http://www.vectorbd.com/bfd/packet/hfpacket.txt
09/04/11

AEA's PK-64 and PK-232 TNCs use 2110 Hz as Mark and 2310 Hz as Space, about in 
the same range as the traditional RTTY tones at 2125 and 2295 Hz (plus 
and minus 15 Hz).  

more here:

http://www.aprs.net.au/hf/hf-modem-tones