On 6 jun, 19:00, Richard Clark wrote:
On Sun, 6 Jun 2010 07:42:31 -0700 (PDT), Wimpie
wrote:

However a PA is not a 50 Ohms source

Hi Wimpie,

You say this like others, with the air of "knowing." *However, when I
ask in response of those who "know" what the PA is NOT, what IS it?

Give me the Z value of your transmitter. *Specify all initial
conditions.

We have had lengthy correspondence with Walt Maxwell's very rigorously
measured Kenwood TS830s that demonstrates a Z of 50 Ohms, or nearly
that as is practicable (say +/- 20%); and yet your voice was missing
from this discussion with evidence to the contrary.

73's
Richard Clark, KB7QHC

Hello Richard,

Try the following experiment:

Measure the forward power of your PA at a convenient load. Use a
directional coupler for that, not a voltage meter calibrated for
power. The "SET" or "CAL" position of a VSWR meter can be used as
forward power indicator.

Now make some mismatch (for example VSWR=2 at different phase) and
read the forward power. Did it change? If so, the output impedance is
no (longer) 50 Ohms. If possible, disable automatic protection to
avoid changing drive level. I don't know the value for my FT7B, but I
know forward power changes with load variations (as I use it as
"measuring instrument" sometimes).

Virtually all power amplifiers I designed do not have a large signal
output impedance of 50 ohms under significant load change. For some I
measured it because of a discussion on this between colleges.

Measuring method used: change in resistive load, from voltage change
you can calculate the current change, hence the output impedance. One
note, except two, all where solid state.

All high efficiency designs (class E, D) that I did have output
impedance far from the expected load impedance. With "far" I mean
factor 2 or factor 0.5. I did not measure that (as it is not
important in virtually all cases), but know it from the overload
simulation/measurement and I did the design myself.

The reason for not being 50 Ohms (after matching) is that when you
change the load, the active device will go into voltage or current
saturation. This is not a hard process, so for small load variation
(low VSWR values), forward power will not change much. I think this is
especially true for vacuum triode PA where you have significant tube-
internal feedback. For large variation (for example VSWR = 2.5,
reflected power 18%), you will notice change in forward power for most
power amplifiers.

High efficiency CW amplifiers use saturated switches (for example
half, full bridge or push-pull), so behave (seen at the active device)
as a voltage source. Depending on the total phase shift of the filter
sections this may convert to a current source behavior (seen at the
output). I had to spent much time to avoid destruction of some
circuits in case of mismatch.

If you can drop me a link to the discussion of the TS830s, I would
appreciate that.

Best regards,


Wim
PA3DJS
www.tetech.nl
Remove abc first in case of PM.