On 12/14/2013 11:18 AM, Jim Lux wrote:
Interestingly, the output Z of your amplifier is probably not 50 ohms
resistive in any case, but most autotuners don't "match to 50 ohms",
they "match to minimum reflected power" or "match to minimum VSWR", so
they take out the effect of changing amplifier Z too.
Jim, I am not sure I agree with your statement, at least not in the case
of standalone auto tuner. While I am not familiar with ins and outs of
practical auto tuner design, I do presume they use a 50 ohm directional
coupler at the RF input to compute the VSWR presented to the RF source
device. The tuning algorithm presumably aims to minimize this 50 ohm
referenced VSWR (again presuming the coupler is 50 ohms) in the same way
an operator aims to minimize VSWR on his VSWR meter when adjusting a
manual tuner. I would argue that is for all practical purposes "matching
to 50 ohms" or perhaps more correctly "matching as close as practically
possible to 50 ohms". This WOULD NOT take out the effect of the changing
output impedance of the amplifier.
If the auto tuner were part of the amplifier, then it would be possible
for the tuner to using additional information (like drive power and DC
current) to account for the changing amplifier output impedance.
73, Mike W4EF............
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