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图5为220 GHz二次谐波混频器集成模块的实物图,以及内部石英微带线与SBD的装配情况。由实物照片可见,内部微带线底部采用导电银胶粘贴至沟槽底部且居中;共有三处金丝跳线,由于人工装配,部分金丝跳线存在明显角度和位置偏差。测试表明金丝跳线至玻璃绝缘子针尖部分牢固性一般,用少许导电胶来对该键合部分固化,以提高可靠性。但导电银胶形貌的不可控,会影响模块整体的传输函数。此外,金属腔体的加工精度、导电银胶涂抹均匀性,ADS中采用非SBD模型,以及SBD实际器件结构与模型结构的差异等因素,都会最终影响混频器的变频特性及噪声特性。从四通道模块整体结构可见,四通道的通道间距减小至7 mm,极大的缩短了接收机通道间的间距,其中两个IF输出端从模块的顶端引出,另外两个IF输出端从模块的底部引出(照片中没有显示),且模块可以线性无死区拼接,提高了线阵扫描系统的集成度。
图 5 220 GHz二次谐波混频器集成模块单通道实物图
Figure 5. Single channel image of 220 GHz sub-harmonic mixer integrated module
混频器的变频特性测试时,LO信号频率fLO设定为110 GHz。通过观察LO输入功率与Con-Loss的关系,发现本振功率PLO=7 dBm (5 mW)时,Con-Loss较低且稳定。对射频输入信号200~240 GHz的输入功率进行标定,固定LO输入信号频率fLO和功率PLO后,测试获得射频输入频率与单边带(Single Side Band, SSB)Con-Loss间的关系,测试结果如图6所示,这里给出了模块中三个通道的测试情况。测试结果表明:在本振频率fLO=110 GHz时,fRF在200~240 GHz的SSB Con-Loss为8.6~13 dB;在204~238 GHz的SSB Con-Loss为8.6~11.3 dB,该混频器IF在16~20 GHz时会急剧恶化,这是IF滤波器特性和前后阻抗匹配问题所导致的。此外,对频率点fLO和工作功率PLO进行了调整,fLO=108 GHz,PLO=3 dBm (2 mW)时,fRF在200~236 GHz的SSB Con-Loss为8.4~12 dB;在200~234 GHz的SSB Con-Loss为8.4~11 dB。测试结果与国内其他太赫兹混频器性能比较结果如表1所示,可见文中设计的二次谐波混频器的SSB Con-Loss性能较好,且在LO功率PLO为2 mW即可满足混频器工作,降低了集成系统对本振源的要求。
图 6 混频器在(a) fLO=110 GHz,PLO=7 dBm和(b) fLO=108 GHz,PLO=3 dBm条件下的SSB Con-Loss测试结果
Figure 6. Measured results of SSB Con-Loss for mixer under (a) fLO=110 GHz @PLO=7 dBm and (b) fLO=108 GHz @PLO=3 dBm
表 1 太赫兹混频器性能对比
Table 1. Performance comparison of terahertz mixers
基于该混频器集成模块,搭建220 GHz被动式接收机系统,对该接收机系统的噪声温度Tsys和NETD进行测量。该系统的前端中包括了WR4.3标准波导口太赫兹天线、220 GHz分谐波混频器集成模块、110 GHz三倍频器模块及Ka波段功率放大器模块。由信号源给Ka波段功率放大器模块提供初始基频信号。太赫兹天线被动接收高频信号后,与110 GHz的LO信号在混频器集成模块中发生谐波混频后输出IF信号,由于IF信号功率很低,约−75~−60 dBm左右,所以IF引出后接入到两级低噪声放大器(Low Noise Amplifier, LNA)模块(其中第一级输入频率1~9 GHz,噪声因子1.0 dB,增益17 dB;第二级输入频率1~25 GHz,噪声因子2.5 dB,增益41 dB)进行信号放大,经过1~7 GHz的带通滤波器和无源检波二极管输出直流电压信号,进行视频放大、积分电路以及输出集采分析。具体接收机链路结构以及拓扑结构见图7。
220 GHz接收机系统的噪声温度Tsys和NETD测试,采用高低温Y因子测试方法:太赫兹冷噪声源TL为常温噪声源,温度为室温,然后测试系统输出电压VL;高温噪声源TH采用红外热源加热吸波材料,该部分已经过定标,可设置高温温度,然后测试系统输出电压VH。测试过程中,通过置换常温噪声源和高温噪声源形成Y因子法所需的高低温噪声源。经过多次切换测试,最终计算得到接收机系统的噪声温度Tsys:
$$ {T_{{\rm{sys}}}} = \frac{{{T_{{\rm{{H}}}}} - Y{T_{{\rm{{L}}}}}}}{{Y - 1}};\;\;Y = \frac{{{V_{\rm{H}}}}}{{{V_{\rm{L}}}}} $$ (1) $$ {\rm{NETD}} ={T_{{\rm{sys}}}}{\left[ {\frac{{\rm{1}}}{{B\tau }} + {{\left( {\frac{{\Delta {{G}}}}{G}} \right)}^2}} \right]^{1/2}} $$ (2) $$ {\rm{NETD}} = \frac{\delta }{\theta } $$ (3) 为确定接收机噪声温度Tsys最低的工作点,需要对信号频率fLO和功率PLO不断调节。其中信号频率fLO由外加的信号源进行调节设置;功率PLO由110 GHz三倍频器模块的外加偏置电阻调节输出功率。而NETD除了受接收机的噪声温度Tsys影响,还受IF带宽B、积分时间τ以及系统增益变化量(∆G/G)等因素限制,如公式(2)所示,所以在满足数据输出频率的同时增大积分时间可以改善NETD。但是实际接收机系统的Tsys在中频带宽B范围内并不是常数,且混频器和LNA都存在增益温漂,所以实际测试的NETD要比公式(2)计算的理论值大。实际测试时,主要通过公式(3)计算NETD值,其中δ为接收机系统标准差,接收机系统响应度θ为(VH-VL)/(TH-TL)。测试结果发现,当信号频率fLO为108 GHz,PLO=5 dBm(3 mW)时,接收机系统的噪声温度Tsys最低,约为1 450 K。接收机系统的Y因子测试数据如表2所示,当积分时间电路设置为700 μs,获得NETD约1.3 K,该测试系统为太赫兹辐射计的实际应用做好铺垫。
表 2 220 GHz太赫兹接收机系统测试结果
Table 2. Test results of 220 GHz terahertz receiver system
TL/℃ VL/V TH/℃ VH/V δ/V NETD/K θ/mV·K-1 Y Tsys/K 22.8 3.738 55.2 3.808 0.002 8 1.352 2.14 1.018 4 1 450
220 GHz sub-harmonic mixer integrated module
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摘要: 在220 GHz二次谐波混频器的设计基础上,提出中频传输波导的垂直转换结构,实现了四通道混频器集成模块方案,缩短了混频器单通道的横向尺寸,为太赫兹接收机系统多通道线阵列集成提供了可行性方案。为优化系统模型的准确性,基于TCAD对肖特基势垒二极管进行三维半导体器件建模计算,依据提取的关键特性参数进行混频器的高频电磁波仿真。通过对该设计方案进行测试,结果表明:当本振频率为110 GHz,功率为7 dBm,射频输入200~240 GHz,混频器的单边带变频损耗为8.6~13 dB,在204~238 GHz的单边带变频损耗为8.6~11.3 dB。当本振频率为108 GHz时,驱动功率仅需3 dBm。此外,基于该混频器模块构建的220 GHz接收机系统,积分时间为700 μs时其温度灵敏度为1.3 K。Abstract: Based on the design of the 220 GHz sub-harmonic mixer, the vertical conversion structure of the IF transmission waveguide was proposed, and the four-channel mixer integration module was realized, the transverse size of the single channel of the mixer was shorten effectively. It provided a feasible scheme for the multi-channel linear array integration of the terahertz receiver system. In order to further optimize the accuracy of the system model, the three-dimensional semiconductor device modeling calculation was carried out for the Schottky-barrier diode based on TCAD, and the high-frequency electromagnetic wave simulation of the mixer was carried out according to the extracted key characteristic parameters. Through the test of the design scheme, the test results show that when the local frequency is 110 GHz, and the power is 7 dBm, the conversion loss of the mixer is 8.6-13 dB as the RF input is 200-240 GHz, and the conversion loss is 8.6-11.3 dB at 204-238 GHz. When the local frequency is 108 GHz, the driving power only needs 3 dBm. In addition, the 220 GHz receiver system based on the mixer module has a temperature sensitivity of 1.3 K as the integration time is 700 μs.
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Key words:
- Schottky-barrier diodes /
- 220 GHz /
- sub-harmonic mixer /
- linear array integration /
- conversion loss
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表 1 太赫兹混频器性能对比
Table 1. Performance comparison of terahertz mixers
表 2 220 GHz太赫兹接收机系统测试结果
Table 2. Test results of 220 GHz terahertz receiver system
TL/℃ VL/V TH/℃ VH/V δ/V NETD/K θ/mV·K-1 Y Tsys/K 22.8 3.738 55.2 3.808 0.002 8 1.352 2.14 1.018 4 1 450 -
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