Wayne.B wrote in
:

Do they put 50 boards in parallel?




No, it actually has something like 248 boards, in parallel. There is one
board that always puts out 100 watts, the only board turned on at the
negative 100% modulation peak. The other boards are controlled by a long
driver board mounted outside the RF shield and hooked to them with big
ribbon cables. The modulation audio simply turns all but the one on and
off as the audio waveform is converted to the digital data. With no
modulation, half the boards produce a 50KW output carrier. As the audio
waveform goes up, more boards switch in up to 100% modulation peak, then
back to half and down to only the one 100W output, then back up. The
output amounts to a sum of all these boards' square wave switching
output. The big square wave is sent to a tuned circuit that filters out
the harmonics with more low-pass filters to keep the neighbors happy.
The output, of course, is an amplitude modulated sinewave that is quite
smooth, considering it comes from a digitized square wave. Being
actually a switching power supply at the RF carrier frequency, the boards
are 95+% efficient like the power supply in your computer....making the
company VERY happy as the electric bill comes down from 130KW load, the
rest wasted as heat, to 55KW load with only 5KW of heat coming off with
simple cooling fans. Simply amazing switcher technology, considering
AM's analog roots. Harris has a pretty good lock on the market for AM
transmitters, now.

The boards come in 100W, 200W, 500W and 1KW power levels, I think.
Binary switching them as the audio waveform is digitized makes quite
small power steps hardly recognizable in the modulated output.

Listen to almost any clear channel AM station in the USA and you'll be
listening to a Harris DX-50 these days. Hell, WWL on 870Khz was on the
air through the hurricane with theirs....near New Orleans.