I found [this][1] quite useful paper, which describes the design of a commercial 802.15.4 transceiver. It says:

> "The modulated and spread I/Q baseband signals are applied to the
> digital-to-analog converters (DAC), whose outputs are low
> pass-filtered and up-converted directly to RF by a single-sideband
> modulator."

So my understanding now is that the 2 MHz modulated baseband sinewave is simplify [multiplied together][2] with 2.4 GHz carrier signal, effectively modulating the amplitude of the 2.4 GHz sine wave. Can someone confirm that this is correct?


  [1]: http://www.microwavejournal.com/articles/652-transceiver-design-for-ieee-802-15-4-and-zigbee-compliant-systems
  [2]: http://www.microwavejournal.com/legacy_assets/images/0509_31_img04.jpg