Design of an optically transparent RF circuit

The wireless data transmissions between a reader unit and the communication module are based on the backscatter modulation, which is well known from the RFID principle. Therefore, Amplitude Shift Keying (ASK) is used for the bidirectional communication procedure. In this case, an envelope demodulator and a modulator circuit are needed. The concept for this analog frontend is descriped in [1, 2].

Further investigations focused on the design of an optically transparent RF circuit [3]. First, an optically transparent carrier material is needed. In this case, quartz glass is best suited since it has good optical characteristics as well as dielectric properties. There are many possibilities to realize transparent conductive strip lines. Nowadays, optically transparent conductive materials are widely used in applications such as photovoltaics, displays and touch screens. Frequently used materials for antennas are Transparent Conducting Oxides (TCOs) such as Indium Tin Oxide (ITO), Silver Nanowires (AgNWs) or meshed grid lines made of conventional metal layers. Our investigation showed that metal grid lines are best suited for the realization of RF structures.

The optically transparent analog frontend fabricated on a quartz glass is shown in Fig. 1. For the reception of the reader’s commands an envelope demodulator is realized by a Keysight HSCH-9161 zero bias beam lead Schottky diode and a 0.3 pF capacitor. In order to generate two reflection states an Avago VMMK-1225 high electron mobility transistor (HEMT) is used in a common source configuration. The usage of a HEMT offers low power consumption (6.3 μW @ 0.7 VGS) and a good backscatter modulation performance. The overall transparency is about 85 % (based on an area of 10 × 4.4 mm²) which is determined by the simulation software Zemax OpticStudio 15.5.