On Jun 5, 1:19*pm, Richard Clark wrote:
On Fri, 5 Jun 2009 07:18:17 -0700 (PDT), wrote:

Uh huh... and all manufacturers use high precision components, and the
impedance at one end of the filter isn't affected by the impedance at
the other end?

Hi Jim,

I haven't the slightest idea why your objection demanding "high
precision components" is necessary. *Do you have anything that is
quantifiable to sustain this concern? *Give us a Monte Carlo result of
those quantified precisions and their impact on Z.

I'm not going to bother. If you care, you can do so, or point to a
published summary, rather than spending hundreds of words reciting how
to do something we both know how to do.

Again.. without published data from the transceiver maker (whether
derived by measurement or analysis, it matters not) or data from
somewhere else, I made the initial assertion that since using the
tuner to measure impedance depended on the source being 50 ohms (or at
least, known Z), one should not blindly assume that a rig has a output
Z of 50 ohms. You provided one set of data for your rig (35 to 70
ohms), which does actually bound the problem, assuming that yours is
representative of the general class. It might be, it might not.
There's no "trivial" (as in spending no more than 5 minutes) way to
know.

For myself, I don't care, today, what my rig's output Z is, because
everything I use it with doesn't care much (e.g. the auto matching
network finds a match, and whether it's matching 50, 30, or 70 ohms,
there's not much difference). If I were doing something different
(measuring Z with a tuner) I would care, and I'd measure it.

Given decades of lock-step design that conforms to accepted practices,
why would anyone have to measure something to KNOW what it is? *

Funny thing, then... there's remarkably little published (as in
findable with google) data on the output impedance of solid state
transmitters. Yes, the designs are pretty cookbook, but there's a
dearth of published test or analysis (I maintain, of course, it's
because nobody really cares much in actual application situations).

For other RF power amp applications (like plasma etchers, RF heating,
etc) there IS data, but those devices aren't ham transmitters.

I did find a couple master's theses that have some data (but over a
very small frequency range around 7MHz) because they used a ham rig as
a source for a bridge scheme of some sort.


Motorola has for years recited at least three different means to
obtain large signal transistor output Z, and has characterized
individual transistors over frequency in charts.
Sure..


And for those same 30+ years of HF solid state rigs, their power
transistors have had (and still do) output "native" Z of several Ohms.

Would that the active device has a Z that is constant, but it's not.
Sure, the MRF454 data sheet says the output Z is 1+.2j ohms (or
something like that) at 30MHz, but is it still that at 1MHz?

Just happened to be a data sheet I have handy... As you say, others
have more data.
(for large signals, no less)


Looking at a more modern power FET for amplifier use, the IXZ210N50L..
There's a whole page of S parameters, and S22 goes from 0.88@-51deg at
2MHz to at 14.32 MHz to at 30 MHz... that's
at Ids =200mA.. bump Ids to 500mA, and the magnitudes stay about the
same, but the phases change, by tens of degrees.

Again, Motorola specifically rejects small signal parameterization for
power applications. *This is, perhaps, your problem with
characterizing amplifier issues.

Those are actually large signal parameters.. that's a 150V transistor
running at several amps drain current.

Again, it just happens to be a datasheet I had laying around.



I have selected, inspected, and validated transistors to Mil Spec and
found very few wandered from commercial specification. *You must
inhabit a very different realm where production lots contain product
that are "substantially different." *Do you have some quantification
for "substantially?" *Or is this another example of a technician's
shrug?

Substantially, as in Output C being off by a factor of more than 2.
But that could also be packaging effects, which are easier to quantify
by experiment than analysis. That's what breadboards are all about.

I would imagine that for parts used in amateur radios, this is all
thoroughly thrashed out, and there would be no big surprises. Just
that the "as implemented" data isn't readily available in 10 minutes
of searching.

No simple transformer is going to make that look like a constant 50
ohms.

Ah, are we now down to parsing this to "exactly 50 Ohms" where in your
objections you offer few quantifications? *Does 49 Ohms invalidate the
premise and score a home run for the opposing team?

Hmm depends on what sort of accuracy you want in your impedance
measurment, eh? If all you care about is 15-20% accuracy, a pretty
big variation will be ok.


I'd love to see some real data for ham rigs.

Mine (Drake TR-7 and Kenwood TS-430s) exhibit values that vary around
50 Ohms with a low of 35 Ohms and a high of 70 Ohms in the margins.
Those rigs also suffer in those margins. *

so the VSWR looking back from the tuner into your transmitter is
1.4:1? * A return loss of around 15dB... what's that work out to... an
error of about 10-15% in the "measuring impedance with a tuner"
technique... *not bad, but not great, either, especially stacked up
with the other uncertainties..

Not great? *You have already suggested it was unknowable,

I never said it was unknowable. I said it wasn't readily available or
known. Clearly one can measure it, and then know it. And now we do,
at least to 1 sig fig sorts of accuracies.. which is better than we
were 24 hours ago.

and others
state it was immaterial. *It gives me pause to have given a concrete
result to now find what was unknown is now "uncertain" and what was
immaterial now counts for little at "not great."

Immaterial in the usual amateur application (feeding a tuner which
feeds a transmission line which feeds an antenna).. not immaterial
when using the tuner to measure Z.



Although you have to admit that a 2:1 impedance variation isn't a
particularly outstanding "constant impedance load"

This characteristic that is "not particularly outstanding" was
formerly deemed impossible to determine and immaterial by others.

You're confusing "data not easily available in 5 minutes on the web"
with "impossible to determine".




Your comments to the original poster, then, could have been reduced to
one response of one line telling him to abandon his quest by this same
logic.

yes, probably.