Abstract
Based on the hybrid integration method， a 335 GHz unbalanced frequency tripler is designed with a symmetrical tapered gradient line matching structure. Under the condition of ensuring single-mode transmission， the matching structure can not only fix the diode position， but also increase the matching effect， and thus solve the problem of narrow 3dB bandwidth for high-frequency band multiplier. The measured results show that the output power of the frequency tripler is all greater than 5 mW in the frequency range of 330-356 GHz. The maximum output power even reaches 11.2 mW at a driving power of 220 mW. The solid-state terahertz local oscillator， as the core device， can drive the 670 GHz sub-harmonic mixer in the superheterodyne receiver.
Terahertz （THz） technology have broad application prospects in radio astronomy， biomedicine， national defense， aerospace and other fields
In this paper， a 335 GHz unbalanced frequency tripler was designed and implemented based on the varactor diode. Through the analysis and design of the input and output circuit， the short circuit of the unnecessary harmonic idle circuit was realized. The tapered line structure was applied to the diode matching circuit， which not only facilitates the assembly of the diode， but also broadens the working bandwidth to a certain extent and reduces the power loss. The designed frequency tripler was manufactured， assembled and tested， and the superiority over the gradient line matching structure is verified.
The 335 GHz frequency tripler consists of input/output circuit， diode unit circuit and matching circuit. The design of each part is described below.
The input part is composed of a DC bias filter， a low-pass filter and an input waveguide-to-suspended microstrip transition to form a three-port network. To widen the bandwidth， two sections of reduced-width waveguide are added. The DC filter and lowpass filter are designed with a compact resonant cell structure. The value of the ground capacitance is greatly reduced by forming a fringe capacitance from the microstrip to the metal wall. By adjusting the value of the equivalent inductance， the size of the resonator is adjusted. At this point， the value of the equivalent capacitance also changes， and then affect the filter rejection degree. The output structure also adopts the probe coupling method. The quasi-TEM mode of the microstrip line is converted into the TE10 mode of the waveguide， and the coupling bandwidth is increased by reducing the broad side of the output waveguide.
When the order of step matching structure increases to infinity and the length of each section is infinitely shortened， the discrete section can be replaced by a continuously tapering transmission line， and it has a shorter total length than the former in the same working bandwidth. Therefore， to further increase the output power and 3 dB bandwidth， a hyperbolic taper gradient line structure is adopted in the matching circuit design of 335 GHz frequency tripler. The matching part inserted between the two transmission lines with impedance Z1 and Z2 is represented by a gradient line of length . The left half of its structure is shown in
Fig. 1 Hyperbolic tapered transmission line structure
图1 双曲锥形传输线结构
A wave traveling from the transmitting end to the receiving end of the tapered non-uniform line undergoes continuous reflections as it travels outward along the portion. These reflections are the result of continuous variations and are directed towards the transmitter. When the wave reaches the receiving end， it encounters an impedance match. Therefore， without any further reflection， it will enter the distant uniform line. Following the convention of dealing with RF transmission line problems， it is assumed that the line is lossless and that the nominal propagation factor is a purely imaginary number， independent of position. That is， ， is the phase shift constant.
Since the tapered asymptote is in symmetric form， the nominal characteristic impedance Z varies with the hyperbolic tangent on the cross section. When the wave propagates along a uniform line， according to the research results of Herbert J. scholars， the reflection coefficient expression is obtained as
Fig. 2 Variation of with for Hyperbolic structure and 335 GHz frequency tripler diode unit
图2 双曲线结构中ρ随l/λ的变化以及335 GHz三倍频器二极管单元
The design of this frequency tripler adopts the method of combining field and circuit. The frequency multiplier efficiency and output power are taken as the optimization goals， and considering bandwidth for comprehensive consideration. Combine the actual processing capacity and control its range and accuracy at the same time. Finally， the complete circuit is established to calculate the full wave electromagnetic field. The complete model of the frequency tripler is shown in
Fig. 3 Architecture of 335 GHz frequency tripler
图3 335 GHz 三倍频器结构
The fundamental wave signal is input through the standard WR8 waveguide in TE10 mode， and converted into a quasi-TEM wave by the input waveguide-suspended microstrip E-probe， then the third harmonic signal is output by the WR2.8 waveguide. In order to facilitate assembly， the suspended microstrip transmission line is made into a "step" form. The Schottky varactor diodes are flip-mounted on the 30 µm thick quartz substrate， which is welded with the frequency tripler cavity through a conductive adhesive. The simulation results are shown in
Fig. 4 335 GHz frequency tripler simulation result
图4 335 GHz三倍频器仿真结果
The physical diagram， internal structure and test environment of the 335 GHz frequency tripler are shown in
Fig. 5 Assembly and test environment of 335 GHz frequency tripler, (a) entire modules, (b) measurement platform
图5 335 GHz 三倍频器装配及测试环境, (a) 整体模块, (b) 测试平台
The test results of the tripler are shown in
Fig. 6 Test results of 335 GHz local oscillator based on frequency triple technology, (a) output power vs. frequency, (b) efficiency vs. input power at 349.5 GHz
图6 基于三倍频技术的335 GHz本振源测试结果, (a) 输出功率与频率关系, (b) 349.5 GHz频点处倍频效率与输入功率关系
With a fixed bias voltage of -6 V， the efficiency variation curve with input power at 349.5 GHz is shown in
In this paper， a hybrid integrated frequency tripler is studied. This approach is easier to implement than the integrated circuit， and the cost is relatively low. It can be seen that there are deviations between the test results and simulation results， which are due to the following reason： When the diode operates at high frequency， the value of the series resistance Rs and zero bias junction capacitance will change due to the increase in temperature. As one of the important parasitic parameters of the diode， its influence on the overall performance of the frequency multiplier cannot be ignored. In order to verify the correctness of the designed frequency multiplier， the W-band driving power is fixed at 23 dBm， the fixed value of Cj0 is 43 fF， and the value of Rs varies between 4 and 8 Ω. The simulation results of changing the Rs value in the circuit are shown in
Fig.7 Relationship between output power and parasitic parameters, (a) output power vs. Rs, (b) output power vs. Cj0
图7 输出功率与寄生参数关系, (a) 输出功率与Rs关系， (b) 输出功率与Cj0关系
Therefore， correcting the value of the important parasitic parameter in the diode model will improve the overall performance degradation of the multiplier caused by it.
A frequency tripler is designed in the form of hybrid integration based on a 30 µm thick quartz substrate. From the theoretical analysis， a frequency multiplier design method based on a symmetric conical asymptotic matching structure is proposed， and the 335 GHz tripler is fabricated， assembled and tested. The measured output power of the frequency tripler is greater than 5 mW at 330-356 GHz， and the peak output power is 11.2 mW at 349.5 GHz under the 220 mW input power. The measured results verify that the design can increase the matching effect while fixing the diode position. At higher frequency bands， the effects of assembly accuracy on multiplier performance are issues that need to be addressed in future terahertz device designs and will be solved in monolithic design.
References
Zheng X， Liu C. Recent development of THz technology and its application in radar and communication system［J］. Journal of Microwaves， 2011， 27（1）：1-5. 10.1016/S1000-9361(11)60014-9 [Baidu Scholar]
Wang C， Lin C， Chen Q， et al. A 10-Gbit/s wireless communication link using 16-QAM modulation in 140-GHz band［J］. IEEE Transactions on Microwave Theory & Techniques， 2013， 61（7）：2737-2746. 10.1109/tmtt.2013.2262804 [Baidu Scholar]
Bowman T C， El-Shenawee M， Campbell L K. Terahertz imaging of excised breast tumor tissue on paraffin sections［J］. IEEE Transactions on Antennas & Propagation， 2015， 63（5）：2088-2097. 10.1109/tap.2015.2406893 [Baidu Scholar]
Jasteh D， Hoare E G， Cherniakov M， et al. Experimental low-terahertz radar image analysis for automotive terrain sensing［J］. IEEE Geoscience & Remote Sensing Letters， 2016， 13（4）：490-494. 10.1109/lgrs.2016.2518579 [Baidu Scholar]
Thomas B， Brandt M， Walber A， et al. Submillimetre-wave receiver developments for ICI onboard MetOP-SG and ice cloud remote sensing instruments［C］. In：Geoscience and Remote Sensing Symposium （IGARSS）， 2012 IEEE International. IEEE， 2012. 10.1109/igarss.2012.6351306 [Baidu Scholar]
Kangas V， D'Addio S， Klein U， et al. Ice cloud imager instrument for MetOp second generation［C］. In：2014 Specialist Meeting on Microwave Radiometry and Remote Sensing of the Environment （MicroRad）. IEEE， 2014. 10.1109/microrad.2014.6878946 [Baidu Scholar]
Yuan L， Mehdi I， Maestrini A， et al. A broadband 900-GHz silicon micromachined two-anode frequency tripler［J］. IEEE Transactions on Microwave Theory & Techniques， 2011， 59（6）：1673-1681. 10.1109/tmtt.2011.2123112 [Baidu Scholar]
Wang H， Pardo D， Merritt M， et al. 280 GHz frequency multiplied source for meteorological Doppler radar applications［C］. In：2015 8th UK， Europe， China Millimeter Waves and THz Technology Workshop （UCMMT）. IEEE， 2015. 10.1109/ucmmt.2015.7460628 [Baidu Scholar]
Jiang J， Chen P， Yue H E， et al. 0.68 THz and 1.00 THz triplers based on discrete Schottky diodes and quartz glass［J］. Journal of Infrared and Millimeter Waves， 2019， 48（2）：154-159. [Baidu Scholar]
TIAN Yao-Ling， HUANG Kun， CEN Ji-Na， et al. High power single and power-combined 100~115GHz Schottky balanced doublers［J］. Journal of Infrared and Millimeter Waves（田遥岭，黄昆，芩冀娜，等。大功率单路和功率合成式100~115GHz 肖特基平衡式二倍频器。红外与毫米波学报），2021，40（1）： 13-18. [Baidu Scholar]
Maestrini A， Tripon-Canseliet C， Ward J S， et al. A high efficiency multiple-anode 260-340 GHz frequency tripler［C］.In：Seventeenth International Symposium on Space Terahertz Technology. 2006. 10.1109/icimw.2006.368544 [Baidu Scholar]
CHEN Zhong-Fei. Research on 340 GHz solid-state frequency multiplier technology［D］. University of Electronic Science and Technology of China（陈忠飞。340 GHz固态倍频技术研究。电子科技大学）， 2016. [Baidu Scholar]
MENG Ze-Zu. Research on 330 GHz Terahertz Multiplier Chain［D］. University of Electronic Science and Technology of China（蒙泽祖。330 GHz太赫兹倍频链路研究。电子科技大学）， 2017. [Baidu Scholar]
Lv Xiaolin. 0.3 THz Monolithic Integrated Frequency Multiplier ［D］.University of Electronic Science and Technology of China（吕肖林。0.3THz单片集成倍频器。电子科技大学）， 2020. [Baidu Scholar]
Deng X， Tian Y， Huang K， et al. A 315–340 GHz frequency amplifier/multiplier chain with 8.5 dBm peak output power［C］. In：2020 IEEE MTT-S International Wireless Symposium （IWS）. IEEE， 2020. 10.1109/iws49314.2020.9360180 [Baidu Scholar]
Scott H J. The hyperbolic transmission line as a matching section［J］. Proceedings of the Ire， 1953， 41（11）：1654-1657. 10.1109/jrproc.1953.274192 [Baidu Scholar]
Klopfenstein R W. A transmission line taper of improved design［J］. Proceedings of the Ire， 2007， 44（1）：31-35. [Baidu Scholar]