The Hittite HMC1048 mixer IC datasheet indicates pins 4 and 6 are N/C, although pin 5 = I/F output, which can reach several GHz of frequency. This seems very odd because adjacent pins on the other two ports ( RF and L/O ) are grounded, which good circuit hygiene would require. Furthermore, pins 10 and 12 are grounded ( according to datasheet ) although pin 11 has no signal on it... there is no need for adjacent grounds for pin 11. The pin descriptions ( and pin numbers ) on pdf page number 9 agree with the pictorial pin numbers on pdf page number 1, but this still makes no sense for a chip running as high as 18 GHz. The datasheet is current, document number v04.0614.
I am laying out a PC board for this device, I need an answer before I can submit it for fab. Not urgent, but soon... a few days.