I saw some papers mentioned the design of Gilbert Mixer. As the attachment, they use a parallel LC Tank below the degeneration inductors.
Paper[1] said, "a parallel LC resonator, tuned on the second RF harmonic, as an ac current source in the emitter of the input transistor pair"
Paper2] said, "The on chip LC Tank is tuned at the input RF frequency to provide the necessary common-mode rejection at RF frequency"
so i am a little confused, which harmonic ( first or second) should the Tank be tuned?
and what exactly the tank works? can anybody explain more detaily? thanks a lot.
[1]5.8GHz and 12.6GHz Si bipolar MMICs ISSCC1997
[2]Frequency-scalable SiGe bipolar RF Front-end design JSSC vol36, 2001.6