The Basic Idea
Instead of building lots of different radios to support multiple transmit frequencies, most 802.11 devices use a common technique to reuse radio components. The raw 802.11 data signal is first modulated with a baseband signal. It is important to note that at this level, all frequencies look the same. After this procedure, I and Q analog outputs are then converted to the final output frequency and then transmitted.
I and Q Review
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Local Oscillator
The Local Oscillator is a frequency generator that provides a reference signal used by both the receiver and the transmitter during the modulation process.
How this is used in the BCM43xx
The BCM43xx contains a programmable mixer which is calibrated to reduce LO feedthrough. The mixer is calibrated by setting it to a known state and the LO is set to a calibration frequency. The calibration states of the mixer are tested and measured for LO feedthrough for each setting until the minimum LO feedback is produced. This state is then used for the mixer during the next transmission until the next calibration is required.
These functions measure the local oscillator
The actual calibration is done by adjusting the LO.