# Positioning Energy-Neutral Devices: Technological Status and Hybrid RF-Acoustic Experiments
## 3 Hybrid Acoustic-RF Approaches for Positioning Energy-Neutral Devices
### Hardware Implementation, Power Consumption and Backscatter Demodulation
#### 3.4.1 Hardware Implementation and Power Review  
Previous implementations [53; 54; 55] demonstrate the backscatter capabilities for IoT applications based on custom ICs. Their small form factor and ultra-low power consumption, often orders of magnitude lower than systems built with lumped components, are attractive features. Due to time and resource constraints in prototyping for R&D, we used  
Figure 6: Frequency spectra of the combined direct and backscattered received RF signals without the local oscillator at the backscatter device (left). The blue and orange lines in both figures represent the spectra for, respectively, a small and a large resolution bandwidth. By adding the local oscillator, the backscattered signals were shifted away from the carrier, improving distinctiveness and enabling reception at lower spectral resolution. off-the-shelf components, selecting options with low-energy consumption in both the active and sleep usage. An overview of this system is depicted in Figure 7 and can be divided into two parts: (I) the acoustic section in which the analog ultrasound signals are captured and transformed into a digitized version, and (II) the RF section, which uses these digital signals to switch the load of the antenna. The acoustic section consists of:  
* MEMS microphone (Vesper VM1000): key specifications in addition to its low-power consumption include its short wake-up time (below 200 us), high sensitivity, and wide frequency response. In-house measurements show that frequencies over 80 kHz can be received sufficiently, much higher than stated in the audible domain-focused datasheet (Note that this frequency range is true for the tested microphone's batch. No guarantee can be given that other future batches of this microphone type will behave similarly). * Amplifier (TI TLV341): this low-noise amplifier has a high UGB, providing a gain up to 34.8 dB for a 40 kHz signal in a single stage. Amplifying and filtering before zero-crossing by the comparator is more noise resilient and therefore gives better results. The single-supply rail-to-rail capability of this opamp makes for an easy implementation and for maximum signal amplitude swings. * Comparator (TI TLV7031): the rise and fall time are specified below 5 nanoseconds, giving a quasi-instant change of state when a zero-crossing occurs. The output of the comparator containing the digitized microphone signal is connected to an SPDT-switch (AD ADG839). This acts as a multiplexer, passing either the local oscillator signal (AD LTC6906) with a frequency of \(f_{clo}\) or a low-voltage signal. The local oscillator frequency is set with two resistors, and can take values between 10 kHz and 1 MHz. Implementing double sideband modulation, we accommodate for a total bandwidth of 80 kHz defined by the maximum chirp frequency \(f_{a,max}=40\) kHz. With the local oscillator bandwidth limitation, this allows for only 12 backscatter nodes to be active at the same time in the same acoustic area. The last part of this RF hardware section are the RF load switches (ADG904). A dipole PCB antenna consists of two \(\lambda/4\) copper strips with a small gap between them. For this antenna, at least two load switches are required to correctly short and open these two sides. The ADG904 has four multiplexer channels. This means that there are two more multiplexer inputs available on both of these chips, and other resistors, inductors and capacitors could be used as a complex serial load between the two copper traces, enabling additional antenna loads for improved reading distance, as proposed in [56]. An LDO voltage converter is missing from the schematic overview in Figure 7a. This LDO regulator converts a higher input voltage coming from an energy buffer into a lower voltage for a DC-offset in the acoustic signal and for powering the low-voltage components. Figure 7: (**a**) Block diagram and (**b**) PCB layout of the backscatter module.