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). Impact of LO Leakage in the New IF Frequency Range of the ALMA Wideband Sensitivity Upgrade Chau-Ching Chiong, Chen Chien, Teddy Huang, Yuh-Jing Hwang Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan LO Leakage in ALMA System Except Band-1, ALMA LO signal is generated by frequency multiplication from low frequency YTO (YIG-tuned oscillator) at ~ 12 to 18 GHz via various schemes. It starts from 12 GHz to avoid overlap with the IF frequency range. However, for some observation, as in the case of Band-6, when the LO starting frequency is close to the IF, as reported in Spurious Responses from IF Signals above 12 GHz (Napier 2010), LO2 will still down-converts significant LO “leakage” signal to the IF output because the roll-off response in the IF spectrum is not sharp enough. LO2 mixes LO signal at USB with sky signal at LSB (Napier 2010) Band-6 LO “leakage” at IF output (Napier 2010, in AIV-2057) Origin of the “Leakage” Signal However, the frequency multiplier is not ideal. At the output of the multiplier, not only the signal at the desired frequency (x6 or x3) but also the unwanted harmonics are generated. Band 6 LO scheme (From Saini et al. 2021). Filters are placed after frequency multipliers to suppress unwanted harmonics . Start freq. 13 GHz Output after x6 (GHz) Output after x3 (GHz) x1 x2 x3 13, 26, 39 x2 x3 x4 x5 x6 x4 52 104 156 208 260 312 x5 65 130 195 260 325 390 x6 78 156 234 312 390 468 x7 91 182 273 364 455 546 x8 104 208 312 416 520 624 x9 117 234 351 468 585 702 x10 130 260 390 520 650 780 These harmonics will also reach the mixer with various power levels. The “leakage” signal will be generated by mixer via mixing nearby harmonics. In this Band-6 x18 scheme, x1 signal can be generated by (x14-x15-x16), (x20-x21) etc. Bandpass filter YTO GHz 13 x7 GHz 91 x6 78 x5 65 x8 104 … WR10 cutoff 52 x4 GHz x18 234 x20 260 … WR3.4 cutoff 208 x16 … 195 x15 182 x14 156 x12 … x6 x3 x21 273 Harmonics in Band 6 LO generation. Numbers with strikethrough represent cutoff by waveguide (WR-10 and WR4.3). The mixer output spectrum is dominated by the LO leakages. We found a close relation between the leakage signal power and the 7th / 6th power ratio, which is thought to be the dominant scheme to generate 1st signal at IF output (left figure). Besides fundamental tone, other harmonics (x2, x3…) can also be found at the mixer IF output port (right figure). (x2, x3, x4) signal shows different slopes when plotting with the x6/(x7, x8, x9) suppression ratio, respectively. This implies multiple mixing schemes other than x6 are involved. Proposed Band 4+5 LO Scheme The starting frequency of the Band 4+5 in LO chain is 24-32 GHz (> IFhigh (18 or 20 GHz in WSU) + baseband bandwidth). The harmonic suppression in the frequency multiplier has to be strictly controlled. Bandpass filter does help, but cannot suppress all harmonics due to wide tuning range in WSU. Balanced frequency multiplier is the solution for achieving high harmonic suppression. Verification by x6 W-band Frequency Multiplier A commercial x6 W-band frequency multiplier by OML (S10MS-AG) is used to in