FSK Modulation

I am currently working on an RFID project and I have been able to successfully create an em style tag. I am now working on HID style tags more specifically the isoprox II which I think from my reading uses Manchester + FSK. I am not really sure how to implement FSK modulation on a Arduino at 125khz so I am asking if anyone can clue me in or tell me what I should read to learn this.