Hello Peng,
Thank you for the useful insights. As to your points,
1. Yes, the input RF port is well-matched for our desired operating frequency range. It was also verified with a network analyzer. The LO port is also seeing the proper power level from the synthesizer.
2. There was no conversion gain, rather there was only a conversion loss!
3. Indeed, the transformer we had already considered is the same ETC1-1-13 that works upto 3 GHz. However, we finally found the issue that has been plaguing all along --> The transformer-balun was only a 1:1 impedance transformer, whereas the Mixer IF port demands a differential impedance between 100 ohm and 250 ohm!
So, once the balun was altered to a 4:1 impedance transformer (corresponding to 200 ohm), the issue appears to have disappeared!!
Thanks a lot once again.