400hz comes from 2 onboard MG Sets and it is used for Synchros and Servos. The 400hz gives greater accuracy transmitting that kind of angular signal. If you want to know more check out the NEETS Navy Electricity and Electronics Training Series. I was a navy ET and we used the 400hz for radar and communication antennas. The folks who really knew it inside and out were the IC Men (Interior Communications) and there are quite a few in our trade.
