Understanding LO signal in mixer circuit

[anvoice]

Hello, I'm trying to build a quadrature amplitude modulator (also a demodulator which is roughly the same thing in reverse) using two mixer circuits and a combiner/splitter circuit. I am planning to use an AD9959 signal generator, and an AD831 mixer. The AD9959 can produce two In phase and two quadrature signals (also has a clock port), and from what I understand, for the transmitter, up-conversion is performed by connecting a single I and a single Q signal to the IF ports on each AD831, and a signal will be available on the RF port.

What I have trouble understanding is the function of the LO (Local Oscillator) port. I understand it is typically driven by a sinusoid or a square wave, but where do I source this signal? Do I generate it via a microcontroller (in which case I'm guessing it has the form of a square wave) or does it come from elsewhere? Is it the same for both mixers in the modulator (and the two in the demodulator on the receiver side), or do I generate it individually for each?