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随着对二次谐波混频器的研究不断深入,出现了一种新型结构的二极管芯片——反向并联二极管芯片。通过将两只肖特基二极管反向并联,二次谐波混频可以得到很好的谐波混频特性,图1为芯片图片及其电流特性。
反向并联二极管工作原理已经得到详细分析[7],这里只列出结果:
$$ \begin{aligned} &i=a_{1} \cos \omega_{\mathrm{LO}} t+a_{2} \cos \omega_{\mathrm{s}} t+a_{3} \cos 3 \omega_{\mathrm{LO}} t+ \\ &a_{3} \cos \left(2 \omega_{\mathrm{LO}}+\omega_{\mathrm{s}}\right) t+a_{5} \cos \left(2 \omega_{\mathrm{LO}}-\omega_{\mathrm{s}}\right) t+\cdots \end{aligned} $$ (1) 式中:ωLO为本振信号的角频率;ωs为射频信号频率;an为n阶常系数。当本振(LO)频率与射频(RF)频率的一半接近时,低频分量只有2ωLO–ωs,只需加一个低通滤波器取出中频信号(IF)即可,电路变得简单。由于电路输出只有本振偶次谐波分量,奇次谐波在两个二极管环路运行,输出端的本振谐波分量减半,因此这种电路还具有输出噪声小的特点。这种二次谐波混频电路要求反向并联的两只二极管高度一致,通过将两只二极管一同制造在同一块芯片上,很好地满足了这一要求。
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混频电路制作在厚度为50 μm的石英基片(介电常数为3.78)上,反向并联结构的二极管芯片使用导电银胶通过倒装焊(flip-chipped)工艺粘在石英基片上的微带线上,实物和内部结构照片如图5所示。
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谐波混频器变频损耗(conversion loss)的测试方案如图6(a)所示。射频信号由笔者课题组研制的340 GHz 32倍频组件提供,在320~360 GHz频带范围内均有接近1 mW的功率输出,倍频器输出端连接功率衰减器,起到隔离和将射频输入功率调到接近仿真优化数值的双重作用;本振信号由所研制的16次倍频构成的170 GHz组件提供,在160~180 GHz频带范围内输出功率均大于10 mW,输出端加衰减器调节到本振所需的输入功率。
图 6 (a)测试系统框图;(b)变频损耗测试平台照片; (c)变频损耗仿真和测试曲线
Figure 6. (a) Block diagram of the test platform; (b) Photo of the conversion loss test platform; (c) Simulation and measured results of the conversion loss
变频损耗测试平台如图6(b)所示,RF和LO功率采用太赫兹功率计(PM5)进行标定,测试时RF信号在0.1~0.01 mW范围,功率计取2 mW档,读取小数点后三位作为RF信号标定值,为防止功率计精度对测试影响,进行两次测量取均值作为RF信号功率值。功率读值精度对混频器变频损耗指标测试很关键,这是由于RF信号功率较小,当取值为小数点后两位时,会出现0.005 mW和0.014 mW值都被读为0.01 mW,功率计的这种四舍五入取值会对变频损耗计算值造成高达3 dB的误差,在测试时需要重视。输出IF信号用频谱进行读取,其单位为dB,频谱取值已为多次扫描取平均值,因此中频输出数值取小数点后一位即可。
LO信号输入功率太低则变频损耗差,太高则会对混频二极管造成损坏,设计和测试表明LO信号功率值在5~8 mW范围变化时,变频损耗指标基本不受LO信号功率值影响,混频器正常工作。测试结果如图6(c)所示,在330~360 GHz频带范围内,变频损耗小于9 dB,达到同时期国外VDI公司的产品水平[14]。测试结果与仿真结果相比差损略差,分析原因是二极管芯片和石英电路装配时与仿真模型有差距,致使实际插损略高。
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接收机等效噪声温度是检验接收机性能的一项重要指标,表征其可探测到信号的最低功率数值。在太赫兹频率超外差接收机应用中,一般接收信号通过天线后直接连接混频器,因此接收系统的等效噪声温度取决于所用太赫兹混频器的性能。
Y因子测试法是目前测量接收机系统等效噪声温度最为通用的测试方法之一[14]。Y因子定义为在不同环境温度下(通常为冷温度Tcold 及热温度Thot)接收机输出功率的比值,如公式(1)所示。测试时设定Tcold为室内常温,将混频器接入接收机系统,首先测试常温时的噪声功率Ncold;黑体加热到标定温度(Thot)稳定后,测得高温时的噪声功率为Nhot,按照公式 (2)得到接收机系统的噪声温度(TDUT):
$$ Y = N_{{\rm{hot}}}/N_{{\rm{cold}}} $$ (2) $$ T_{{\rm{DUT}}}=(T_{{\rm{hot}}}-YT_{{\rm{cold}}})/(Y-1) $$ (3) 通过Y因子噪声测试方法对研制的谐波混频器进行了噪声温度测试,结果如图7所示。测试结果表明,谐波混频器在30 GHz带宽的范围内,双边带噪声温度低于1250 K,在310 GHz处最低为780 K,与国外同频段相比具有一定差距。当测试使用不同增益天线时,对测试结果影响较大,这主要是因为黑体发射的噪声信号被接收机接收的越多,测试噪声温度效果越好,因此在测试时尽量选用一些增益高的接收天线;谐波混频器的本振功率也与噪声温度有关,合适的本振功率可以有效地降低噪声温度。
A fixed-tuned 340 GHz sub-harmonic mixer
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摘要: 基于最新研制的小阳极结反向并联肖特基二极管芯片,设计和制造了320~360 GHz固定调谐分谐波混频器。混频器的结构采用的是传统电场(E)面腔体剖分式结构:将二极管芯片倒装焊粘在石英基片上,再用导电银胶将石英电路悬置粘结在混频器下半个腔体上。电路设计采用场路相结合的方法:用场仿真软件建立混频电路各个功能单元的S参数模型,将它们代入非线性电路仿真软件中与二极管结相结合进行混频器性能整体仿真优化。最终测试结果表明,谐波混频器的双边带在4~6 mW的本振功率驱动下,在320~360 GHz超过12%带宽范围内,双边带变频损耗均小于9 dB;混频器在310~340 GHz频带范围内,双边带噪声温度最低为780 K。声温度最低为780 K。Abstract: The design and fabrication of a fixed-tuned 320-360 GHz sub-harmonic mixer, featuring a newly developed small anode junction anti-parallel pair of planar Schottky diodes chip, are presented. A traditional E-plane split-block waveguide architecture was adopted in the mixer's design: the diodes chip was flip-chipped onto a quartz-based microstrip circuit and suspendedly glued to the bottom half of an equally split waveguide block with silver epoxy. A method of combination of field and circuit was applied to simulate and optimize the performance of the mixing circuit. Every functioning part of the mixing circuit was calculated with field simulating software to create its own S-parameters package, which was then combined with the diodes' barriers to be used by nonlinear circuit simulating software to simulate and optimize the performance of the mixer. The final test indicates that the ndicates that the mixer's double side band (DSB) conversion losses were lower than 9 dB, over 12% of bandwidth (320-360 GHz), with 4-6 mW of local oscillator (LO) power input; and at room temperature a minimum DSB equivalent noise temperature of 780 K was measured for RF frequency between 310 GHz and 340 GHz.
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Key words:
- fixed-tuned /
- sub-harmonic mixer /
- anti-parallel /
- conversion loss
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