[Amps] Harris DAX and other solid state considerations

[Rob Atkinson, K5UJ]

The advantages of solid state amps seem to center on among other things, the 
idea that they can be made of rf modules and augmented for various power 
levels as the user desires.  Yet no one has discussed the power supply 
issues with this.   It seems the options would be a separate supply for each 
module (not very practical to my way of thinking) or a big supply capable of 
powering several modules to the legal limit.  But what if you only wanted 
500 w.?  Would you want to buy a 1.5 or 2 kw+(U.S. export) level supply?  
Probably not.  Now the manufacturer is into making a bunch of different 
power supplies.   Sounds a little too complicated for me.

Harris DAX (This is from the Harris website):
<<<The Harris DAX-5 is one of the first in a dynamic family of 1-6kW AM 
transmitters designed from the ground up to provide superior IBOC and analog 
performance. DAX’s exceptional linearity and bandwidth will not only deliver 
the cleanest analog sound in this power range, but also the most accurate 
reproduction of the IBOC signal. Accurate IBOC signal reproduction with low 
bit error rate is essential to maximizing digital coverage.
With DAX, Harris - the company that has pioneered every AM modulation 
standard in use today - introduces a brand new 
high-efficiency/high-performance modulation technique. Called Digital 
Adaptive Modulation, this new modulation technology uses a digitally 
generated AM waveform with DSP based adaptive correction to give the user a 
high performance transmitter in a cost effective platform. Digital Adaptive 
Modulation samples the modulated output and corrects for non-linearity. The 
result is the cleanest, purest analog or IBOC signal in this power level.

When you combine Digital Adaptive Modulation with redundant “hot-swappable” 
RF modules, and a number of proprietary features, the result is 
irresistible: unmatched performance, reliability and cost-effectiveness 
today plus a “plug-and-play” migration path to IBOC - all at a competitive 
price. >>>

The Harris DAX and DX series, to clear up what sound like misconceptions, 
generally operate as follows.  The rf section consists of many p.a. modules 
that can be rapidly switched on and off.  They produce rf at different power 
levels.  Each is fed to a combiner.  by selectively turning on different 
modules in combination it is possible to generate rf in different small 
power steps covering a wide range of output wattage levels, that gives a 
close approximation to an analog amplitude.
Baseband signal from studio arrives at transmitter.  It is digitized and fed 
to a proprietary logic circuit
that, based on the amplitude encoded in the data, will switch on the 
appropriate modules to provide the proper output level for the given 
baseband level at any instant.

The combiner couplings smooth out the leading edge spikes of the module 
switching.

In theory, sure it would be possible to come up with something like this for 
a ham's dsb carrier and ssb station but as someone else mentioned and I 
think this is correct, this method (I believe it is class D) is not 
authorized for U.S. hams, but more important, anything like this would be 
way way more moolah than any ham would want to pay.  Now we are back to the 
different economic circumstances for hams and broadcasters.

Rob Atkinson
K5UJ

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