I agree completely with all of that, but with the discontinuance of the K3s it seems that Elecraft doesn't really consider the K4 to be strictly a base station rig. Their continued insistence upon everything being 12 volt capable is a bit of a puzzlement for me.
73, Dave AB7E On 11/4/2019 1:37 PM, Joe Subich, W4TV wrote:
The issue is that 12V transistors are simply not capable of providing the "clean" voltage swings necessary to generate 100 - 120W peak power. A typical transistor/FET has a series output impedance of around 1 - 1.5 Ohms. Thus if the peak current is ~10A per device, the transistor is operating from cut-off to saturated - with the unavoidable distortion (clipping/saturation) at each end of that range. Simply put, insistence on more than about 60 W PEP from a pair of 12V devices is *asking for IMD* and "active predistortion" only masks that issue with a significant cost in complexity and DSP hardware. Equipment designers should give up on 12V devices for "base station" radios and go to 48V devices - even at 200 W PEP output levels. The designers should stick to 60 W output for 12V devices. That level is more than enough to drive an amplifier in the KPA-500 class to rated output and close (if not sufficient) for even a KPA-1500 and can be improved, if necessary, with much less complex precorrection systems. Redesigned with bias levels and transformers optimized for the lower power levels, perhaps with increased negative feedback, the K3/K3S could exhibit *much* - at least 10 dB - better IMD at 50 - 60 W PEP output (although efficiency would probably suffer for those who just absolutely need to run the rig from a 12V battery pack on a picnic table). 73, ... Joe, W4TV
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