When using the MAX2223, note that the VCO frequency is not the same as the LO frequency. For LO frequencies below 1125 MHz, the VCO runs at 4×LO due to a divide-by-4 output. For example, at 950 MHz LO, the VCO is at 3800 MHz. At 925 MHz LO, it’s 3700 MHz; at 1124 MHz, it’s 4496 MHz. At 1125 MHz and above, the divide-by-2 is used, so the VCO is 2250 MHz at 1125 MHz LO and 4350 MHz at 2175 MHz LO. This gives a VCO tuning range of 2250–4496 MHz. Some margin should be assumed at both ends, and overlapping bands likely ensure smooth transitions.