On Apr 1, 2:38 pm, Owen Duffy wrote:
I am intrigued that many people have attempted to measure the equivalent
source impedance of a transmitter with such varying results.

On the one hand is the assertion that a transmitter adjusted for optimum
operation is comparable with a linear source, and the source impedance
must therefore be the conjugate of the load.

....

I have a lot of trouble with that one, especially the "must therefore"
part. What is "optimum operation"? Is it delivering the most power
to the load, or is it delivering the RATED power to the load, at some
particular efficiency and level of distortion? I'd claim it's the
latter.

There are lots of examples of "optimum" load NOT being "conjugate-
matched" load. A typical stereo amplifier has an output impedance
that's a fraction of an ohm, but the amplifier is optimized to deliver
power to loads in the vicinity of 4 to 8 ohms, most often. The power
lines delivering power to a home show a source impedance that's a tiny
fraction of an ohm, but with everything in the house turned on, the
net load might be as low an an ohm--in rare cases a bit less than an
ohm. The load placed on a typical battery is similarly many times the
battery's internal resistance, except in the case of a heavy load on a
battery near the end of its charge. And lest you think that all
sources are optimized for load resistances higher than the source
resistance, I can change the feedback on that stereo amplifier without
changing the power output stage design, so the amplifier is still best
at delivering power to loads in the 4-8 ohm range, but now the output
impedance with new feedback is around 100 ohms.

So WHY should we expect a transmitter to represent a source impedance
particularly close to the load impedance, or to its complex conjugate?

I've gone through analyses similar to what you what you reported in
the remainder of your posting, with an output network whose Q I varied
(in the analysis), and come to similar conclusions. Just as you say,
Owen, when I do that, it's all clear and not magical at all. And the
source resistance can be made to be what I want through feedback, if I
wish. In some of the work I do, it's important to have a virtual ZERO
impedance at a particular node, but that's generally done using an AGC
loop, so the very short term dynamic impedance at that node may be
something considerably different from zero. But if you do power
measurements with varying loads, it will appear that the impedance
there is very close to zero. (Then you can put a 50 ohm resistor from
that node to a precision 50 ohm line, and have a very good 50 ohm
source; you can put another 50 ohm resistor from that node to another
line and have two matched sources, for testing other circuits...part
of a vector network analyzer.)

Cheers,
Tom

"K7ITM" wrote in news:1175469074.999185.17760
@d57g2000hsg.googlegroups.com:

On Apr 1, 2:38 pm, Owen Duffy wrote:
I am intrigued that many people have attempted to measure the
equivalent
source impedance of a transmitter with such varying results.

On the one hand is the assertion that a transmitter adjusted for
optimum
operation is comparable with a linear source, and the source impedance
must therefore be the conjugate of the load.

...

I have a lot of trouble with that one, especially the "must therefore"
part. What is "optimum operation"? Is it delivering the most power
to the load, or is it delivering the RATED power to the load, at some
particular efficiency and level of distortion? I'd claim it's the
latter.

Tom, I chose the word optimum for a reason, and I agree with you.

The design process does not find a drop dead maximum power in the way
that loading a source with a variable impedance finds a maximum power.
The rated power of an amplifier is a compromise, and dependent on the
available voltage and current, required linearity / IMD, active device
characteristics (eg saturation effects), dissipation limits (anode,
control grid etc), harmonic output, efficiency to name just a few. To
complicate crude experiments to determine maximum power output, the valve
is usually operated close to saturation, so small load changed result in
severly non-linear behavior. Further, apparent output impedance is
affected by the regulation of the DC supply, which is many transmitters
is better for short term current demands than sustained load.

For example, I have a Ameritron AL811H amplifier with 4 x 811A. The
operating point for SSB telephony is different to AM due to anode
dissipation limits. Some would suggest that when optimised for each of
the SSB telephony and AM operating points (ie different anode load
resistances) into a 50 ohm load, that the equivalent source impedance
*must* be 50 ohms, and that it happens without specific design
provisions.

Owen