ASK modulator based on switchable FSS for THz applications

Authors


Abstract

[1] An amplitude shift keying modulator that is based on a switchable frequency selective surface (FSS) is described for THz communication. The FSS uses a square loop aperture geometry, with each unit cell having four PIN diodes across the aperture at 90 degree intervals. To minimize the effect of bias lines on the overall frequency response of the FSS, a crossed-shape negative dc bias line, which is connected to the center of the FSS unit cell, has been placed on the rear surface of the dielectric substrate. Positive dc biasing is provided from the front side of the FSS structure. Simulation results are presented for two possible operating frequencies and two principal polarizations at normal and oblique incidence. These show that a free space modulator based on the proposed FSS structure would provide about 19 dB transmission loss at 600 GHz between on and off states of the PIN diodes for both transverse electric and transverse magnetic polarizations.

1. Introduction

[2] A terahertz (THz) communication system has been proposed as a means of achieving high-speed data transmission especially over short range such as indoor applications [Piesiewicz et al., 2005; Bird, 2006]. For a reliable and compact indoor THz communication system, efficient and high-speed modulators along with other electronic components are very important. Unfortunately, such modulators are not readily available which makes the realization of a THz indoor wireless communication system challenging. Successful development of these modulators holds the key for future THz communication. Recently, the amplitude modulation approach has been used to transmit a music signal at THz frequencies [Ostmann et al., 2004]. A two-dimensional electron gas (2DEG) modulator is used in this research with a channel bandwidth of few KHz. The improvement in this particular technique might allow achieving a bandwidth of few MHz, but this cannot provide data rates in the multigigabit range that are intended for future wireless communication. Another THz wave modulator based on a silicon oxide/polyaniline photonic crystal is proposed by Li [2007]. In this research the modulation is based on a dynamic shift of the photonic band gap by the applied external electric field with a 3 dB modulation bandwidth of 10 kHz at 1 THz. Therefore, this technique cannot be considered for very high throughput communication systems due to bandwidth restrictions. Some potential modulators described in the literature use a tunable phase shift with a liquid crystal device [Chao et al., 2004] and metamaterials [Paul et al., 2009; Ziolkowski and Cheng, 2004]. The approach proposed here is an amplitude shift keying (ASK) modulator based on a switchable frequency selective surface (FSS).

[3] FSSs are used as spatial filters for microwave, millimeter and submillimeter wave electromagnetic signals [Munk, 2000; Rogier et al., 2000]. A transmission-based ASK modulator is shown in Figure 1. Initially an unmodulated THz signal is incident on the switchable frequency selective surface (FSS) [Kiani et al., 2010a, 2010b; Sanz-Izquierdo et al., 2009; Kiani et al., 2008, 2007]. For reasons described by Bird [2006] a carrier of 600 GHz is chosen in the present work. The amplitude of the outgoing wave is reduced or increased depending on the characteristics of the FSS. This in turn will determine the properties of the modulated signal, such as modulation depth and ultimately the signal performance in the presence of noise. A unit cell of the proposed FSS is shown in Figure 2. PIN diodes are used in a FSS to switch between the on and off states. Typical models of the diode in these states are shown in Figure 4. A square loop aperture is used as the FSS element to obtain high bandwidth and good angle of incidence stability. Four PIN diodes are placed orthogonal to each other in a symmetrical arrangement on a unit cell of the FSS. The purpose here is to show that this ASK modulator could provide data rates in the gigabit range, as opposed to the modulators described by Li [2007], Chao et al. [2004], and Paul et al. [2009], and achieve acceptable RF performance. The entire structure could be fabricated on an indium gallium arsenide (InGaAs) substrate, which is a suitable material for operation at sub-THz frequencies.

Figure 1.

Proposed method for achieving ASK modulation using switchable frequency selective surface.

Figure 2.

The front view of the switchable FSS unit cell.

2. Description of Switchable FSS Modeling

[4] The front and the rear views of the active band-pass FSS unit cell are shown in Figures 2 and 3. A single layer active FSS could be designed on a InGaAs substrate, which has a dielectric constant and loss tangent of 11 and 0.008, respectively (http://cobweb.ecn.purdue.edu/mslhub/MaterialsDBase/MATERIALS/InGaAs/ingasframes.html). The thickness of the substrate is 2.5 μm. A square loop aperture is used as the FSS element to obtain high bandwidth and good angle of incidence stability [Munk, 2000]. The dimension of the square loop aperture is selected in such a way that the unloaded aperture resonates at a lower frequency (484 GHz) than the nominal operating frequency (600 GHz). The reason for this is that the inclusion of the PIN diodes and the dc bias lines in the periodic structure adds extra reactance to the band-pass FSS and this increases the resonance frequency. Four diodes are placed orthogonal to each other on a unit cell of the FSS to obtain a stable frequency response at oblique TE/TM incidence. The outer and the inner edge diameters of the square loop aperture are 74 μm and 72 μm, respectively. This makes an aperture width of 2 μm. The periodicity of the FSS elements is 75 um. The interelement spacing is kept small to enhance the angle of incidence stability [Munk, 2000]. Negative dc biasing is provided from the reverse side of the dielectric substrate by means of symmetrical cross-shaped bias lines. Their width is 1 μm. They are aligned with the orthogonal position of the PIN diodes on the front side of unit cell as shown in Figure 2. The metallic square plate on the front side of unit cell is connected to the center of the cross shaped negative bias lines with the help of a through pin having a diameter of 0.2 μm. The positive dc biasing is provided from the outer end of square loop aperture on the front side of FSS. As an ASK modulator, the performance of the switched FSS is determined by the ratio of transmission between on and off states of the FSS. Therefore, the overall RF performance of the switched FSS is of significance.

Figure 3.

The rear view of the switchable FSS unit cell.

3. PIN Diode Modeling

[5] The diode modeling is an important aspect of the design as the diode has a considerable effect on the overall performance of the switchable FSS. Several different diodes were considered and a diode was chosen with the most suitable specifications for the project. The equivalent circuit model of the PIN diode is also taken from the work by Alekseev et al. [1996] in which the diode in its on and off states are represented by combination of RLC series and parallel circuits (Figure 4). The typical values used for forward bias (on) are Ls = 21 pH, Rs = 0.5 Ω, Rd = 4.5 Ω, and Cp = 4 fF, while for reverse bias (off) the series resistance of Rd = 4.5 Ω is replaced by a capacitance of Cd = 2 fF in the circuit model. However, in the on state, only the forward resistance (Rs+Rd) is considered. This InGaAs-based PIN diode has a low turn-on voltage (0.46 V), low insertion loss (<1.2 dB up to 38 GHz), and high switching cutoff frequency (17 THz), as necessary for microwave and millimeter-wave switching and limiting applications. Since the FSS substrate is also InGaAs, the fabrication of the proposed FSS should prove possible.

Figure 4.

(a) Reverse bias and (b) forward bias equivalent circuits of the PIN diode used in FSS modeling.

4. Theoretical Results

[6] In this section, theoretical results for the switchable FSS are presented. It can be observed that the design has a fairly stable frequency response for both perpendicular (TE) and parallel (TM) polarizations at normal and oblique incidence.

4.1. FSS Without Diodes

[7] First, the FSS was simulated without PIN diodes to achieve a stable frequency response at normal and oblique incidence for both TE and TM polarizations. The frequency domain solver CST Microwave Studio (http://www.cst.com) was used for simulating the FSS unit cell. Metal in the FSS modeling was modeled as a perfect electric conductor (PEC). The pin (wire) which connects the cross-shaped dc bias lines to the center of the metal plate on the front side of unit cell was ignored in the simulations because CST meshing for such a thin wire created difficulties at oblique angles. However, it was found that the connecting pin has little effect on the resonance frequency of FSS. For both TE and TM polarizations, the transmission and reflection characteristics of the FSS at 0 and 45 degree incidence angles are shown in Figure 5. A stable frequency response was observed, as the resonance frequencies are 484 GHz (both TE and TM at 0°), 483 GHz (TE-45°) and 482 GHz (TM-45°). The reflection coefficients at these resonance frequencies are −24 dB, −26 dB and −23 dB, respectively. As far as transmission is concerned, an insignificant loss can be observed which is 0.3 dB at 0°, 0.4 dB for TE-45° and 0.2 dB for TM-45° angles of incidence. The −10 dB bandwidths are 32 GHz at 0°, 26.5 GHz at TE-45° and 48 GHz at TM-45°. It is noted here that TM-45° gives better bandwidth performance than TE-45°, however, when it comes to switching between on and off states the TE-45° gives better isolation than the TM-45° one as described later. Therefore, a trade-off would have to be made in a practical implementation of the FSS.

Figure 5.

Theoretical transmission and reflection characteristics of switchable FSS for both TE and TM polarizations without PIN diodes.

4.2. FSS With Diodes in Off State

[8] Next, four PIN diodes are placed orthogonal to each other on the unit cell of FSS and simulated. The lumped elements in CST MW Studio were used to model RLC series-parallel circuit as shown in Figure 4a. The FSS resonated at 600 GHz at normal incidence, but there is another resonance at 320 GHz due to the inclusion of extra reactance in the FSS model. Therefore, the switchable FSS modulator could be used at two frequencies. The sources, detectors and other related electronic components may be more easily available at 320 GHz than at 600 GHz. The theoretical results at both frequencies are described below.

4.2.1. At 600 GHz

[9] For both TE and TM polarizations, the transmission and reflection characteristics of the FSS at 0 and 45 degree incidence angles are shown in Figure 6. The resonance frequencies are 600 GHz (both TE and TM at 0°), 611 GHz (TE-45°) and 603 GHz (TM-45°). With the inclusion of PIN diodes, the angular stability is slightly affected. However, this may not be an issue if a 45° orientation of the FSS modulator is preferred in the THz communication system. The FSS in this case can be tuned to any desired frequency by taking a 45° angle of incidence as a reference. The reflection coefficients at these resonance frequencies are −29.7 dB, −33.1 dB and −25.5 dB, respectively. In this case the insertion losses are 0.4 dB at 0°, 0.5 dB for TE-45° and 0.4 dB for TM-45° angles of incidence, respectively. The −10 dB bandwidths are 29.5 GHz at 0°, 20.1 GHz for TE-45° and 32.1 GHz for TM-45°. Once again with the diodes in place the bandwidth of the TM-45° incidence wave is more than for TE-45° incidence one. Also, there are undesirable weak resonances at 550 GHz and 320 GHz for TM-45° which needs to be addressed in future research. Further work is required to determine whether these are genuine resonances due to the FSS model or a problem with the software such as meshing or convergence.

Figure 6.

Theoretical transmission and reflection characteristics of switchable FSS for both TE and TM polarizations when PIN diodes are in off state.

4.2.2. At 320 GHz

[10] The results for 320 GHz are also shown in Figure 6. The resonance frequencies are 320 GHz (both TE and TM at 0°), 329 GHz (TE-45°) and 327 GHz (TM-45°). The reflection coefficients at these resonance frequencies are −23.5 dB, −24.2 dB and −25.7 dB, respectively. In these instances the insertion loss is 0.2 dB at 0°, 0.4 dB for TE-45° and 0.6 dB for TM-45° incident waves. The −10 dB bandwidths are 10.6 GHz at 0°, 6.3 GHz for TE-45° and 12.5 GHz for TM-45°, respectively.

4.3. FSS With Diodes in On State

[11] The circuit shown in Figure 4b is used for switching PIN diodes from off to on state. Once again, lumped elements are used for modeling. The results are as follows:

4.3.1. At 600 GHz

[12] Figure 7 shows the theoretical transmission and reflection characteristics of switchable FSS when PIN diodes are in on state. The reflection coefficients for all angle of incidence and polarizations are almost 0 dB. However, the transmissions coefficients are −18.9 dB for 0°, −22.4 dB for TE-45° and −16.6 dB for TM-45° angles of incidence. Therefore, the FSS transmission can be switched by 18.5 dB at 0° while for TE- and TM-45° incidence it can be switched by 21.9 dB and 16.2 dB, respectively. As described earlier, it can be noticed that the switching quality at oblique TE incidence is better than TM. However, for TE polarization, the bandwidth is less than the oblique TM case. Therefore, in a practical system where the incident field is arbitrarily polarized, the bandwidth would be determined by the TM polarized component.

Figure 7.

Theoretical transmission and reflection characteristics of switchable FSS for both TE and TM polarizations when PIN diodes are in on state.

4.3.2. At 320 GHz

[13] The theoretical transmission and reflection results for this case are also presented in Figure 7. In this case as well, the reflection coefficients for all angles of incidence and polarizations are almost 0 dB. The transmissions coefficients are −25.8 dB for 0°, −29.2 dB for TE-45° and −23.2 dB for TM-45° angles of incidence. Therefore, the FSS transmission can be switched by 25.6 dB at 0° while for TE- and TM-45° incidence; it can be switched by 28.8 dB and 22.6 dB, respectively. The isolation between the transmitting and nontransmitting state of FSS is better at 320 GHz than 600 GHz.

5. Discussion

[14] Support is given to the simulation results presented in section 4 through the work of other research [Alekseev et al., 1996, 1997] and also simulation results obtained with another software package (Ansoft HFSS), compared with our results which were obtained using CST MW Studio.

[15] Please note that in our ASK modulator, we have used the diode presented by Alekseev et al. [1996]. The equivalent circuit model and the measured results are described in this reference. It is observed from the measured data in the work of Alekseev et al. [1996] that about a 20 dB transmission loss was achieved at 40 GHz when the PIN diode is switched between on and off states. The insertion loss is less than 1 dB.

[16] In other work by the same research group [Alekseev et al., 1997], an alternative equivalent circuit model of the PIN diode is represented. Comparing this circuit model with Figures 4a and 4b, it is observed that there is a little difference in the forward resistance of the diode in the on state (while the values of Ls and Cp are same). In addition, this diode was used for W band switching applications for which measured results are presented.

[17] Note in addition that in the ASK switchable FSS described here, we have used two diodes in one direction (TE polarization) and two in the other (TM polarization), which means that Figure 8b in the work of Alekseev et al. [1997] applies in the present configuration. To predict the switching characteristics of our ASK FSS modulator at 70 GHz, the simulations were run at frequencies from 70 to 750 GHz and the results are presented in Figure 8. It is observed that at 70 GHz about a 30 dB transmission loss was achieved which agrees with the results in Figure 8b of Alekseev et al. [1997]. However, there are differences in the results as the data presented by Alekseev et al. [1997] was for a different circuit configuration. Nevertheless, the results provided by Alekseev et al. [1996, 1997] tend to support our simulation results for a free-space modulator.

Figure 8.

Computed transmission and reflection characteristics of the ASK FSS modulator switching at 70 GHz.

[18] Further, to support the simulation results presented in Figures 6 and 7, Ansoft HFSS v12.1 was used to model and simulate the same design and some results obtained are presented in Figures 9 and 10 along with those of CST MW Studio. These two sets of results illustrate the agreement that is obtained.

Figure 9.

Theoretical transmission and reflection characteristics of switchable FSS using CST and HFSS at normal incidence.

Figure 10.

Theoretical transmission characteristics of switchable FSS using CST and HFSS at normal incidence.

6. Conclusion

[19] Design considerations and initial modeling results have been presented for a switchable FSS operating at 600 GHz for very high throughput communications. This free-space modulator provides about 19 dB transmission loss at 600 GHz between on and off states of PIN diodes at normal incidence and allows switching at modulation frequencies in excess of 70 GHz. The performance of the switchable FSS as a modulator is determined by the ratio of transmission between on and off states of the switch. The modulator could be used in two different configurations. For normal incidence it could be used as a transmission type while by titling the modulator at 45°, a reflection type modulation could also be achieved. Further work is now in progress to determine the experimental performance of the proposed FSS as a modulator. In this work we have used PIN diodes for switching. To obtain a higher transmission through and reflection from between the on and off states of the modulator, the use of MEMS and NEMS as switches could also be considered.

Acknowledgments

[20] We would like to extend our gratitude to Thomas Benke of Leap Australia for his support and guidance for setting up switchable FSS model in HFSS v12.1 to verify the results obtained using CST MW Studio 2010. Many thanks to Frank Demming of CST for his professional help during the course of this project.

Ancillary