Impact of LO Leakage in the New IF Frequency Range of the ALMA Wideband Sensitivity Upgrade

We reviewed the LO scheme in the current and the on-going upgraded receiver bands in ALMA system. Except Band-1, the fundamental frequency of the existing local oscillators in the ALMA receivers ranges from 12.4 to 25.8 GHz followed by various type of frequency multiplier. The frequency of these signals will overlap the new IF frequency range in the ALMA WSU (4-16, 4-20 GHz etc.). Therefore, spurious signal may enter in the new IF range due to the leakage. To investigate the phenomena, leakage power level at the IF output and the power ratio of the harmonics after frequency multiplication are measured in a W-band system. As expected, the "leakage" power level is proportional to the relative power ratio of the 5th/7th harmonic to 6th signal. It is thus recommended to choose the LO starting frequency as high as possible (> highest IF frequency + baseband bandwidth).