On 4 mayo, 23:11, walt wrote:
On May 2, 8:09*pm, Cecil Moore wrote:
On May 2, 5:23*pm, Wimpie wrote:
I am very sorry Cecil, but I still don't see the point where the
discussed method may go wrong.
Everyone seems to be charging ahead, willy-nilly, without seeing the
point which is that there are other effects present besides
reflections.
Therefore carrying out a single-port measurement with a slightly off-
carrier frequency (to create non-coherence) under required output
conditions, will result in a meaningful output impedance.
Nope, it won't because virtual impedances don't cause reflections.
Only physical impedance discontinuities cause reflections. The rest of
the redistribution of RF energy is caused by the superposed
interaction between forward and reflected waves, i.e. interference
effects. Most hams do not understand the role of interference in the
redistribution of RF energy. Hope this helps.
http://micro.magnet.fsu.edu/primer/j...interference/w...
Please pay close attention to the last paragraph. "... when two waves
of equal amplitude and wavelength that are 180-degrees ... out of
phase with each other meet, they are not actually annihilated, ... All
of the photon energy present in these waves must somehow be recovered
or redistributed in a new direction, according to the law of energy
conservation ... Instead, upon meeting, the photons are redistributed
to regions that permit constructive interference, so the effect should
be considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."
You guys are presuming that reflections are the only thing you are
seeing and that is just not true. You are also seeing interference
effects without realizing it so your conclusions are doomed to failure
unless you can differentiate between constructive/destructive
interference and reflected waves. Since there has been no mention of
interference effects, I am forced to conclude that you guys are
ignorant of such effects.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK
During 1991 Warren Bruene used the RPG method in which he believes he
measured the source resistance of an RF power amp, which he calls
'Rs'. I have never agreed that his method measures the source
impedance, or that his data has any relevance to anything.
Consequently, I am not impressed with the discussion going on here
concerning applying a signal back into an operating RF power amp to
determine the source impedance. Please define 'source impedance'--
where is it located in the amp? At the plate? At the output of the pi-
network?. And how do you know the data obtained using this method is
correct? Have you verified it by comparing it with data obtained using
another method?
I made a statement in an earlier post that when measuring the output
impedance using the 'load pull' method we're not concerned with the
absolute maximum power that can be delivered, but instead, limiting
the 'maximum' power delivered to that which can be delivered with a
specific level of grid drive, one which allows the power *to be
limited to that of a normal operating level. Tom disagrees with this
position, that it is really the ABSOLUTE MAXIMUM power delivery that
should be considered. As you can see, I don't agree with Tom.
*I don't know how many on this thread have actually reviewed the
portion of my Chapter 19 that presents the step-by-step procedure I
used in determining the output impedance of the Kenwood TS-830S tx,
which shows precisely the output impedance appearing at the output of
the pi-network.
To summarize the procedure that I maintain will provide an accurate
measurement of the output impedance appearing at the output terminals
of the pi-network is as follows:
1) Adjust the loading and tuning controls of the amp to deliver all
the available power to a complex load in the amount normally used in
operation with the setting of the grid-drive level required to obtain
that output power.
2).Measure the impedance of the complex load.
3) The output impedance, or 'source' impedance of the amp appearing at
the output terminals of the pi-network is the complex conjugate of the
load impedance.
Now, when you measure the source impedance using the externally-
injected signal, does the data from that measurement agree with that
of the load-measuring method? If it does, then I'll agree that the RPG
method is valid. If it doesn't I'll continue to have considerable
doubt as to its validity. But I'd still like to know where the
resistance measured by this method is located in the amp.
Walt
Hello Walt,
Except for bias or supply voltage change due to load change, load
pulling does give similar results as off-carrier signal injection. I
did this in simulation for various circuits (linear and non-linear).
You may remember that I put something in a document (discussion of
last year, it is still on my website).
You can get different results in case of soft power supply or bias
supplies. In case of manual load pulling, bias/supply voltage may
change (think of change in grid current due to change in RF plate
voltage). There is sufficient time for all voltages and currents to
settle. I added a section on the envelope response due to bias
current/voltage change.
When you use the injection method (for example with 130 Hz offset), it
is like load pulling where you switch the load 200 times/s (more
specifically you rotate the phase of the reflection coefficient as
seen by the PA). In such a situation bias and supply voltages will
settle to an average value resulting in (slightly) different results.
While not relevant for here, but nice to mention, stiffness of bias
supplies has influence on IMD also.
With kind regards,
Wim
PA3DJS
www.tetech.nl