A Tunnel Reflection Amplifier for RFID Antennas

F. Amato, G. Durgin School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta, Georgia 30332–0250 [email protected], [email protected]

Abstract—Long range back-scatter RFID systems have in- energy level below the potential barrier, it can still be found creased the importance of a high signal-to-noise ratio received behind the barrier. In a highly doped p-n junction, there are at the reader. The signal-to-noise ratio is directly related to the energy levels in the valence band of the p-type material at the reflection of the loads used on the tag which is traditionally same energy as some levels in the conduction band of the n- limited between -1 (short) and 1 (open). By using a tunnel diode type material separated by a thin potential barrier [2]. From biased in the region, it is possible for the tag the theory, there is a finite probability that to amplify (Γ > 1) the back scattered signal. The paper illustrates this idea, explains how the tunnel diode has to be implemented, an can be found beyond the potential barrier; such and compares the theory to early results. probability increases as the barrier becomes thinner. Further requirement is that there should be unoccupied levels on the I.INTRODUCTION side to which the particle will tunnel. There are two main areas where the tunnel found Applying a forward to the junction causes a flow usage: switching circuits and sinusoidal applications for neg- of from the n-type side to the p-type side causing ative resistance amplifiers. In a reflection-type amplifier a an increase of the tunneling current followed by a current propagating signal incident on the negative resistance region of decrease; although the tunneling effect will end at a certain the tunnel diode (Fig. 2) is reflected with increased amplitude point, the p-n junction current will keep flowing and it will [1]. Such application is of high interest in RFID semi-passive increase with the applied voltage. A negative resistance region antennas as it would allow to increase signal-to-noise ratio (Fig. 2) makes the tunnel diode interesting to be used as a received at the reader. single port amplifier [1]. Back-scatter systems use a reader that transmits a contin- First part of the work consisted in studying the behavior uous wave (CW) signal to the tag which receives the CW of some tunnel diodes provided by Aeroflex/Metelics (Model signal. The CW signal enters through the tag antenna and MBD5057-E28) and comparing them with the expected results between two loads to modulate data into the CW and theory. To measure the Direct Current curve shown in Fig. waveform which is reflected back out of the tag antenna. 2, the tunnel diode, in series with a 1 Ω , has been The modulated signal then propagates back through the air connected to a DC power supply; a multimeter has been used to the reader where the signal is demodulated and processed to measure the current across the circuit and a voltmeter has by a computer. Traditionally, a short and an open are used as been used to monitor the voltage across the resistor in order to the two loads in order to keep them as far apart as possible be able to compute the exact voltage acting on the tunnel diode. from each other to maximize signal-to-noise ratio for positive Fig. 3 shows the schematics of the experimental setup used to resistance loads. If the negative resistance of a tunnel diode is test the tunnel diode. For each 0.02 V step of the DC power used for back-scatter technology, the signal-to-noise ratio can supply the current in the multimeter and the voltage at the be dramatically improved at the expense of more consumed port of the resistor have been saved and the data subsequently power to bias the tunnel diode (Fig. 1). processed to find the voltage across the tunnel diode; Fig. 2 shows the results of the DC curve. The negative resistance region is between 56 mV and 280 mV which is where the tunnel diode must be biased to behave as a negative resistor. The currents ranged from 561 µA to 129 µA. The slope shown in Fig. 2 is the inverse of the resistance which is calculated in Eq. 1. The resistance is -518, but this is only valid over the region from 56 to 280 mV as previously stated. The negative resistance of the tunnel diode is:

0.28 − 0.056 = −518Ω (1) Fig. 1. General back-scatter system with tunnel diode 129 · 10−6 − 561 · 10−6

II.THEORY III.EXPERIMENT A tunnel diode works using the quantum mechanical phe- The aim of the work is to demonstrate the feasibility of a nomenon of tunneling: although an electron (or a hole) have reflection type amplification using a tunnel diode. A reflection- Fig. 5. Schematic of the designed reflection amplifier

Fig. 2. DC curve of a real tunnel diode purpose, an appropriate amplifier circuit has been designed and realized to be used at the center frequency f0. Fig. 5 shows the schematic of the circuit in which the inductance L has been used to allow to bias the tunnel diode at the required voltage. The experiment consisted in connecting port 1 of the with a signal generator providing a source of -10 dBm at f0 = 4.1 GHz, port 2 with the amplifier connected to the biased tunnel diode and port 3 with a spectrum analyzer to measure the output signal. If port 2 is terminated in a negative resistance and a signal wave enters port 1, all the energy emerges at port 2, is reflected and amplified by the negative resistance, re-enters port 2 and emerges at port 3. The gain of the amplifier will be the square of magnitude of the reflection coefficient at port 2 [1].

Fig. 3. Circuit used to draw the DC curve of the tunnel diode G = |Γ|2 (3) type amplifier can be implemented by connecting a signal The performed experiment measured the input and the generator and the tunnel diode to a circulator in order to be able output power from the amplifying circuit at f0 = 4.1 GHz; to observe the amplified signal incident on a spectrum analyzer the tunnel diode has been biased at a DC voltage (0.18 also connected to the circulator [3]. The setup is shown in Fig. V) that puts it in the negative resistive region. The power 4. A circulator isolates the reflection amplifier from both the from the signal generator is reflected back to the spectrum generator and the spectrum analyzer; the reflection amplifier analyzer and the received signal level has been observed. A terminates the transmission line of characteristic resistance R0 DC block at the spectrum analyzer port has been used to in a negative resistance |R|, giving a reflection coefficient avoid DC current to leak across the amplifying circuit. Early which is negative and greater then 1. results showed an amplification of the input signal of 3.6 dB at the working frequency while biasing the tunnel diode in − |R| − R the negative resistance region; no amplification effects where Γ = 0 (2) − |R| + R0 observed while biasing the tunnel diode at a voltage outside the negative resistance region (above 0.2 V). To use a tunnel diode as a reflection amplifier the following Through these first experiments, it has been possible to criteria need to be met: biasing the diode in the negative demonstrate the possibility of using a tunnel diode to amplify resistance region of the DC curve and have a transmission line back-scattered signals of semi-active RFID tags. Further stud- with a negative resistance at the center frequency f0. For this ies and investigations will follow with the aim of integrating a tunnel diode reflection amplifier in a RFID experimental tag antenna.

ACKNOWLEDGMENT This work has been possible thanks to the support of the US-Italy Fulbright Commission.

REFERENCES [1] J. O. Scalan, Analysis and Synthesis of Tunnel Diode Circuits, John Wiley & Sons Ltd., 1966. [2] R. F. Pierret, Fundamentals, Addison Wesley, 1996 [3] M.K. McPhun, U.H.F. tunnel-diode amplifier, Electronics Letters, 1969. Fig. 4. Experimental setup