Abstract
A D-band（110~170 GHz） direct detection radiometer front-end which consists of detector module， low noise amplifier module and standard gain horn antenna. The D-band detector is designed and fabricated based on zero-bias Schottky barrier diode HSCH-9161 and the measurement shows that the voltage sensitivity is larger than 400 mV/mW between 110~140 GHz， larger than 120 mV/mW in D-band and the maximal value reaches about 1 600 mV/mW@110 GHz. The D-band LNA module is packaged with self-designed MMIC and the module measurement shows the peak gain is 10.8 dB@139 GHz， the gain higher than 7.8 dB from 137 GHz to 144 GHz， the measured input return loss （S11） and output return loss （S22） are better than -5 dB and 8.5 dB in operating frequencies， respectively. Finally， the imaging experiments based on this front-end are carried out.
The millimeter-wave covers the frequency from 30 GHz to 300 GHz with the properties of relatively small wavelength and low-loss propagation in different frequency window
A D-band direct detection radiometer front-end is designed and carried out imaging verification in this paper. The radiometer front-end is built on detector module，low noise amplifier module and standard gain horn antenna. The D-band detector module is designed and fabricated based on zero-bias Schottky barrier diode HSCH-9161 and the D-band low noise amplifier （LNA） module is packaged with self-designed InP-based LNA MMIC. Finally， imaging experiments is carried out based on this radiometer front-end and have a good result， which lay a solid foundation for the practical application of the terahertz radiometer.
For improving the sensitivity and achieving good signal-to-noise ratio （SNR）， a low noise amplifier is an essential need of every radiometer system. The better that noise performance is the weaker signals can be received and analyzed which correlates directly with the sensitivity or range of imaging systems. Improving the noise performance of a receiver is main job of a LNA. The amplifier circuit has been manufactured using 100-nm gate length InP HEMT technology in Ref.13. In this paper， the selected amplifier has been assembled in an E-plan split-block waveguide package. The package design mainly includes transitions from microstrip to rectangular waveguide. The outer dimensions of the block are 30 mm×20 mm×25 mm and the LNA chips and chip capacitors are attached to the block with conductive epoxy glue.
Fig.1 (a) The split-block module with MMIC, (b) the photo of the LNA module
图1 (a)内部装配图, (b)低噪声放大器模块照片
The LNA module was measured using Rohde & Schwarz ZVA50 network analyzer with Rohde & Schwarz ZC170 （110~170 GHz） frequency extenders. The measurement results for the packaged LNA are shown in
Fig.2 LNA packaged measured S-parameters, on-chip S21
图2 LNA封装S参数与在片S21对比
The millimeter wave detectors play an important role in many millimeter wave systems such as power detection devices， direct detection receivers and imaging system. With the development of millimeter wave technologies， there is pressing need of compact and highly sensitive detector that operated at room temperature. Schottky diodes with the properties of low parasitic capacitance and series resistance provides an efficient solution for millimeter wave detection under room temperatur
A D-band detector was designed based on zero-bias Schottky barrier diode HSCH-9161 and
Fig. 3 Schematic diagram of D-band detector
图3 D波段检波器原理图
Fig.4 Small signal linear model of HSCH-9161
图4 HSCH-9161小信号线性模型
All passive networks including waveguide to microstrip， matching network and low pass filter of the detectors are analyzed and designed by HFSS and fabricated on 50-µm quartz substrate. The simulation model and simulated result of input waveguide to microstrip transition is shown in
Fig.5 (a) Simulation model of input waveguide to microstrip, and (b) the simulated result
图5 (a) 输入波导-微带探针过渡仿真模型，(b) 仿真结果
Fig.6 (a)Simulation model of low pass filter, and (b) the simulated result
图6 (a)低通滤波器仿真模型，(b)仿真结果
Finally， the quartz substrate was mounted to the waveguide block with conductive epoxy glue. The outer dimensions of the block are 20 mm×19 mm×15 mm and
Fig.7 Photo of the detector
图7 检波器实物图
The diagram of voltage sensitivity measurement setup for the detector is shown in
Fig.8 Diagram of sensitivity measurement setup for detector
图8 检波器测试系统框图
The voltage sensitivity （Rv） of the detector can be calculated by
| ， | （1） |
where Vout is the voltage measured by digital multimeter and Pin is the incident RF power presented by Erickson power meter. The sensitivity of the detector is estimated by this method and the result is shown in
Fig.9 The measured sensitivity of the detector
图9 检波器响应率测试图
When the detector is working， the effective detection current mainly refers to the current through the junction resistance Rj. Due to the junction capacitance Cj and Rj are in parallel， it plays a certain bypass function， resulting in a decrease in sensitivity. Also， the series resistance Rs divides the valid voltage. The operate frequency f of the diode can be defined as
| . | （2） |
As the frequency increases， the value of Rj decreases. Because the diode selected in this paper is suitable for W-band， as the frequency increases， the Rj will drop sharply， and the cut-off frequency is close to the D-band， so that most of the signal energy received by the diode is applied to Rs， which decreases Rj. The diode nonlinearity basically disappears， eventually causing its performance to be severely degraded.
An imaging system that based on LNA module， detector module and standard horn antenna has been designed to demonstrate the imaging performance of our radiometer front-end. Based on the above module， the schematic diagram of imaging experiment and the measurement setup are shown in Figs.
Fig. 10 Principle of imaging experimental
图10 成像实验原理
Fig. 11 Photo of the measurement setup
图11 成像测试链路图
First， we perform the imaging experiments only use detector and antenna and the imaging result is shown in
Fig.12 Imaging result of the radiometer front-end without LNA
图12 未加低噪声放大器的成像结果
Then the imaging experiments are performed with LNA module and detector module and the imaging result is shown in
Fig.13 Imaging result of the radiometer front-end with LNA
图13 加入低噪声放大器之后辐射计前端的成像结果
The noise equivalent temperature difference （NETD） is an important figure of merit for determining radiometer sensitivity. NETD is related to the thermal responsibility of the system and can be defined using system parameters as
| ， | （3） |
where τ is the integration time， B is the RF bandwidth， TS is the scene temperature and TR is the receiver noise temperature. Due to the bandwidth of LNA module is about 20 GHz， according to Eq.3， when τ = 1 ms and B = 20 GHz， the ideal NETD of the system are estimated to be about 0.2 K. For better image quality， increasing the bandwidth and decreasing the noise figure of LNA and choosing a diode that suitable for D-band for improving the voltage sensitivity is necessary.
In this paper， a D-band direct detection radiometer front-end which consists detector module，low noise amplifier module and standard gain horn antenna is designed and the imaging experiments have been carried out. The D-band detector module is designed and fabricated based on zero-bias Schottky barrier diode HSCH-9161 and the D-band LNA module is packaged with self-designed InP-based LNA MMIC. The imaging experiments are carried out based on this radiometer front-end and the results are compared with or without LNA module. After adding the LNA module the contrast to the image is improved， and the imaging result is better. This radiometer front-end lays a solid foundation for the practical application of the terahertz radiometer.
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