On 9 jun, 02:07, Richard Clark wrote:
On Tue, 8 Jun 2010 12:17:37 -0700 (PDT), Wimpie
wrote:
When we go back to my first posting in this thread (the first reply)
where I stated that amplifiers do not show 50 Ohms in general, is not
that strange and doesn't violate Walt's conclusions.
Hi Wimpie,
Time to rewind:
On Thu, 27 May 2010 19:32:40 -0700 (PDT), walt wrote:
The source resistance data reported in Secs 19.8 and 19.9 were
obtained using the load variation method with resistive loads. Note
that of the six measurements of output source resistance reported in
Table 19.1, the average value of the resistance is 50.3 ohms obtained
with the reference load resistance of 51.2 ohms, exhibiting an error
of only 1.8 percent.
Considering these two explicit initial conditions and findings:
1. Using a Kenwood TS-830S transceiver as the RF source, the tuning
and loading of the pi-network are adjusted to deliver all the
available power into a 50 + j0-ohm load with the grid drive adjusted
to deliver the maximum of 100 watts at 4 MHz, thus establishing the
area of the RF power window at the input of the pi-network, resistance
RLP at the plate, and the slope of the load line. The output source
resistance of the amplifier in this condition will later be shown to
be 50 ohms. In this condition the DC plate voltage is 800 v and plate
current is 260 ma. DC input power is therefore 800 v ? 0.26 a = 208 w.
Readings on the Bird 43 wattmeter indicate 100 watts forward and zero
watts reflected. (100 watts is the maximum RF output power available
at this drive level.) From here on the grid drive is left undisturbed,
and the pi-network controls are left undisturbed until Step 10.
2. The amplifier is now powered down and the load resistance RL is
measured across the input terminals of the resonant pi-network tank
circuit (from plate to ground) with an HP-4815 Vector Impedance Meter.
The resistance is found to be approximately 1400 ohms. Because the
amplifier was adjusted to deliver the maximum available power of 100
watts prior to the resistance measurement, the averaged resistance RLP
looking into the plate (upstream from the network terminals) is also
approximately 1400 ohms. Accordingly, a non-reactive 1400-ohm resistor
is now connected across the input terminals of the pi-network tank
circuit and source resistance ROS is measured looking rearward into
the output terminals of the network. Resistance ROS was found to be 50
ohms.
Quite explicit. *Rated power into the rated Z load revealed a
conjugate basis Z match (afterall 50=50) or an image basis Z match
(50=50). *The pi-network simply transformed the source/load. *Anyone
skilled with the concept of the Smith Chart can immediately follow
that logical construct.
We even get the plate resistance which conforms to within published
specifications for the pair of tubes.
We even get the near classic result of a conjugate basis Z match
efficiency (however, only in the first pass approximation).
How asking the same question:
* "Does Walt's data support the evidence of a Conjugate Match?"
migrates into responses to other issues is a strange dance. *I ask "up
or down," and everyone wants to vote sideways.
When you don't touch the plate and load capacitor and change from 50
Ohms to a load with VSWR = 2, you violate Walt's conditions
I have not the vaguest idea where you got the idea that I changed
anything. *The totality of my discussion never budged from steps 1 and
2 and I deliberately and clearly confined all matters there and
repeated them more than occasionally.
In case of doubt use the forward power measurement technique,
I have, and the data I've asked from you was not intended for some
one-time design for a unique application. *This is an Ham radio group,
and the object under consideration is an Ham transmitter. *Feel free
to substitute freely among a similar class (frequency range, power
output) such that others might compare their own. *
when it
changes under varying load, your amplifier doesn’t behave as a 50 Ohms
source.
This is the most curious statement that eventually comes down the
pike. *It is like I am talking to a novice to explain:
* * * * "That is what the tuner is for!"
Maybe I had to be more precise to
mention the exceptions: forward power control loop as mentioned by
Roy,
What Roy is describing is nothing close to Walt's hypothesis that I
have confined to a single point observation. *No one has advanced
claims made that demonstrate:
1. *Constant Z across all loads;
2. *Constant Z across all frequencies;
3. *Constant Power across all loads;
4. *Constant Power across all frequencies.
No competent Ham expects that of any Ham transmitter.
Walt reported a 50 Ohm source Z from a tube rig. *I have measured a 50
Ohm Z from two of my own transistor rigs. *The difference in
technology is not an issue. *I have also been responsible for
measuring RF power and source/load specifications within the chain of
national standards laboratories. *Stretching the term RF, this spanned
from DC to 12GHz for power levels up to 100W. *The sources across the
board exhibited 50 Ohms Source Z.
Your own national lab is represented at:http://www.vsl.nl/
They undoubtedly use the same reference citations as our National
Institutes for Science and Technology.
***********
Putting the entirety of this discussion aside, and returning to your
very special project with an 9 Ohm source of RF at 8 Mhz. *How
efficient was its operation feeding a 50 Ohm line to a 50 Ohm load?
Can you document this with schematics, parts description, and measured
data at key points from drain through to the load? *This was the level
of available resources that came with Walt's discussion.
73's
Richard Clark, KB7QHC
Hello Richard,
We can continue mentioning measurement results, do other claims, but
that may not converge. Your results show 50 Ohms output impedance,
mine show other. Such results cannot be verified easily, so this
remains food for endless discussions.
I decided to use another approach that can be verified by people
having a (spice) simulator. I did simulations for some circuits that
can be expected in the amateur world and aren't exotic. I put them
over he
http://www.tetech.nl/divers/PA_impedance.pdf . Now people
can figure out what it IS and get their own educated opinion. I used
low frequencies to avoid discussion about the validity of the
simulations (parasitic inductances, etc).
From these simulations it is clear that small changes in load or drive
level result in large change of output impedance. This knowledge
resulted in my statement: "an RF PA isn't a 50 Ohms source", what was/
is heavily disputed by you.
Of course you can design to have better output VSWR under varying load
or drive, but this is generally not done for systems that just have to
deliver the power (transmitters).
If you want to reproduce some of the results yourself, we can compare
our spice netlists. I recently added a real class C example as this
was heavily discussed in another thread.
Best regards,
Wim
PA3DJS
www.tetech.nl
When you remove abc, PM will reach me.