phone interview twice, one from HR, one from hiring manager.

there is a phone interview from manager. I told him my background and my job expect. He told me his hiring plan and the product in design. It just last 15 minutes. after one week, I got a on site technical interview. I meet 4 group members. They ask me about my previous experience. and ask some basic CMOS analog design question. some questions are very tough. I can not give the answer. But I give out my thinking. I don't know if it is correct or not. But it is a solution. after one week, I got the offer.