5.9 A 37μW dual-mode crystal oscillator for single-crystal radios

The emerging Internet of Things (IoT) market is fueled by reductions in power, cost and size of wireless sensors. Wireless nodes reduce average power by using intermittent data transmission, which is synchronized by a continuously operating sleep timer in each node. In some applications, such as disposable sensors, low cost and small physical size are more important than achieving the lowest possible power consumption. Crystal cost and size is a limitation, particularly because each node requires two crystals. The first crystal, in the MHz range, is used to generate the PLL reference clock. The second crystal, usually 32.768kHz, is used to generate an accurate sleep timer used for synchronizing data transmission. A radio SoC may occupy 4×4mm 2 board area while a standard size for each crystal is 3.2×2.5mm 2 . Therefore, the size of the crystals can be larger than the SoC itself, limiting the applications.