A low noise 1.8V LDO is integrated to provide the I/O supply for the output buffers. The LDO output is externally available to power a clock source such as a TCXO. A clean supply is provided to the clock buffers and the clock source for optimum phase noise performance. The input range of the LDO allows the device to be powered directly from a single cell Li battery. The LDO is enabled by either of the CLK_REQ_N signals. When disabled, the device enters a low power shutdown mode consuming less than 1μA from the battery. The LDO requires an output decoupling capacitor in the range of 1μF to 10μF with an equivalent series resistance (ESR) of at least 0.1Ω for compensation and high-frequency PSR. This capacitor must stay within the specified range for capacitance and ESR over the entire operating temperature range. A ceramic capacitor can be used if a small external resistance is added in series with the capacitor to increase the effective ESR. An input bypass capacitor of 1μF or larger is recommended.