On 27 abr, 23:18, Cecil Moore wrote:
On Apr 27, 1:43*pm, Wimpie wrote:

In other words, the amplifier sees a constant VSWR, but with
changing phase. Small frequency difference results in slow phase
change of VSWR.

From the IEEE Dictionary:

"impedance -

(1)(A) The corresponding impedance function with p
replaced by jw in which w is real. Note: Definitions
(A) and (B) are equivalent.

(1)(B) The ratio of the phasor equivalent of a steady-
state sine wave voltage ... to the phasor equivalent
of a steady-state sine wave current ...

(1)(C) A physical device or combination of devices
whose impedance as defined in definition (A) or (B)
can be determined. Note: This sentence illustrates
the double use of the word impedance ... Definition
(C) is a second use of 'impedance' and is independent
of definitions (A) and (B)."

The pinging experiment seems to be measuring a physical impedance (1)
(C) the nature of which is unclear. When the amplifier is outputting
power, it seems that the source impedance would be a V/I ratio (1)(B)
which doesn't respond to incoherent signals. Seems to me, you guys are
pinging something other than the source impedance.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Hello Cecil,

You may try to figure out how the signal injection method functions
(it is a form of active load pulling).

Can you agree with:

it doesn't matter whether:
-power reflect towards the amplifier is caused by load mismatch, or
-power is sent towards the amplifier by means of a phase synchronized
source.

This source is phase synchronized with the PA's exciter, so we have a
steady state system.

We assume small load mismatch (or low injected power towards the PA)
so that the operating point of the PA just changes slightly (to allow
linear approximation).

Now we insert a coupler between the amplifier and the load. This
coupler will measure the forward voltage generated by the PA, plus the
reflected part of the voltage that originates from the phase
synchronised source. Depending on the phase relationship, it can be
more or less then the forward voltage of the PA. If the PA shows 50
Ohms, the coupler's output would not change due to the signal
injection (as no signal is reflected by the PA).


We note the forward coupler's output voltage (both phase and
amplitude).

Now we change the phase relationship between the exciter and the
source that transmits some power toward the PA. Lets change 180
degrees and keep the amplitude the same. We again note the coupler's
output voltage (both phase and amplitude).

The voltage that is reflected by the PA equals half the complex
voltage change because of the phase change. Off course you have to
correct the readings because of the coupler loss. If you know the
signal that is send toward the PA, you can now calculate the complex
output impedance of the PA for small load change around 50 Ohms.

Instead of changing the phase of the source manually, you can do that
continuously and note the couplers output continuously. If you change
the phase of a signal continuously (with certain constant rad/s), the
result is a decrease or increase of frequency.

Assuming some reflection by the PA, the complex output from coupler
rotates around a certain point. That certain point is the result of
the PA's output power and the rotating vector is the result from the
injected signal that is reflected by the PA (back towards the load).

With a VSA you can discriminate between the voltage component from the
PA itself and the reflected component (with slightly different
frequency). With a normal spectrum analyser, you can only determine
the magnitude of the PA's reflection coefficient (or VSWR as you
like). Given the dynamic range of today's equipment, you can inject a
very low level signal that may mimic load mismatch well below VSWR =
1.1.

With respect to the impedance concept, we as amateurs do not use
steady state signals, as they contain no information. We modulate them
and are still using the impedance concept, despite the definitiones
you showed.

As long as the signal that is injected is well within the pass band of
the PA and it sufficiently low to allow linear approximation, the
concept of superposition and concept of impedance still holds. But if
you feel more confident with the manual phase change, or using two
known loads with known slight mismatch, I have nothing against it.
But if you have a VSA, some couplers and signal source at hand, it
may save lots of time.

With kind regard,

Wim
PA3DJS
www.tetech.nl