Phase characteristic design of sub-harmonic mixer in a RF direct digital modulation

A common fourth-harmonic mixer used in RF direct digital modulation is presented in this paper. An important phenomenon has been observed in the mixer that a gentle variance in the LO power or IF power can result in sharp change of the RF signal phase. The effect has not been mentioned before, as well as theoretical explanation. The phase characteristic of an even-harmonic mixer is analyzed and design technique to control the phase characteristic of the mixer is presented. The test results demonstrate the feasibility of the design technique and good performances of the improved circuit.