On 3 mayo, 02:09, 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.
The signal injection is just a way of emulating a non 50 Ohm
termination where you need to change load and or cable length. By
using slightly off-carrier frequency, you emulate a changing phase of
reflection coefficient. That emulated reflected signal goes to the PA
and interferes with the forward signal (produced by the PA). That
interference is required as this modifies current and voltage at the
PA's active device.
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."
To be honest, I, and my ham friends, never had to use photon theory to
solve problems in both amateur and professional RF-Engineering. So I
think this doesn’t contribute to the discussion.
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.
Worth to try yourself:
1. Set up a simple linear circuit in simulation (for example AC
voltage source some impedance in series). You can use whatever (free)
program you like. Use a program that allows transient (non-linear)
simulation as you are working with two frequencies at the same time.
I used Beige Bag spice A/D professional version 4 (not free)
2. Determine the output impedance via load pulling (for various loads
if you like)
3. Do the same with the "VNA" setup (with the PA producing RF output).
You need to observe the envelope variations or you have to insert a
narrow band filter to suppress the carrier. I prefer observing the
sinusoidally changing envelope (as this shows distortion
immediately (intermod products) and saves me the settling time for the
filters).
In simulation you have ideal current meters, so if you use voltage and
current directly (and drop the directional couplers) you can skip the
conversion from reflection coefficient to impedance.
Here you will see that the output impedance is independent of VSWR
(as we started with a linear circuit) and both methods give same
results.
If you like, you could now implement a real PA in simulation and do
the simulation again. Note that tuning the amplifier in simulation is
(very) time consuming. You will notice that, depending on the
amplifier you implemented, impedance will change with increasing VSWR
presented to the amplifier (valid for both methods).
I did all the above with various circuits to prove the fitness of the
method. For some circuits there was a difference in result. After
evaluation, that difference was caused by changes in (bias) supply
during "manual" load pulling (that initially did not happen during the
slightly off-carrier signal injection). With stiff bias conditions,
the difference disappeared to within the accuracy limits.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK
With kind regards,
Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me in most cases.