On Apr 26, 2:49*pm, Wimpie wrote:
On 26 abr, 22:21, Jim Lux wrote:



Measuring the output impedance (for relative small change in load) is
possible, but is not a simple task. Very likely other people will
comment on this.

ON9CVD's website I linked to has a very simple technique.. 50 ohm dummy
load and a 220 ohm resistor you can switch in.

At 100W (into 50 ohms), the 220 ohms would only dissipate 22W. *You
could get some of those non-inductive resistors from Caddock and series
them up to do something like this.

BTW, this is a simplified version of what's called a "load pull" test....
which makes me wonder if one could cobble up a quick test set that could
be controlled by a computer to do automated output Z measurements of an
HF transceiver over a reasonably wide range... One approach would be to
use a RS-232 controlled antenna tuner and, maybe, a antenna relay box
with several different load resistances).

The challenge (having actually looked at doing this with a LDG AT200PC)
is that the Z of the tuner isn't very well defined. *It's a pretty big
calibration project in itself.

Maybe, though, one could build a few test dummy loads.. say a 25 ohm and
a 75 or 100 ohm, along with your vanilla 50 ohm, and the antenna switch
(like an RCS-8V). *Basically, you're building a "high power resistor
substitution box"

You'd want some sort of nice inline watt meter (like an LP100) to make
the measurements.

Hello Jim,

Other method is injecting a slightly off-carrier frequency signal into
the amplifier (this emulates a constant small VSWR shown to the PA
(wtih 50 Ohms load), but with continuous varying phase). Because of
the difference in frequency, one can measure the forward (towards the
PA) and reverse (reflected by PA) signal with a two channel VSA.
This will give you the PA's complex output impedance.

Tom (K7ITM from my head) did this with a HP89410 with couplers.

With kind regards,

Wim
PA3DJSwww.tetech.nl

Yep, 'twas me. Using the 89410, I can resolve a signal removed from
the transmitter's output by a very small frequency offset, so it's
well within the bandwidth of the transmitter. With synchronous
averaging, the injected signal could be one or two Hz away from the
transmitted carrier, though it helps a lot to have a transmitter
running from a really "clean" (low phase noise) source if you're going
to do that. The injected signal can be many tens of dB lower in
amplitude than the transmitted signal. As I recall, I was using a
signal a kHz or so away from the carrier, still well within the
bandwidth of the RF power amplifier. One thing you have to be careful
about is either disabling ALC, or operating well outside the ALC loop
bandwidth; you don't want the ALC screwing up your results.

As Wim points out, this setup presents a load to the RF power
amplifier that's indistinguishable from an R+jX load that's
continuously varying, tracing a path around a little circle on a Smith
chart (a very tiny circle, when using signals that are very small
compared with the transmitted power). The rate the circles are traced
out is just the frequency offset between the transmitter and the test
signal.

Cheers,
Tom
K7ITM