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Abstract-A new type of sigma-delta modulator that operates in a special mode named limit-cycle mode (LCM) is proposed. In this mode, most of the SDM building blocks operate at a frequency that is an integer fraction of the applied sampling frequency. That brings several very attractive advantages: a reduction of the required power consumption per converted bandwidth, an immunity to excessive loop delays and to digital-analog converter waveform asymmetry and a higher tolerance to clock imperfections. The LCMs are studied via a graphical application of the describing function theory. A second-order continuous time SDM with 5 MHz conversion bandwidth, 1 GHz sampling frequency and 125 MHz limit-cycle frequency is used as a test case for the evaluation of the performance of the proposed type of modulators. High level and transistor simulations are presented and compared with the traditional SDM designs.

Electronic devices in contact or in close proximity to the human body can use its conductive properties to establish body coupled communication (BCC) between each other. This human centric communication paradigm can be used for wireless body-area networks to reduce the impact of interference on/from RF systems, to avoid the fading effect that the body has on radio systems and to enable power efficient, high data-rate wireless links.BCC, without skin contact, can be realized via two electrode RX/TX devices capacitively coupled to the human body (Fig. 11.5.1); TX generates a variable electric field while RX senses the variable potential of the body with respect to the environment. The signal transfer along the body has high-pass characteristics with a corner frequency determined by the input impedance of the RX device. A signal attenuation of less than 70dB has been measured between devices placed at various positions on the human body (Fig. 11.5.1,[1,2]). Concerning interference, the body-channel is especially affected by interference below 1MHz while for higher frequencies the observed interference level is below -75dBm (Fig. 11.5.1). However, in the FM band the body starts acting as an antenna and this level may rise to -30dBm [4]. The BCC implementation in [3] used wideband digital TX signals and was very low-power but a cognitive FSK approach [4] was preferred to it due to the afore-mentioned high levels of FM interference above 50MHz.Here, as in [3], we couple a wideband digital signal to the human body but instead of a 50Ω input resistance for the RX we use high resistance so that the RX can effectively sense the 1 to 30MHz band, thus avoiding the FM band. Moreover, for further interference suppression, we designed a novel correlation-based RX that attenuates any signal that is poorly correlated to the expected one.The transceiver architecture is shown in Fig. 11.5.2. Figure 11.5.3 shows functional and measured waveforms. The TX performs Manchester encoding on the digital data-stream so that at least one voltage transition occurs per bit. The RX input stage is a clamped low-noise amplifier (LNA). For each bit-period the electrodes are shorted for a short time (20% duty cycle) and the DC level of the amplifier is restored. For the rest of the period the input resistance is very high and the input signal is amplified. This approach suppresses low frequency interference while the fast voltage transitions from the TX are amplified when they occur during the RX period. The bandwidth of the LNA is limited to 30MHz so that high frequency interference is attenuated. The interference is further suppressed by correlating the output of the LNA with a one-bit data template with transitions in the middle of the RX period (Fig. 11.5.3). For this purpose, the received signal is multiplied by -1 in the first half of the period and by +1 in the second half and the result integrated. The computed data correlation is maximal for digital-like transitions occurring in the middle of the RX period, while any other signal is str...

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