Hi baohulu, I suppose you are a young engineer from China who are looking for the way to be a good designer. But firstly, you must be patient and be honest.
Everybody here understood what aaron_do said, but you cannot???
Here, I have two questions too,
1. You have talked many times that you need to handle 8MHz IF bandwidth, what a wide bandwidth??? Indeed I have designed passive mixer with ten times IF BW of yours, and I followed a similar methedology that aaron_do mentioned, that really helps.
2. You use Low-IF Rx to design Silicon TV Tuner system??? That's really not a good choose, maybe you should debate this point with your system guy, but not waste your time here and make us frustrating. aaron_do just want to share his insight and experience with you.

Sorry for my not answering your questions. You can mail me if you are still struggling, but definitely I haven't such a good patience as aaron_do held.

BTW, I am also Chinese, and I knew so many designers at Chinese Mainland. They are young, smart, hardworking, graduated from top University, and continuously accumulated experiences. But many of them still need to learn how to be patient and modest, just as those their Mom told them once and once again. The world belongs to the youth, but will wait until they become matured.