# Passive Harmonic RFID System for Buried Assets Localization
## Harmonic Tag
### System Operation and Tag Description  
The system is based on a harmonic reader configuration where the tag is placed over a pipeline as shown in Figure 1. The depth of the pipes of different services depends on the application, but the tag can be buried closer to the surface to facilitate its detection. This is an important difference compared with detection techniques based on GPR systems where the measured backscattered pulses correspond to the depth at which the pipes are buried. The reader interrogates the tag at the UHF frequency, \(f_{0}\) (a tone between 865 and 868 MHz). The tag is composed of a frequency doubler that generates the second harmonic at \(2f_{0}\), which is modulated (modulation frequency \(f_{m}\)) and transmitted by an antenna tuned at the second harmonic. This modulated signal is then detected and filtered by the reader where undesired reflections at \(f_{0}\) coming from the ground or other objects (clutter) are eliminated. Hence, the harmonic frequency component (\(2f_{0}\pm f_{m}\)) allows for detecting the presence of the buried tag. In this work, the tag is modulated using a low-frequency oscillator for tag identification and facilitates its detection. The modulation frequency is adjusted depending on the asset or pipeline to be detected. Therefore, the measurement of the modulation frequency allows for identification of the tag. In addition, the introduction of the tag modulation concept simplifies the design of the reader as the high isolation between the fundamental frequency and second harmonic is not required. The tag is inspired from a previous harmonic tag used as a humidity sensor [38], but with some modifications that have been performed to stabilize the modulation frequency, and the antennas have been redesigned for the underground operation. The block diagram of the proposed tag is shown in Figure 2. A prototype of the tag is implemented on standard 1.6-mm thick FR4 substrate, which is shown in Figure 3. The reader transmits a continous-wave (CW) signal at frequency \(f_{0}\) using a circularly polarized antenna. The tag receives this signal using a dual-polarized patch antenna tuned at \(f_{0}\). One of  
Figure 1: Block diagram of the proposed harmonic system (**a**) and an image of a typical installation (**b**). the outputs of this antenna is connected to the frequency doubler and a linear-polarized transmitter antenna at \(2f_{0}\), and the other output to an RF-to-DC converter. The converter's output is used to bias a low voltage oscillator that modulates the tag response by changing the bias of the Schottky diode at the frequency doubler. To this end, an oscillator based on two low-power NAND gates (74 AUP1G00, Plano, TX, USA) and a quartz crystal at the modulation frequency (32 KHz) are used in this work. The high-quality factor of the quartz resonator allows using low-resolution bandwidth filters (in the receiver) that reduce the noise and make the tag detection easier, because the oscillation frequency is constant independently of the harvested voltage in contrast with the previous tag [21]. The schematic of the oscillator is shown in Figure 4. The frequency doubler is based on a zero-bias Schottky diode (Avago Technologies, San Jose, CA, USA, model HSMS-2850) [21]. The fundamental signal at \(f_{0}\) is filtered using a high-pass filter (output filter in Figure 2) implemented with a \(\lambda/4\) stub at \(2f_{0}\). The second harmonic is blocked at the frequency doubler input using a short-circuited \(\lambda/4\) stub at \(f_{0}\) and a bypass capacitor. The output of the frequency doubler can be modulated by changing the diode DC bias point. To this end, a bias resistor (\(\rm{R_{bias}}\)) is inserted at the output of the oscillator. The quarter wavelength line end is connected to the ground at RF using a bypass capacitor. The high value of the resistor (1 k\(\Omega\)) also blocks the RF signal and limits the DC current consumption, while the DC return is allowed through the resistor. Figure 3: Photography of the implemented tag.