Pins 4/6, which are next to the LO are shown as N/C on the data sheet, but pins 10/12 are
shown as ground, although pin 11 is N/C. This doesn't make sense; is there an error on the data sheet?
The data sheet for the HMC1048 is correct, pins 4,6 and 11 and NC.
The reason that you have GND on pins 10 and 12 is to connect more GND from DIE to the evaluation boards. There is no GND on pins 4 and 6 because some coupling inside the package and also they found for these IF frequencies the GND is not 100% necessary.
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