On Mon, 1 Sep 2003 10:19:39 +0100, "Ian White, G3SEK"
wrote:

Richard Clark wrote:
On Sun, 31 Aug 2003 19:42:42 +0100, "Ian White, G3SEK"

Motorola publishes the equivalent series (or parallel) resistance for
the MRF-xxx (pick your own that corresponds to the actual device used
for the finals in your own transmitter).

Sorry, that horse won't run. RF power transistor data sheets specify the
load impedance that needs to be presented *to* the transistor from the
outside world, in order for the device to function as specified.

Clearly you did not look at such a sheet, and certainly not from
Motorola.

The MRF421 is used in two of my HF rigs and you have yet to offer what
you have. You didn't look did you? Have you ever looked? Have you
ever repaired a Finals' deck? Ever design one that is comparable?

From that specification sheet(s):
Figure 7 - Series Equivalent Impedance, Zin
shown with both Smith Chart and tabular results over the range of 2
MHz to 30 MHz:
Freq. Zin
30 0.7 - j.5
15 1.39 - j1.1
7.5 2.8 - j1.9
2.0 5.35 - j2.2

Figure 9 - Output Resistance versus Frequency, Zout
slightly less than 2 Ohms at 1.5 MHz to 1.0 Ohms at 30 MHz shown in a
clear and unambiguous chart over the entire range of frequency and
resistance described as Rout, Parallel Equivalent Output Resistance,
(Ohms).

Both specifications are observed for the Transistor powered at 12.5V
with a collector quiescent current of 150mA, and supporting an output
power of 100W PEP.

Sometimes they state the conjugate of the required load impedance, and
sometimes they don't say which.

Sometimes? You don't seem to sure, and you offer nothing specific.


That is not the output impedance *of* the transistor itself (except in
certain special cases). However, the data sheets sometimes do
ambiguously call it the "output impedance". It's a confusing mess.

The data sheets are quite ordinary to the designers however. I've
never sat through a design seminar where any RF design engineer has
made such a statement as yours (much less the outright howler of
circularity below). In fact their queries related to exactly these
specifications. How is it that your experience is different?


However, I'd hoped by now that we were all agreed about this particular
dead horse. It's buried somewhere in the Google archives of this
newsgroup.

The rest of the argument about transforming the impedance from the
device collector/drain anode to the output socket is correct, but it's
being applied to the wrong impedance. The answer you'll find at the
output socket will always be 50 ohms, because all you've done is lead
your horse around in a very tight circle.

Odd that in a chain of signal flow, that you see it being circular.
I've yet to observe a commercial rig with its antenna strapped back
into its driver input. Reading schematics is a fine art, and I have
taught more than 200 students how to do it with far more complex
Transceivers, like Collins equipment KWT-6 (AN/URC-32 HF) or Ship's
Radar AN/SPS-10, VHF/UHF AN/SRC20-21 and so on.

The quality of documentation for those pieces of equipment include
mini-treatises covering the engineering details of design, and
specific to these issues of matching. You would be well advised to
garner such gear, as you could, with attending documentation to
augment your education. One still popular item that I taught also
comes from Collins in the form of the R-390A.

I'd better stop now, before some seriously tasteless horse metaphors
come to mind.

Hi Ian,

Regrettably, you would be outclassed in metaphor usage by an old salt
who could express the word **** as a verb, noun, adjective, adverb,
conjunction....

For those lurkers who get short-shrifted of quality in these
half-debates I offer another Motorola reference: "RF Device Data
Volume II." Observe in AN2821, "Systemizing RF Power Amplifier
Design," in the sub-section "Amplifier Design":
"After selection of a transistor with the required
performance capabilities, the next step in the design
of a power amplifier is to determine the large-signal
input and output impedance of the transistor."
...
"Having determined the large-signal impedances, the
designer selects a suitable network configuration
and proceeds with his network synthesis."

ALL may note this is exactly what has been described by me. ALL may
note nothing of equal scope and depth is offered in rebuttal - if one
were to elevate those responses to such a lofty description.

73's
Richard Clark, KB7QHC