WASHINGTON (AP) — Federal Reserve Chair Janet Yellen said Wednesday that the central bank has no "fixed timetable" for raising interest rates but she believes the economy is ready for a rate hike by the end of the year. Under the change the Fed is mulling, all banks with more than $50 billion in assets would still have to undergo annual stress tests to see if they would have a sufficient capital buffer to withstand loan losses in a severe economic downturn.