The frequency of the squeal is equal to the difference between the
carrier frequencies. Many of the TV transmitters I work with are now
required the be within +/- 3 Hz of their assigned frequency. We do that
by locking the oscillators to a reference derived from a GPS receiver.
They could easily be doing the same thing.
--
Gene Seibel KB0NNN
http://pad39a.com/gene/broadcast.html
Because I fly, I envy no one.