# Position Sensing with a Compact 0.1 THz Amplitude Modulated Source
## 2 Materials and Methods
### Structure of the THz System Layout and Devices Used  
The schematic diagram of the experimental setup which corresponds with the proposed measurement setup from the previous sections can be seen in Figure 3.  
The THz source used was a Gunn diode source with a central frequency of 100 GHz and modulation frequency of 1 kHz, as shown in Figure 4a. The horn antenna, which is also part of the source system, was designed and fabricated in LMFE to direct the signal in the desired direction.  
The focal lenses were represented by two polyethylene lenses [19]. The lens diameter was d = 70 mm and the focal point was f = 115 mm. The source and lens were mounted on a custom 3D-printed mount and then positioned on a tripod so that the height and angle of the source could be changed. The THz source has an additional lens holder to focus the illumination. The setup described can be seen in Figure 4b.  
The sensing element in the measurement setup is a titanium-based broadband microbolometer coupled to an antenna. The main property of the microbolometer is the thermally dependent electrical resistance [20]. The antenna is tuned to a certain THz band, receives the incident wave power and transmits it to the microbolometer. The higher the electromagnetic signal power, the larger the thermal changes and the larger the resistance difference and, thus, higher voltage drop, on the element. The whole sensor is built on a silicon membrane [20] with the frame on a printed circuit board (PCB). The size of the sensor and antenna depends on the application and frequency range used. The membrane size is in the range from 2.5 \(\times\) 2.5 to 5 \(\times\) 5 mm. In the prese  
Figure 3: Schematic diagram of the experimental setup.  
Figure 2: Zoom of the proposed measurement layout. Propagation of the THz signal when the horizontal plane is in the: (**a**) middle position, (**b**) raised vertically, and (**c**) lowered vertically.  
a broadband log-periodic antenna with a diameter of 3 mm fabricated on a 5 \(\times\) 5 mm membrane was used. The sensor mounted on the PCB used can be seen on the upper part of the PCB in Figure 4c.  
The sensor PCB consists of two voltage regulators, a LT1028 low noise operational amplifier (LNA) and passive elements. The sensor PCB is supplied with \(\pm\)5 V, which is regulated by two voltage regulators. The output of the antenna-coupled titanium-based microbolometer is a 1 kHz analog signal. Due to the low frequency, no impedance matching was required for the input of the LNA. The LNA with a gain of 1000 is needed to amplify the output signal of the sensor (in the \(\upmu\)V). This is required to achieve higher resolution using ADC.  
A linear sampling method with a linear stepper motor from Thorlabs was used to simulate the sensor array. The maximum range of the stepper motor is 50 mm. The stepper motor and the sensor PCB on the holder can be seen in Figure 4d.  
Data acquisition was performed using a custom-made PCB with the ADS8568 from Texas Instruments (ADC) and the Atmel SAMDE18A microcontroller (MCU) from Microchip. The sensor PCB and the data acquisition PCB were connected via a coaxial cable with a SubMiniature Version A (SMA) connector. The MCU controls data acquisition and  
Figure 4: Instruments used: (**a**) AM-THz source with Gunn diode and attached horn antenna, (**b**) AM-THz source and lens holder, (**c**) Sensor PCB, and (**d**) Sensor PCB holder with linear motor.  
conversion of the ADC and performs data synchronization with the PC via universal serial bus (USB).  
To process and display the acquired sensor data on PC, National Instruments LabVIEW software was used. The LabVIEW program also controls the stepper motor and synchronizes the data acquisition with the stepper motor position.