On 12/14/13 1:28 PM, Michael Tope wrote:
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.
FWIW, it would be easy to test, but you need a fair amount of power to
get it to tune: the 20dBm from a signal generator isn't quite enough.
But, say you took your ham rig, ran it through a 10dB pad, giving you a
pretty close to 50 ohm source, regardless of what the rig does. Then,
put a resistor in series or parallel to make it a 25, 75 or 100 ohm
source. Put a dummy load on the other side and let it tune.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|