特性 (a)ACST三倍频器,(b)本文提出的三倍器
The performance of the proposed symmetrical and balanced 510 GHz is compared with previously research. In this frequency range, the conversion efficiency of the proposed tripler is higher than Ref. [
Ref | On-chip Capacitor | Number of diode | Balanced or not | Freq/ GHz | Peak power / mW | Efficiency / % | |
---|---|---|---|---|---|---|---|
[ | yes | 6 | yes | 540-640 | 1.55 | 4.5-9 | |
[ | yes | 8 | yes | 840-900 | 1.46 | 1-3 | |
[ | yes | 12 | yes | 265-330 | 24 | 5-15 | |
[ | yes | 12 | yes | 510-552 | 35 | ~7 | |
[ | yes | 12 | yes | 210-225 | 45 | 3-15 | |
This Work | no | 8 | yes | 490-530 | 16 | 3-11 |
A high-power terahertz Schottky diode based symmetrical tripler configuration at 500 GHz is reported which uses 180-degree power splitter in input and a pair of symmetrical tripler in output to enhance the power handing. The peak conversion efficiency and maximum output power of 11% and 16 mW is presented. The scheme of tripler offers an important option to build more powerful and pure sources at terahertz frequency. This 510 GHz multiplied sources would be served as the local oscillator of a 1 THz heterodyne receiver.
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