Richard Clark wrote:
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?

Of course I have, over several years: looked at data sheets (Motorola
more than any others), at numerous application notes, and at Dye and
Granberg's textbook.

Have you
ever repaired a Finals' deck?

If you mean, have I worked in the mobile radio industry, then the answer
is no.

Ever design one that is comparable?

Oh yes: designed, built, used - and understood.


From that specification sheet(s):
Figure 7 - Series Equivalent Impedance, Zin
[snip]
Yes, Zin is the true input impedance of the device - but "Zout" is not.

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).

"Described" is the operative word. What these terms really mean is a
different matter - see the quotes from Motorola below.


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.

I am very sure. Some manufacturers state the conjugate, others don't.
Even Motorola have changed their terminology over the years: sometimes
it's "output impedance" or "Zout", sometimes it's "Z(subscript OL)",
sometimes it's "Z(superscript *)(subscript OL)".

Motorola's AN1526 was written in the 1990s to clear up this mess. This
is the key paragraph [my comments in square brackets]:

"Almost every RF power device in Motorola's RF Device Data Book has a
section identifying the device's large-signal series equivalent input
and output impedances. Most often, the device output impedance is
referred to as 'the complex conjugate of the optimum load impedance into
which the device operates at a given output power, voltage and
frequency.' That is certainly a statement requiring some careful
thought, especially since the term 'output impedance' is somewhat
misleading [so even Motorola admit that]. ... as described in [AN282]
it is the conjugate of the LOAD impedance at the fundamental operating
frequency which allowed the transistor to 'function properly' [when the
load impedance was varied in a test jig]."

AN1526 goes on to explain this in much more detail. All of it is
consistent with my earlier statement that the parameter sometimes
labelled "output resistance" is actually the LOAD that needs to be
presented *to* the device from the outside world, for optimum
performance.



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?

Maybe it's because I haven't swallowed someone else's half-digested
information, but have done my own thinking and not given up till it
really made sense.


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 never mentioned signal flow - I'm talking about your circular logic!

The output network is designed to transform a 50-ohm load at the output
into the complex load impedance that needs to be presented to the
collector. But you incorrectly claim that the data sheet gives the
"output impedance" of the device itself. You then claim that by
transforming that "device output impedance" through the output network,
you can calculate the output impedance of the transmitter. Since you are
simply back-tracking through the network design calculations, the answer
you get will always be 50 ohms! It's circular logic in the tightest
possible loop.


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":

That's actually AN282A.

"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."
...
What you cut out here was the key reference to "collector LOAD
resistance", presumably because you missed the significance of "load".

"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

AN282, from which you quote, was first published in 1968. The truth
about load impedance is in there to be seen, but I'd be the first to
agree that it's not stated clearly. Motorola then confused the issue by
continuing to talk ambiguously about "output impedance" for at least
another 20 years.

AN1526, from which I'm quoting above, supersedes AN282. It was written
about 25 years later in an attempt to clear up that inherited mess of
loose definitions... but apparently with limited success.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek