Hi jdobler, we are finally replacing the part, we'll see...
They have asked me though to check if the design was right, before mounting the new part. Let me ask you a few questions:
1. The LO input was dc-coupled (0V) with a center-tapped transformer (being the center tap connected to ground). This didn't work, the signal was strangely deformed. I took out the transformer and replaced with a couple of capacitors (ac-coupled), and it started working. Strangely the DC of the signals was around -2.5V.
2. The RF input was dc-coupled (0V) with a center-tapped transformer, and working. Is there any problem in having one dc-coupled and the other ac-coupled? (and is one preferred to the other?)
3. Because I don't use the output amplifier, the AN&AP inputs are unconnected (which I think it shouldn't be a problem), and all the pins from the output amplifier are unconnected too (I wonder if I should connect VFB to OUT, and COM to gnd, even if I don't use them). The datasheet doesn't give a clue.
Thanks for your support. I think some information is missing in the datasheet in this regard 