Transmitter Output Impedance
 

On 4/25/2011 7:35 PM, Sal M. Onella wrote:
This group has presented members with valuable lessons in antennas and
transmission lines, like how to measure, how to match, etc.

Something I haven't seen is a discussion of the source impedance of
the transmitter. My curiosity was piqued today as I took some baby
steps into EZNEC. A particular antenna had such-and-such VSWR if fed
with a 50-ohm cable and a different value if fed with a 75-ohm cable.

No, the antenna still has the same characteristics. You changed the
impedance at a point remote from the antenna.

While this is hardly news, it got me wondering whether a 75-ohm cable
will load the transmitter the same. Doesn't seem like it.

It depends on a number of factors. For example, use a .5 wavelength 75
ohm cable to feed a 50 ohm resistance. Your transmitter happily thinks
it is 50 ohms. Because it is.

My point: Using 75-ohm cable to improve the match at the antenna
won't help me ... IF ... I suffer a corresponding loss due to
mismatch at the back of the radio. My HF radios, all solid state,
specify a 50 ohm load. As necessary, I routinely use an internal
autotuner and either of two external manual tuners. (I'm aware of the
published 1/12 wavelength matching method.)

Wisdom in any form would be appreciated. Thanks.

"Sal"
(KD6VKW)


Transmitters don't have a clearly defined output impedance and, whatever
output impedance is there, doesn't necessarily mean anything.

Your main concern is to provide a 50 ohm load for the transmitter to see.

Actually, it the feed line and load are both 75 ohms, you will only see
a 1.5:1 SWR. Don't worry about it.

Cheers,
John

Transmitter Output Impedance
 

On 5/3/2011 7:27 PM, Cecil Moore wrote:
On May 3, 6:29 pm, John wrote:
I believe his confusion is the one-port vs two-port problem. I don't
have a problem with your explanation. But, I think he is throwing in a
port where he should not.

The second port is actually there on the other side of the black box,
but it has been overlooked. Doesn't it give anyone pause to realize
that the s11 parameter changes by an infinite percentage depending
upon whether it is measured as a one-port system or as a two-port
system?

No. The experiment is designed as a one-port experiment. You have broken
the law and must go to jail without passing GO.

"Having cancer of the colon is better than no colon at all."

Transmitter Output Impedance
 

On 4 mayo, 01:11, dave wrote:
On May 3, 2:01*pm, Wimpie wrote:

On 3 mayo, 02:09, Cecil Moore wrote:

On May 2, 5:23*pm, Wimpie wrote:

I am very sorry Cecil, but I still don't see the point where the
discussed method may go wrong.

Everyone seems to be charging ahead, willy-nilly, without seeing the
point which is that there are other effects present besides
reflections.

I think most of the discussion is ignoring lots of things... first of
all you must answer the question; Does it really matter? *If it does,
then i will assume that you are a designer and know how to apply the
tube characteristics to come up with a design that matches your
selected tube to the expected load... but does that process really
ever describe the 'output impedance'?? *Then you must also consider
the tuning parameters employed... sure, you can measure the output
impedance at a given operating point, but answer the question again;
Does it really matter? *Or do you need to measure it over a wide range
of operating conditions? *Every operator i have seen tune up one of my
amps has done it a little bit different... heck, i don't even do it
exactly the same twice in a row i bet. *So when i am running an amp
into a switchable set of 7 different antenna combinations on a given
band, can tune from one end of the band to the other without touching
the settings, and can make an infinite number of small adjustments to
the drive, tune, and load settings, and on some bands can tweak a
tuner after the amp to 'make it happier', do I really care what the
'output impedance' really is? *As long as the matching network
provides adequate adjustment so i can get out the desired power into
my various loads while keeping the tubes within their operating
limits, do i really care what the 'output impedance' really is at any
one set of conditions that i may never exactly duplicate again? *I
think not. *So this boils down to an academic discussion, and as in
many cases where no one has, or can, make an exact statement of the
problem the specific answer remains elusive. * So consider this, until
you have a complete statement of the problem you will never be able to
derive a value that any two of you will agree on, let alone actually
try to set up a measurement of.

Hello Dave and John,

When you look to my first posting to this thread, you may conclude
that I agree with you.

We had such a discussion about a year ago where I stated that most RF
amplifiers do not have 50 Ohms output impedance. That statement was
heavily disputed by some persons. I tried to support that statement
with simulations, but without any success.

Regarding the "academic discussion" I also agree. In my professional
career where I designed several RF PA's, only 2 times the output
impedance of the amplifier was of importance. In one of these cases I
couldn't meet the specs and had to insert attenuation (some waste of
power…).

The thing I don't like is that some people criticize methods used by
some of the group members without a solid foundation.

Regarding the two-port single-port issue. One can setup a reasoning
based on a two-port setup, but that significantly complicates the
matter without giving any additional insight. I tried to keep it
simple by referencing to a VSWR measurement (with an antenna analyzer)
of an antenna when a strong transmitter is nearby, but it seems I
wasn't clear enough for all people following this thread.

With kind regards,


Wim
PA3DJS
www.tetech.nl
Remove abc first before setting free the pigeon.

Transmitter Output Impedance
 

On Tue, 3 May 2011 19:06:38 -0700 (PDT), Wimpie
wrote:

only 2 times the output
impedance of the amplifier was of importance.

What a strange ellipsis.

In other words if this statement bears upon the discussion, then it
deserves a fuller context. Why was it of importance?

In one of these cases I
couldn't meet the specs and had to insert attenuation (some waste of
power…).

Ed McMahon: "How much power?"

Just trying to get some perspective on this power sourced in the PA.

73's
Richard Clark, KB7QHC

Transmitter Output Impedance
 

On May 3, 9:06*pm, Wimpie wrote:
Regarding the two-port single-port issue. One can setup a reasoning
based on a two-port setup, but that significantly complicates the
matter without giving any additional insight. *I tried to keep it
simple ...

"Everything should be made as simple as possible, but not simpler."
Albert Einstein

Here's my earlier example:

Source-----Z0=50----x----1/4WL Z0=100----200 ohm load

s11 is measured at point x equal to 0.3333 and also 0.3333 at the
load. Nowhere is s11 equal to 0.0000. Put everything to the right of
point x into a black box and s11 measures to be 0.0000 under exactly
the same conditions??? And you guys want all of us to trust that
measurement enough to predict the disputed source impedance of an RF
amp when it cannot even predict the load impedance in the above very
simple passive circuit?

There are reflected waves at point x (s11*a1) that are equal in
magnitude and 180 degrees out of phase with the reflected waves
transmitted back from the load (s12*a2). The two waves undergo
destructive interference at point x which creates a V/I ratio of 50 at
point x. But the absence of *net* reflected energy at point x does not
mean that there are no reflections at point x. There are actually two
sets of reflections at point x that mask any attempt to determine the
actual value of the load impedance by measuring s11 when the system is
installed inside a black box. It is foolish to presume that there are
no similar interference patterns inside an RF amp. In fact, the only
condition where there is no interference inside a simple voltage
source is when there are no reflections or the reflections are
orthogonal to the source signal.

There is a good discussion of the role of interference in the creation
of virtual impedances in section 4.3 of "Reflections", by Walter
Maxwell. Even though a lot RF engineers scoff at the laws of EM wave
physics from the field of optics, the best explanation of interference
I have ever read is the chapter by the same name in "Optics", by
Hecht. Another good chapter in "Optics" is "The Superposition of
Waves".
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Transmitter Output Impedance
 

On 4 mayo, 14:23, Cecil Moore wrote:
On May 3, 9:06*pm, Wimpie wrote:

Regarding the two-port single-port issue. One can setup a reasoning
based on a two-port setup, but that significantly complicates the
matter without giving any additional insight. *I tried to keep it
simple ...

"Everything should be made as simple as possible, but not simpler."
Albert Einstein

Here's my earlier example:

Source-----Z0=50----x----1/4WL Z0=100----200 ohm load

s11 is measured at point x equal to 0.3333 and also 0.3333 at the
load. Nowhere is s11 equal to 0.0000. Put everything to the right of
point x into a black box and s11 measures to be 0.0000 under exactly
the same conditions??? And you guys want all of us to trust that
measurement enough to predict the disputed source impedance of an RF
amp when it cannot even predict the load impedance in the above very
simple passive circuit?

There are reflected waves at point x (s11*a1) that are equal in
magnitude and 180 degrees out of phase with the reflected waves
transmitted back from the load (s12*a2). The two waves undergo
destructive interference at point x which creates a V/I ratio of 50 at
point x. But the absence of *net* reflected energy at point x does not
mean that there are no reflections at point x. There are actually two
sets of reflections at point x that mask any attempt to determine the
actual value of the load impedance by measuring s11 when the system is
installed inside a black box. It is foolish to presume that there are
no similar interference patterns inside an RF amp. In fact, the only
condition where there is no interference inside a simple voltage
source is when there are no reflections or the reflections are
orthogonal to the source signal.

There is a good discussion of the role of interference in the creation
of virtual impedances in section 4.3 of "Reflections", by Walter
Maxwell. Even though a lot RF engineers scoff at the laws of EM wave
physics from the field of optics, the best explanation of interference
I have ever read is the chapter by the same name in "Optics", by
Hecht. Another good chapter in "Optics" is "The Superposition of
Waves".
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Hello Cecil,

Basically, it doesn't matter what is inside the box. It can be fully
characterized by its impedance versus frequency (small signal
approach).

I am fully aware of that there are (back and forth) reflections in the
quarter wave line inside the black box. However in case of a black
box, you don't know that (black box principle).

Same thing happens in most narrow band antennas. The antenna wire
itself may be subjected to (for example) VSWR = 10, though the
impedance can be 50 Ohm (and therefore doesn’t introduce reflection in
a 50 Ohms feed line). I know that this 50 Ohms is the result of
interfering waves/signals, but an MFJ 259B antenna analyzer, or my
FT7B doesn't (and doesn't need to know it).

One could only guess what is a black box based on its impedance versus
frequency curve, S11 curve or Time domain response (I had to do this
in school).

So if we decide to open the black box and we put the reference plane
inside the box, we get a new thread, something like: "why has a PA
certain output impedance?", or "what is the large signal output
impedance of valves, BJT, FET, etc?".


With kind regards,

Wim
PA3DJS
www.tetech.nl

Transmitter Output Impedance
 

John KD5YI wrote:


Transmitters don't have a clearly defined output impedance and, whatever
output impedance is there, doesn't necessarily mean anything.

Your main concern is to provide a 50 ohm load for the transmitter to see.

actually, to provide an *acceptable* load for the transmitter that
maximizes radiated RF power. That might not be 50 ohms.. and I don't
know that we actually care what it is, unless we're designing amplifiers.

50 ohms is basically a "standard test condition" so that you can compare
amplifiers, and it happens to be convenient that everything is made with
the same standard impedance. That way, your test load (which will
dissipate a fair amount of power in some tests) can be located somewhere
different, and connected by a length of transmission line with the same
impedance, so the test at the amplifier output reference plane is still
valid.

Transmitter Output Impedance
 

On May 4, 10:47*am, Wimpie wrote:
Basically, it doesn't matter what is inside the box. It can be fully
characterized by its impedance versus frequency (small signal
approach).

If what is inside the box doesn't matter, why waste time trying to
measure what is inside the RF amp box? What can be "fully
characterized" is the box's relationship to everything outside of the
box. Exactly what is inside the box cannot be ascertained at all while
we handicap ourselves with a blinders. When we actually look inside an
RF amp, we find active non-linear devices upon which we all have been
warned about trying to use linear assumptions.

It doesn't matter what is inside the box when *conditions outside of
the box* are being considered. EE201. What is inside the box matters
considerably for *conditions inside the box*. For instance, replacing
a 50 ohm dipole with a 50 ohm lumped circuit doesn't change things
between the load and the source. But it causes an almost infinite
change from the load out to the rest of reality.

Anything else I could say on this subject would be repetition.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK

Transmitter Output Impedance
 

On May 2, 8:09*pm, Cecil Moore wrote:
On May 2, 5:23*pm, Wimpie wrote:

I am very sorry Cecil, but I still don't see the point where the
discussed method may go wrong.

Everyone seems to be charging ahead, willy-nilly, without seeing the
point which is that there are other effects present besides
reflections.

Therefore carrying out a single-port measurement with a slightly off-
carrier frequency (to create non-coherence) under required output
conditions, will result in a meaningful output impedance.

Nope, it won't because virtual impedances don't cause reflections.
Only physical impedance discontinuities cause reflections. The rest of
the redistribution of RF energy is caused by the superposed
interaction between forward and reflected waves, i.e. interference
effects. Most hams do not understand the role of interference in the
redistribution of RF energy. Hope this helps.

http://micro.magnet.fsu.edu/primer/j...interference/w...

Please pay close attention to the last paragraph. "... when two waves
of equal amplitude and wavelength that are 180-degrees ... out of
phase with each other meet, they are not actually annihilated, ... All
of the photon energy present in these waves must somehow be recovered
or redistributed in a new direction, according to the law of energy
conservation ... Instead, upon meeting, the photons are redistributed
to regions that permit constructive interference, so the effect should
be considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."

You guys are presuming that reflections are the only thing you are
seeing and that is just not true. You are also seeing interference
effects without realizing it so your conclusions are doomed to failure
unless you can differentiate between constructive/destructive
interference and reflected waves. Since there has been no mention of
interference effects, I am forced to conclude that you guys are
ignorant of such effects.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK



During 1991 Warren Bruene used the RPG method in which he believes he
measured the source resistance of an RF power amp, which he calls
'Rs'. I have never agreed that his method measures the source
impedance, or that his data has any relevance to anything.

Consequently, I am not impressed with the discussion going on here
concerning applying a signal back into an operating RF power amp to
determine the source impedance. Please define 'source impedance'--
where is it located in the amp? At the plate? At the output of the pi-
network?. And how do you know the data obtained using this method is
correct? Have you verified it by comparing it with data obtained using
another method?

I made a statement in an earlier post that when measuring the output
impedance using the 'load pull' method we're not concerned with the
absolute maximum power that can be delivered, but instead, limiting
the 'maximum' power delivered to that which can be delivered with a
specific level of grid drive, one which allows the power to be
limited to that of a normal operating level. Tom disagrees with this
position, that it is really the ABSOLUTE MAXIMUM power delivery that
should be considered. As you can see, I don't agree with Tom.

I don't know how many on this thread have actually reviewed the
portion of my Chapter 19 that presents the step-by-step procedure I
used in determining the output impedance of the Kenwood TS-830S tx,
which shows precisely the output impedance appearing at the output of
the pi-network.

To summarize the procedure that I maintain will provide an accurate
measurement of the output impedance appearing at the output terminals
of the pi-network is as follows:

1) Adjust the loading and tuning controls of the amp to deliver all
the available power to a complex load in the amount normally used in
operation with the setting of the grid-drive level required to obtain
that output power.

2).Measure the impedance of the complex load.

3) The output impedance, or 'source' impedance of the amp appearing at
the output terminals of the pi-network is the complex conjugate of the
load impedance.

Now, when you measure the source impedance using the externally-
injected signal, does the data from that measurement agree with that
of the load-measuring method? If it does, then I'll agree that the RPG
method is valid. If it doesn't I'll continue to have considerable
doubt as to its validity. But I'd still like to know where the
resistance measured by this method is located in the amp.

Walt

Transmitter Output Impedance
 

On 5/4/2011 4:11 PM, walt wrote:
On May 2, 8:09 pm, Cecil wrote:
On May 2, 5:23 pm, wrote:

I am very sorry Cecil, but I still don't see the point where the
discussed method may go wrong.

Everyone seems to be charging ahead, willy-nilly, without seeing the
point which is that there are other effects present besides
reflections.

Therefore carrying out a single-port measurement with a slightly off-
carrier frequency (to create non-coherence) under required output
conditions, will result in a meaningful output impedance.

Nope, it won't because virtual impedances don't cause reflections.
Only physical impedance discontinuities cause reflections. The rest of
the redistribution of RF energy is caused by the superposed
interaction between forward and reflected waves, i.e. interference
effects. Most hams do not understand the role of interference in the
redistribution of RF energy. Hope this helps.

http://micro.magnet.fsu.edu/primer/j...interference/w...

Please pay close attention to the last paragraph. "... when two waves
of equal amplitude and wavelength that are 180-degrees ... out of
phase with each other meet, they are not actually annihilated, ... All
of the photon energy present in these waves must somehow be recovered
or redistributed in a new direction, according to the law of energy
conservation ... Instead, upon meeting, the photons are redistributed
to regions that permit constructive interference, so the effect should
be considered as a redistribution of light waves and photon energy
rather than the spontaneous construction or destruction of light."

You guys are presuming that reflections are the only thing you are
seeing and that is just not true. You are also seeing interference
effects without realizing it so your conclusions are doomed to failure
unless you can differentiate between constructive/destructive
interference and reflected waves. Since there has been no mention of
interference effects, I am forced to conclude that you guys are
ignorant of such effects.
--
73, Cecil, w5dxp.com
"Halitosis is better than no breath at all.", Don, KE6AJH/SK



During 1991 Warren Bruene used the RPG method in which he believes he
measured the source resistance of an RF power amp, which he calls
'Rs'. I have never agreed that his method measures the source
impedance, or that his data has any relevance to anything.

Consequently, I am not impressed with the discussion going on here
concerning applying a signal back into an operating RF power amp to
determine the source impedance. Please define 'source impedance'--
where is it located in the amp? At the plate? At the output of the pi-
network?. And how do you know the data obtained using this method is
correct? Have you verified it by comparing it with data obtained using
another method?

I made a statement in an earlier post that when measuring the output
impedance using the 'load pull' method we're not concerned with the
absolute maximum power that can be delivered, but instead, limiting
the 'maximum' power delivered to that which can be delivered with a
specific level of grid drive, one which allows the power to be
limited to that of a normal operating level. Tom disagrees with this
position, that it is really the ABSOLUTE MAXIMUM power delivery that
should be considered. As you can see, I don't agree with Tom.

I don't know how many on this thread have actually reviewed the
portion of my Chapter 19 that presents the step-by-step procedure I
used in determining the output impedance of the Kenwood TS-830S tx,
which shows precisely the output impedance appearing at the output of
the pi-network.

To summarize the procedure that I maintain will provide an accurate
measurement of the output impedance appearing at the output terminals
of the pi-network is as follows:

1) Adjust the loading and tuning controls of the amp to deliver all
the available power to a complex load in the amount normally used in
operation with the setting of the grid-drive level required to obtain
that output power.

2).Measure the impedance of the complex load.

3) The output impedance, or 'source' impedance of the amp appearing at
the output terminals of the pi-network is the complex conjugate of the
load impedance.

Now, when you measure the source impedance using the externally-
injected signal, does the data from that measurement agree with that
of the load-measuring method? If it does, then I'll agree that the RPG
method is valid. If it doesn't I'll continue to have considerable
doubt as to its validity. But I'd still like to know where the
resistance measured by this method is located in the amp.

Walt

It seems to me that we maybe should not be talking about source
impedance but maybe about 'regulation' or some other equivalent word.

For an open-loop source made from real components, I think one will
always find that there is a dE/dI number for output loading that we can
call source resistance.

It does not always mean that there is a physical resistance in the
circuit. It is simply a measure of the ability of the device to provide
an unvarying voltage under conditions of varying load.

Does this make any sense?

John