A 24-GHz Front-End Integrated on a Multilayer Cellulose-Based Substrate for Doppler Radar Sensors

Abstract

This paper presents a miniaturized Doppler radar that can be used as a motion sensor for low-cost Internet of things (IoT) applications. For the first time, a radar front-end and its antenna are integrated on a multilayer cellulose-based substrate, built-up by alternating paper, glue and metal layers. The circuit exploits a distributed microstrip structure that is realized using a copper adhesive laminate, so as to obtain a low-loss conductor. The radar operates at 24 GHz and transmits 5 mW of power. The antenna has a gain of 7.4 dBi and features a half power beam-width of 48 degrees. The sensor, that is just the size of a stamp, is able to detect the movement of a walking person up to 10 m in distance, while a minimum speed of 50 mm/s up to 3 m is clearly measured. Beyond this specific result, the present paper demonstrates that the attractive features of cellulose, including ultra-low cost and eco-friendliness (i.e., recyclability and biodegradability), can even be exploited for the realization of future high-frequency hardware. This opens opens the door to the implementation on cellulose of devices and systems which make up the "sensing layer" at the base of the IoT ecosystem.

Keywords: Doppler radar sensors; Internet of things (IoT); all-natural electronic; circuits on cellulose; flexible substrates; green electronics; paper-based substrates; substrate integrated circuits.

Figure A1

Via-through connection in a multilayer cellulose circuit: structure (a) and simulated scattering parameters (b). The main geometrical parameters are the hole diameter $D_h=1.05\mathrm{mm}$, and via diameter of $0.2\mathrm{mm}$.

Figure A2

Branch-line coupler and patch antenna array. Branch-line: manufactured prototype (a) and experimental results (b). The coupler diameter is about $4.4\mathrm{mm}$. After Reference [39]. Antenna array:manufactured prototype (c) and experimental results (d). The antenna dimensions are $20\times 20{\mathrm{mm}}^2$. After Reference [40].

Figure A3

Manufactured layout (a) and experimental results (b) of the Schottky diode mixer. No components are soldered to the printed circuit board (PCB) shown in (a). The active area of the structure is about $4.6\mathrm{mm}$ in diameter. Measured and simulated conversion loss are compared in the plot; these results are obtained by sweeping the LO power between $-10\mathrm{dBm}$ and $5\mathrm{dBm}$ with $f_{RF}=24\mathrm{GHz}$, $f_{LO}=23.95\mathrm{GHz}$, $f_{IF}=50\mathrm{MHz}$, and $P_{RF}=-30dBm$. After Reference [46].

Figure 1

Block diagram of the radar sensor. The circuit uses a branch-line power divider to couple the voltage-controlled oscillator (VCO) to both the antenna and the mixer. If the target moves, a Doppler frequency shift $f_{\delta }$ is produced in the reflected wave and is detected by the mixer circuit. The resistance $R_{IF}$ and the capacitor $C_{IF}$ constitute a low-pass filter and terminate the mixer output port. LO: local oscillator; RF: radio frequency.

Figure 2

Fabrication process. (a) Photo-resist deposited on the metal layer and patterned using a mask, UV and a developer. (b) Wet etching of the metal surface. (c) Adhesive laminate after the etching of the metal layer: the adhesive material underneath is exposed. (d) Application of the sacrificial layer and removal of the protection layer. (e) Circuit transferred to the host substrate. (f) Sacrificial layer removal; the last step also removes the adhesive material. M: metal, A: adhesive, P: protection, R: photo-resist, S: sacrificial layer, SUB: host substrate.

Figure 3

Multilayer substrate. (a) Cross-section of the multilayer substrate structure adopted for the fabrication of the 24-GHz radar front-end. (b) Materials. Bulk copper with conductivity ${\sigma }_m=5.8\times {10}^7\mathrm{S}/\mathrm{m}$ is adopted to implement all the metal layers. The substrate parameters as follows: $h=230\mathsf{\mu }\mathrm{m}$, $t_a=30\mathsf{\mu }\mathrm{m}$, $t_m=35\mathsf{\mu }\mathrm{m}$. The photo-paper and the acrylic adhesive relative permittivity are: ${\epsilon }_r=2.9$ and ${\epsilon }_{r,a}=1.3$ respectively. The photo-paper loss tangent is: $\tan \delta =0.08$.

Figure 4

Fabricated 24-GHz front-end on a multi-layer cellulose-based substrate. (a) Antenna side. (b) Active circuit side. (c) Demonstrator including external VCO, intermediate frequency (IF) amplification and triggering stages. The used substrate area is $20\times 27{\mathrm{mm}}^2$. The realized cellulose circuit has the size of a postage stamp.

Figure 5

People detection results. (a) Experimental setup. (b) Analog output corresponding to a person at an 8-m distance with a relative speed of 1.7 m/s. (c) Analog and digital outputs associated to a person at a 3-m distance that slowly moves. In the last case the relative speed is of only 0.4 m/s. These signals are measured after the amplification and, possibly, after the triggering stages of the demonstrator (see Figure 4).

Figure 6

People detection results. (a) Time domain output signal corresponding to a person at a 6-m distance with a relative speed of about 1.5 m/s. (b) Frequency domain output signal obtained performing the FFT of panel (a). The main peak corresponds to a Doppler frequency of $244\mathrm{Hz}$.