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.
No, they use some sort of transformer SWR bridge, with diode detectors
(and some use a phase detector too(SGC), to know which direction to
tune). It's not a directional coupler in the sense that there's a
sufficiently long line in terms of wavelengths and you couple off the
traveling wave like you do in microwaves.
More like a clever transformer scheme as seen in a raft of SWR meters.
These things measure forward and reverse power, but don't really have
a characteristic impedance in and of themselves. What's 50 ohms about
them is that the "calibration" is correct for 50 ohms.
Think about it.. how are you going to build a real directional coupler
that's 1" long for 160 meters? What you can do is build a form of
hybrid which has two outputs which are related to the forward and
reverse power.
http://www.sgcworld.com/Publications/Manuals/239man.pdf
has the schematic on page 26
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.
Sort of.. They have a routine that takes the ADC values for Fwd and Rev
and turns that into a number that is similar to VSWR, and they minimize
that. I don't have the listing handy, but it's soemthing like
(Vfwd-Vreverse)/fwd, and drive it to zero. The scaling from Vfwd to
actual forward power may change, but optimizing this will get you to a
match.
Since they never need to display actual VSWR, they don't bother to
normalize it or anything. They just have a known threshold for when to
stop adjusting.
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............
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|