# Sub-Milliwatt Transceiver IC for Transcutaneous Communication of an Intracortical Visual Prosthesis
## 3 Implanted Transceiver  
### IR-UWB Transmitter  
To achieve the low power consumption, the IR-UWB transmitter can be designed as a radio frequency integrated circuit (RFIC) rather than with discrete microwave components [26]. The IR-UWB transmitter can be implemented as an RFIC using CMOS technology. Since the transmit power of IR-UWB is low due to the FCC restriction [27], a simple CMOS IC is sufficient. This renders the CMOS RFIC transmitter to consume little power and it allows for integration with the downlink implanted receiver in the same IC. Figure 2 shows the overall circuit diagram of the proposed CMOS IC IR-UWB transmitter. The IR-UWB transmitter is comprised mainly of the modulator, the impulse generator and a current-controlled oscillator. The design of the transmit antenna, which will be connected to the chip, should be compact and wide band. Although this is also an important aspect for the overall system, it is outside the scope of this paper. To attain low power consumption and reduce complexity, an on-off keying modulation (OOK) scheme is used. It modulates the short pulses (impulse signals) that come from the impulse generator. The modulator can be implemented in CMOS technology by a simple D-Latch [28], making it a digital component. The data bits and the current-controlled oscillator clock are fed as the control signal and D-input of the latch, respectively. The resulting signal is fed to the impulse generator (see Figure 2). Implementing the impulse generator in CMOS technology can be done with a tunable delay element, followed by squaring of both the signal and its delayed version, and then a NOR-logic gate to form the pulses. The oscillator used for pulse generation in the pulse generator is a five-stage current-starved single-ended ring oscillator as reported in [29]. Figure 3 shows the circuit schematic of the five-stage current-starved single-ended ring oscillator. The difference compared to [29], however, is that five stages was used in our work, and an extra transistor for current control makes it differ from [29]. \begin{table}
\begin{tabular}{c c c} \hline  & **Uplink Transmitter** & **Downlink Receiver** \\ \hline Power consumption & \textless{}10 mW & \textless{}10 mW \\ \hline Data rate & \textgreater{}23 Mbps & \textgreater{}200 kbps \\ \hline Bit error rate & \textless{}10^{-3}\) & \textless{}10^{-3}\) \\ \hline Security & 3–7 mm short link & 3–7 mm short link \\ \hline Frequency band & 3–5 GHz & 4–12 MHz \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of system requirements. For tunable delay elements, a current-controlled shunt-capacitor as in [30] is proposed. Tuning is realised via transistors M1, M2 and M3, while transistors M4, M5 and M6 are used for balance, see Figure 2. Transistor M1 transforms the control current into voltage and biases M2. By changing the input current, the resistance of M2 is tuned. This changes the capacitive loading of M3 on the first stage of the inverter chain. The tunable delay elements add flexibility and help shaping the spectrum of the impulse signals as first explored in [31], our differs with an extra transistor to make it current-controlled rather than voltage-controlled. For proper impulse forming using a NOR-logic gate, the squareness of the signals coming from the tunable delay lines is crucial, and this is done by using three inverters on both the reference line and the delay line for symmetry. Figure 4 shows the IC layout design of the IR-UWB transmitter. From the figure it can be seen that the NOR gate, which is the impulse construction block, is made large enough to drive an antenna that could be connected to it in future work. Large transistor sizes improve the drive capability of the last stage, especially when logic gates are used [32]. Finally, pulse shaping filters are needed, which can be implemented off-chip either by relying on the transmission parameter (S21) from the biological tissue (skin) between the transmit/receive antenna or by a bandpass filter.