# Passive Harmonic RFID System for Buried Assets Localization
## Harmonic Tag
### Tag Characterization  
A reader based on laboratory instruments to characterize the tag has been designed. A block diagram of the experimental setup and a photograph are shown in Figure 6. It is composed of a signal generator (Rohde & Schwarz SMA100A, Moorpark, CA, USA) connected to a power amplifier (PA, Minicircuits ZHL-3010, New York, NY, USA). It is tuned at the fundamental frequency (\(f_{0}\)) and is capable of performing sweeps in frequency and varying its power level. A commercial circular-polarized UHF RFID antenna (Feig) is used in transmission. The receiver is a spectrum analyzer (Rohde & Schwarz FSP30, Cary, NC, USA) with a preamplifier and a Band-Pass Filter (BPF) tuned at the second harmonic band to avoid the saturation of the low-noise amplifier (LNA). A linear-polarized antenna (Geozondas AU-1.0G4.5 GR, Vilnius, lithuania) is used in reception. In addition, a multimeter is connected to the output of the RF-to-DC converter to measure the rectified voltage. An oscilloscope is used to measure the oscillation frequency and to check the oscillation of the tag. Figure 6: (a) Block diagram of the experimental setup based on laboratory instrumentation and (b) photograph. Compared with harmonic tags based on simple diodes [32; 33], the oscillator requires a power source that is obtained from the input rectenna. The RF-to-DC converter is based on a diode voltage multiplier using five stages of series-connected zero bias diodes (Avago Technologies, model HSMS-2852) [38]. The matching network at the input of the circuit is optimized to be matched between \(-20\) dBm and \(-10\) dBm. It consists of an LC matching network and it is tuned with the network analyzer. This circuit needs only 0.45 V DC to oscillate. From the characterization of the RF-to-DC converter that is shown in Figure 7, a threshold power of around \(-20\) dBm is required to bias the circuit. At this threshold power, the efficiency obtained with the oscillator connected to the output is about 15% (Figure 7b). The tag read range will be studied in the next section. To this end, the conversion loss of the doubler must be known. When the tag is modulated with an ideal square wave train, the expected spectrum is the Fourier transform of the waveform pulse at the harmonics of the modulating frequency (\(\hat{f}_{m}\)). Therefore, there is a modulation loss (K) with respect to the unmodulated case [49]. A prototype of doubler for characterization purposes has been manufactured. Figure 8 shows the measured conversion loss when the doubler is externally modulated with a 32 KHz square wave of 0.45 V (minimum DC voltage needed by the oscillator). The conversion loss is obtained from the difference between the measured power at the output at \(2f_{0}+f_{m}\) and the input power at \(f_{0}\), obtaining a value around 34 dB for \(-20\) dBm of input power. Figure 7: **(a)** Measured detected voltage as a function of input power at 868 MHz. The dashed line shows the minimum voltage needed to power up the oscillator (0.45 V). **(b)** Measured efficiency as a function of the input power at 868 MHz. Compared with the tag presented in the work of [38], the modulation frequency is determined by the quartz crystal, and it is not sensible to the variation of the bias voltage. Therefore, as the power reaching the tag overcomes the oscillator power threshold, the modulation frequency detected does not vary with the input power. The frequency modulation of the tag proposed in this work has several advantages. First, it overcomes the inevitable parasitic coupling between transmitter and receiver at the second harmonic. This undesired harmonic signal is mainly generated by the output power amplifier in the reader transmitter. Figure 9 shows a couple of measurements of the receiver spectrum using the setup of Figure 6 considering the tag over the ground located at 70 cm from the reader antennas and without the tag. As shown in Figure 9, a residual level at \(2f_{0}\) is always detected, despite the use of filters, even in the absence of the tag.