HF upconverter circuit question
L5 is a high value inductor and is used therefore as a radio frequency choke i.e. it blocks RF at the normal frequencies your device uses. However it will pass DC and normal intermediate (IF) frequencies thus terminating the mixer correctly in 300 ohms at the IF pin.

C10 and C11 form an output attenuator on the clock signal and this is done, more than likely, to prevent the mixer being over-driven by an excessive clock voltage amplitude.

R2 is less clear - maybe the mixer prefers a clock source that is resistive in nature i.e. the 18 ohms serves that purpose. R3 correctly terminates the DC impedance at the local oscillator (LO) input in 300 ohms.