RadioBanter - View Single Post - Using Tuner to Determine Line Input Impedance

Richard Clark · June 5th 09, 09:19 PM posted to rec.radio.amateur.antenna

On Fri, 5 Jun 2009 07:18:17 -0700 (PDT), wrote:

Now, this is the most curious statement of them all. *Every LPF that
is mounted in any Ham grade HF rig is designed with both a 50 Ohm
input Z and a 50 Ohm output Z. *This is easily verified through the
same page that does the calculations, or through trivial math for the
individual components' Z.

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.

My original point is that, barring measurement, you don't KNOW.
(which is sort of your argument too, eh?)

Given decades of lock-step design that conforms to accepted practices,
why would anyone have to measure something to KNOW what it is? These
are exceedingly basic considerations that do not demand differential
calculus to get to the first pass approximation. A simple algebraic
approximation is going to be quite good. Measurement will confirm.

You recite Motorola without context, I will fill that gap:
"The network theory for power amplifier design is
well known but is useless unless the designer
has valid input and output impedance data for
the transistor."

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. I have enumerated
these in the past to no obvious response from other objectors that
Motorola knows what they are doing (even when objectors recited the
exact same references!), but I will proceed once again:
1. Load pull (which satisfies NOT using small signal parameters);
2. Transistor saturation, current, and supply voltage;
3. transition and diffusion capacitances at the collector junction.

This last method responds to your nervous wondering about low
frequency response (UP TO the highest frequency, Z does not undergo
dramatic changes).

I haven't seen a 13.6V, 100W amplifier deck in the last 30 years that
varies one iota from any other - and for good reason: this stuff has
been cookie-cutter design for decades using transistors with known and
optimized characteristics that haven't varied either. Yes, FETs have
emerged in those years, and so have high voltage PNPs and NPNs. Their
contributions have only shifted the design points, not the topology,
nor the matching principles. With absolutely every new component's
introduction, there have been more than sufficient discussion of
matching principles that describe their output Z.

So, looking at things with which I have practical experience and
measurements.. MMIC amps tend to be be pretty flat over octave
bandwidths, but I don't think they're representative of ham rigs with
either FET or Bipolar output stages (which have to cover multiple
octaves, in any case). *

Why not? *

Because the MMICs are a totally different design model. To start
with, they're also Class A, while most ham rigs run Class AB. They
also tend to be "detuned" for broadbanding, at the expense of
efficiency. (not all MMICs are this way.. I'm talking about the MAR-n
series, for instance)

That is not a reason, nor is it different to any great degree. The
same issues dominate both topologies. Further, efficiency is not
always a goal when it contradicts the need for gain; and both may not
satisfy match (as there is more than one form of match: conjugate
match and Z match). I don't suppose you have anything quantitative
here either, do you?

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?

This is another strange objection. You flatly state it is not, but
you are wholly uninformed about it specifically. Your nervousness is
odd with its faltering expectation for performance at what the
industry generally considers to be a very undemanding frequency. Your
question's implied suggestion is: "is the Z for 30MHz the same as the
Z for 29.9MHz?", which from your limited perspective (you are really
reading a rather thin spec sheet) won't answer either. Study
something more encompassing; Motorola has vastly more informed
resources than your meager recitation. A simple browse of half a
dozen transistors from the same production series provides quite
specific details.

My Drake TR-7 uses MRF421s and those transistors are rated at 100W PEP
(my rig is rated at 140-150W CW) where their output resistance varies
from 1.8 Ohm to 1 Ohm over more than 4 octaves. The spec sheets are
more detailed and do not depart from others of that class. As such, I
do not expect your MRF454 would either, but your qualitative
expectations are hard to account for and no parts engineer would cut a
purchase req from them.

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.

Having actually worked on an amplifier design with similar parts, I
can also say that the datasheet is only a "get you in the ballpark for
the design" tool. The "real parts" (especially when packaged on a
board and attached to the heat sink) are substantially different.

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?

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?

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, 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."

Measurements were done by
pull, by substitution, by looking into the antenna connector with an
RF Bridge and all confirmed by simple reverse design principles.
Variations between any method rarely departed from one another, and
never from the values above. *

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. I
would say that the gulf that I bridged goes a good deal further than
demurrals such as yours. As for the sudden emergence of the
qualification for "outstanding," that can be obtained if you care to
indulge in quantifying what "outstanding" means to you. Am I being
set up with an impossible goal to satisfy your point? This
performance characteristic that I have published has served an active
market for generations; and that in its own right responds to issues
of "outstanding." The market would respond: "sufficiently so."

As to could it be better? Without a doubt, and again, design would
proceed from knowing the transistor source Z, not "by guess and by
golly." This again goes to conventional design considerations that
were enumerated by H.W. Bode in the late 30s.

just because *I* don't want to spend the time measuring
it doesn't mean that the information is of no value to the community.
I would venture that of all the data that hams, collectively, could
measure, this is actually not as useful as some other data.. It just
doesn't have that much impact on day to day operation.

What a tedious reply in the face of correspondence in other topics
that struggle to pass as intelligent discussion. I wonder why you put
such effort to responding here if this has so little impact, no
relative importance, no relation to design, nor application to "using
a tuner to determine Line Input Impedance" (a fully informed topic of
great impact on day to day operation, isn't it?).

Very few hams
adjust their tuner by calculating L and C based on measured data, or
else there wouldn't be a plethora of articles and posts about
"tuning", "pruning", "trimming" and the techniques for doing this, and
arguments about whether a Brand X meter is better than a Brand Y
meter, etc.

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.

Hams, by and large, adjust their tuners by minimizing the reflected
power, and don't much care what the actual component values are. (e.g.
what ham tuner actually has accurate dial calibrations in pF or uH? )

Do I need to quote that correspondent's post? And if I did, what
objection would fall from that? another step descending into
intellectual entropy.

Professionals, on the other hand, do CARE,

and Hams do NOT. By this standard, our group would dissolve to the
merits of photons bouncing waves off each other.

Every problem is reduced to those two options?

Obviously not, but I'll bet that it covers over 90% of hamdom (and a
lower percentage of the folks reading this thread).

90% of hamdom - leaving some several many thousands who are interested
is sufficient enough. Not for you? Not enough readers here?
Diminishing the level of discourse is not going to build audience to
that several many thousand potential, will it?

On the other hand, I have worked with high power Transistor circuits
that have acted exactly as resistors, inductors, and capacitors and
output Z was exactly like an antenna at a given frequency (or rather
input Z, as one design was an active 100W load).

Yes.. but were those run-of-the-mill amateur transceivers? (the
original question)..

Yes.. but indeed - how tedious. I have responded to the specific
question; I have responded to the specific component; I am responding
to correlating designs. Every response is rooted in published and
quantified characteristics.

I have no doubt that it is possible to build
amplifiers with constant Z (to any degree of constancy desired.. heck,
a 1000W amp and a 60db pad gives you a 1mW amplifier with very good
output Z, regardless of what the amp does).

Why does your imposed solution have to come with a crippling hit to
efficiency or match? The only inhibition against designing an
amplifier to meet your unquantified expectations is the cost involved,
not loss of efficiency, not loss of match. You said you have
experience in amplifier design, this cannot be such an impossibility
can it?

But, does a "designed for mass production and cost target" transmitter
fall into that category?
It's not a published spec

You mean you haven't read the spec. I have seen this objection too.
When I've offered just such specs, objectors have then recursed back
into how output Z is unknowable and immaterial as if the topic had
never been encountered before.

ARRL doesn't measure it when they review rigs

Now THERE's an authority! Do they measure efficiency?

73's
Richard Clark, KB7QHC