Sequel of "Impedance Matching when the Transmitter, Line and Load..."

  • #1
Then might be regarded as continuation of discussion https://www.physicsforums.com/threa...ne-and-load-impedances-are-different.1009266/ where unfortunately it's not more possible to post further replies (why?). So I would like
to continue it here, since I think that there are some issues discussed there which require
a little bit more clarification.


@berkeman responded in the linked discussion in #25 as follws:

"I think this is pretty close to correct. If the line segments are long enough to manifest TL
effects (and thus generate reflections), it is best to use matching networks to minimize
reflections and wasted energy. If the joining sections are short, the matching networks are
probably not needed. Note though that even RF connectors are designed for a particular impedance
generally, even though they are electrically short at RF frequencies."

I'm a bit confused about the statement that "If the joining
sections are short, the matching networks are probably not needed."


As far as I understand it correctly, in general case where the source ##S## and load ##L##
have complex impedances ##Z_S## and ##Z_L## there are two standard but different impedance matching methods known:
the maximal power transfer and the minimization of reflected signal waves (they only coincide if the impedances of source and load are real, ie resistive.

But in general setting that's not the case. So if we have such source and load with complex
impedances given and our goal is to match them, we should ask ourself a basic question
depending on the context "What is for us in THIS situation more important? To
maximize the transfered power to the load or to minimize the reflected waves?

As I said in general to obtain both simultaneouly is mathematically not possible
(see my opener #1 in https://www.physicsforums.com/threa...ne-and-load-impedances-are-different.1009266/ )

And now we come back to @berkeman's statement. If we deal with the case that
joining sections (=transmission lines) are short (with resp the used wavelenghts, I guess),
then we know that the wave reflection effects are neglectable.

So in this case the impedance matching should be focussed on the maximization of
power transfer, or not?

So I not understand why berkeman said there that in case of short joining sections NO
impedance matching is needed.

Doesn't this condition only exclude the neccessarity for carring for the signal reflection issues? But then since we can exclude the neccessarity for minimization of wave reflection, we can focus ourself on maximization of power transfer only, or not?

Or did I misunderstood the point there?
 

Answers and Replies

  • #2
Baluncore
Science Advisor
2021 Award
10,346
4,676
Say you have a ±10V signal from an op-amp with a 10 mA output current limit, driving a module having a 10k ohm input resistance. You want to use a short 50 ohm line for the job.

If you terminate the line output with a 51 ohm resistor, the op-amp will need to deliver 10V / 50R = 200 mA. But the op-amp can only deliver 10 mA, so it will require a 200 mA line driver. Then 99.5% of the power will end up in the termination resistor. That is a waste of money, energy, and PCB real-estate.

If instead you ignore the matching, the line will look like a low-pass network, which you can make look to the low impedance op-amp output, more like the 10k load.
 
  • Like
Likes DaveE and berkeman
  • #3
berkeman
Mentor
61,268
11,771
Thread closed temporarily for Moderation...
 
  • #4
berkeman
Mentor
61,268
11,771
Then might be regarded as continuation of discussion https://www.physicsforums.com/threa...ne-and-load-impedances-are-different.1009266/ where unfortunately it's not more possible to post further replies (why?).
It is generally against the PF rules to re-post a locked thread. That thread was closed because of contentious debates that your questions sparked, which is not really your fault. I'll re-open this thread for a bit to see how it goes. Hopefully we can stay on-topic without too much debating this time.

That said, what specifically are you asking these questions for? Do you have a particular system or setup that you want to understand? Or are you just trying to understand general concepts involved in impedance matching? If you could narrow down your questions, that would probably help to avoid any contentious debates about generalities.

the maximal power transfer and the minimization of reflected signal waves (they only coincide if the impedances of source and load are real, ie resistive.
Most real-world situations will involve real impedances. The main one that I know of that does not is matching to an electrically short antenna...

And now we come back to @berkeman's statement. If we deal with the case that
joining sections (=transmission lines) are short (with resp the used wavelenghts, I guess),
then we know that the wave reflection effects are neglectable.
If I have a 50 Ohm source and a 50 Ohm coax cable connecting to a 50 Ohm load for an RF application, I'm not going to sweat it if I only have a 75 Ohm coax adapter/connetor to go from one 50 Ohm thing to another...
 
  • #5
DaveE
Science Advisor
Gold Member
2,027
1,644
Very short transmission lines wouldn't need a termination because there can't really be a wave propagating to cause the undesirable effects of a mismatch (VSWR and such). Compared to the frequencies of interest, everything happens essentially instantaneously; a wave wouldn't have a significant phase shift from one end to the other. The entire source-load impedance can be well modelled as a single impedance network. Of course you may want to choose what those impedances are for various reasons, but I wouldn't call it matching a TL.
 
  • #6
Averagesupernova
Science Advisor
Gold Member
3,967
867
I think the op needs to eliminate transmission lines from his thinking as @DaveE had implied. After @The Tortoise-Man has his head wrapped around everything that happens with various loads on amplifiers with various output impedance, including reactive loads, then transmission lines can explored. @The Tortoise-Man send me a PM if you like.
 
  • #7
hutchphd
Science Advisor
Homework Helper
3,952
3,120
I am very confused by this entire exercise. Perhaps my understanding is amiss so I will ask simple questions: Isn't the impedance of a cable only defined for a cable of infinite length? By what token can you apply it to this circumstance?
The supposition that the two ends of the cable don't see the other external impedances might be ok in certain circumstances but needs justification IMHO.`
 
  • #8
DaveE
Science Advisor
Gold Member
2,027
1,644
Isn't the impedance of a cable only defined for a cable of infinite length?
I think this may be a semantic issue. I can define a characteristic impedance for anything that can resonate, like a simple L-C, or a very short line. Granted, Zo isn't that useful away from resonance for networks or short lines, but it exists mathematically. Anyway, infinity isn't required. It's useful for TLs as a descriptive parameter. For example, in LTspice if you want to simulate a lossless TL, you would normally input the time delay and impedance.

The supposition that the two ends of the cable don't see the other external impedances might be ok in certain circumstances but needs justification IMHO.`
Or it might just be wrong.
 
  • #9
hutchphd
Science Advisor
Homework Helper
3,952
3,120
But the"it looks like a 50 ohm resistive load" is only because the "dissipated" energy is actually heading on down the line to infinity yes? So a shorter section of coaxial cable will look like some sort of "pi" or "T" L-C filter? Are there standard ways to choose the characterizations or is this a black art? Apologies for my ignorance.
 
  • #10
Baluncore
Science Advisor
2021 Award
10,346
4,676
Are there standard ways to choose the characterizations or is this a black art?
You use the length of cable required to connect the modules.
You then consider it more carefully if the length of cable is over about 1/20 wavelength.
Otherwise, you just call it a small lump of capacitance, or a series inductor.
 
  • #11
berkeman
Mentor
61,268
11,771
Isn't the impedance of a cable only defined for a cable of infinite length?
Mainstream textbook reference please.

##Z_0## is well defined in the literature, and I use it every day. One way to measure the ##Z_0## of a TL with length of a couple of wavelengths is to connect a potentiometer at the end and turn the little dial thing until there are no reflections for an input waveform drive. There are no infinities involved in that measurement, no?
 
  • Like
Likes sophiecentaur and DaveE
  • #12
berkeman
Mentor
61,268
11,771
Mainstream textbook reference please.

1638754147411.png

https://web.mst.edu/~kosbar/ee3430/ff/transmissionlines/z0/index.html
 
  • #13
Baluncore
Science Advisor
2021 Award
10,346
4,676
Isn't the impedance of a cable only defined for a cable of infinite length?
It is a transmission line if the capacitance and the inductance are distributed over the length of the line. If you make a ladder network out of many series inductors and parallel capacitors, it will behave like a transmission line, with Zo = √(L/C).
You can calculate Zo from the physical cross section of the line, without knowing the length.
You can measure Zo of 1" of line with a TDR.
https://en.wikipedia.org/wiki/Time-domain_reflectometer
 
  • #14
hutchphd
Science Advisor
Homework Helper
3,952
3,120
Mainstream textbook reference please.
https://en.wikipedia.org/wiki/Coaxial_cable
Please see sec 6 in particular 6.3 where this is derived (for an infinite cable). I think that method of measurement allows the meter to assure a node at the end of the cable and extrapolate. But I am not at all expert here and this is a dark art!!
 
  • #15
hutchphd
Science Advisor
Homework Helper
3,952
3,120
You can measure Zo of 1" of line with a TDR.
But I don't really understand the definitionof ##Z_0## except for an infinite cable....can you help?
 
  • #16
Averagesupernova
Science Advisor
Gold Member
3,967
867
Mainstream textbook reference please.

##Z_0## is well defined in the literature, and I use it every day. One way to measure the ##Z_0## of a TL with length of a couple of wavelengths is to connect a potentiometer at the end and turn the little dial thing until there are no reflections for an input waveform drive. There are no infinities involved in that measurement, no?
I can't recall which, maybe several, but I've seen in some ARRL books that characteristic impedance of a line can be visualized by sticking the probes of an ohmeter on the end of a transmission line that extends to infinity. I'll do a bit of digging.
-
Edit: Radio Amateur's Handbook 1975 edition. It was on top of the pile, didn't look any farther.
 
Last edited:
  • #17
Averagesupernova
Science Advisor
Gold Member
3,967
867
KIMG1583.JPG

KIMG1584.JPG

KIMG1585.JPG

Snapped a few pix of the subject matter. I don't think the ARRL will throw any fuss about me snapping some pix out of a 1975 handbook.
 

Attachments

  • KIMG1583.JPG
    KIMG1583.JPG
    60.4 KB · Views: 9
  • #18
Baluncore
Science Advisor
2021 Award
10,346
4,676
When defining Zo, we need to get away from infinite lines and standing waves.
Simply calculate the capacitance and inductance of 1” of line, then compute Zo = √(L/C).
That will be the dot in the middle of a Smith Chart.

Remember Ohms law? Impedance = Voltage / Current.
For a transmission line, conceptually;
Series inductance reduces the line current, while increasing the line voltage.
Parallel capacitance increases the line current while reducing the line voltage.
Zo is the geometric mean of the two parameters. Zo = √(L/C).
 
  • Like
Likes Averagesupernova
  • #19
Averagesupernova
Science Advisor
Gold Member
3,967
867
When defining Zo, we need to get away from infinite lines...
For us slow-witted folks, visualizing the ohmeter probes on the transmission line that extends forever helps.
 
  • #20
DaveE
Science Advisor
Gold Member
2,027
1,644
But I don't really understand the definitionof ##Z_0## except for an infinite cable....can you help?
I think part of the confusion here may be the difference between characterizing a TL as a circuit element (with a characteristic impedance), and the input impedance of a real TL including it's termination. An ideal matched termination makes the line appear at the input as if it's infinite in length, which would have an input impedance equal to the characteristic impedance. Any other termination requires analysis of the input impedance which is a function of the characteristic impedance, length, and termination. So, the characteristic impedance of just the TL is still well defined, but it's not necessarily the input impedance of the network (TL + termination).
 
  • #21
DaveE
Science Advisor
Gold Member
2,027
1,644
Another way of saying this is that the characteristic impedance of a (finite length) TL is the value of the termination that makes it appear from it's input as if it is infinite in length. i.e. no reflections.
 
  • #22
516
301
My understanding of the characteristic impedance of the transmission line is very simple. It is the ratio of the corresponding voltage and current caused by the wave passing through the transmission line. This has nothing to do with the length of the transmission line, source impedance and load impedance, input and output impedance. Therefore, the characteristic impedance is only related to the material and structure of the transmission line itself. But of course it needs to be noted that there may be waves propagating forward and waves propagating in the opposite direction at the same time on the transmission line.
 
Last edited:
  • #23
Averagesupernova
Science Advisor
Gold Member
3,967
867
This has nothing to do with the length of the transmission line, source impedance and load impedance, input and output impedance.
Of course it has nothing to do with the length of the line so long as we cease our voltage/current measurements before the wave has arrived at the opposite end of the line. Hence the infinite length description. It helps us understand what is happening before saying: "Ok, now we give the line a specific length and describe what happens when we drive a line with a shorted, open, or terminated (properly or improperly), end, etc. etc."
-
It's called characteristic impedance for a reason. Because it's a characteristic of the line, that's it.
 
  • #24
516
301
Of course it has nothing to do with the length of the line so long as we cease our voltage/current measurements before the wave has arrived at the opposite end of the line.
If the load impedance of the transmission line does not match the characteristic impedance of the transmission line, the input impedance at the other end of the transmission line is not equal to the characteristic impedance, as described in Post 21. But this will affect the input impedance (what I call it), not the characteristic impedance. I just want to say that the characteristic impedance is not affected.
 
  • #25
Say you have a ±10V signal from an op-amp with a 10 mA output current limit, driving a module having a 10k ohm input resistance. You want to use a short 50 ohm line for the job.

If you terminate the line output with a 51 ohm resistor, the op-amp will need to deliver 10V / 50R = 200 mA. But the op-amp can only deliver 10 mA, so it will require a 200 mA line driver. Then 99.5% of the power will end up in the termination resistor. That is a waste of money, energy, and PCB real-estate.

If instead you ignore the matching, the line will look like a low-pass network, which you can make look to the low impedance op-amp output, more like the 10k load.

Sorry for my dumbness but I'm sure if I understand your example. What you mean by the formulation
"terminate the line output with a 51 ohm resistor"?

What is this 51 ohm resistor? Does it play the role of a "matching device" in following sense:

51 Ohm resistor Matching Device.png



Or do you have another configuration in mind?
 

Related Threads on Sequel of "Impedance Matching when the Transmitter, Line and Load..."

  • Last Post
Replies
7
Views
830
Replies
15
Views
3K
  • Last Post
Replies
8
Views
1K
  • Last Post
Replies
10
Views
2K
  • Last Post
Replies
7
Views
4K
  • Last Post
Replies
3
Views
4K
  • Last Post
Replies
2
Views
2K
Top