Wireless transmitters, like our environment monitor, use a battery management circuit to provide clean 3.3V power to the processor and wireless radio module. Here is our CO gas, temp and humidity transmitter (VP-TX-180):
The CO sensor is the brown device that looks like a AA battery. To the right are the %RH and Temperature sensors. This circuit works equally well with Zigbee and 900 MHz modules, and the Roving Networks Wi-Fi and BLuetooth modules.
Here is the battery circuit based on the Microchip MCP1256 chip:
The V_BAT signal goes to the PIC (PIC18LF14K22) to monitor the battery voltage. This signal used to trigger a transmission on low battery voltage. The PS_SL signal is a control signal from the PIC to put the MCP1256 into Low Power Mode while the PIC and Radio Module are sleeping. During sleep mode the MCP1256 has a minimal current draw of 0.1uA, with a higher output ripple.