The truth about decibels

[keith]

On Sat, 02 Jul 2005 08:38:28 -0700, RST Engineering (jw) wrote:

It has nothing at all to do with your analysis (which I find rather complete
and correct) but a "radar mile" (out and back) is 10.7 microseconds. Just
to pick the flyspecs out of the pepper {;-)

My guess is that your mile is in air and Mr. PA's (or FH, as some prefer)
is in polypropylene or some such.
--
Keith

 

[Kevin Aylward]

Pooh Bear wrote:

Kevin Aylward wrote:

John Perry wrote:
Pooh Bear (and others) wrote:

600 ohms working was only ever needed for long 'land lines' - and
how many of
them still exist ? It's all concentrated at the local exchange
and distributed digitally via optic fibre these days !


Actually, in some sparsely populated rural US areas there are still
the ancient 600-ohm lines in use. You can recognize them by the
bare wires held up on glass insulators and twisted every few
hundred feet by dropping one to a lower crosstie so it can swap
places on the next crosstie with its return wire. They're now used
mainly for frequency- division multiplexing (FDM), I think, but
there are still a few out there.

I *know* ! It's actually difficult to make a practical twisted pair
that *isn't* around 100 ohms at HF !


In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC ,
the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm impedance
is quite beyond me. Batteries wouldnt last very long if cables were
so bloody dreadful.

Check the AES-3 spec about digital audio links. The original idea was
to use standard mic cables for digital AES/EBU ( now usually referred
to as AES-3 ) connections.

It worked out that standard mic cable has a 110 ohms characteristic
impedance in the MHz region.

But fortunately, its not 110 ohm at DC, otherwise we would have a slight
problem with the phantom power

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Robert wrote:

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:Zzuxe.59226$Vo6.31925@fe3.news.blueyonder.co.uk...
John Perry wrote:
Pooh Bear (and others) wrote:

600 ohms working was only ever needed for long 'land lines' - and
how many of
them still exist ? It's all concentrated at the local exchange
and distributed digitally via optic fibre these days !


Actually, in some sparsely populated rural US areas there are still
the ancient 600-ohm lines in use. You can recognize them by the
bare wires held up on glass insulators and twisted every few
hundred feet by dropping one to a lower crosstie so it can swap
places on the next crosstie with its return wire. They're now used
mainly for frequency- division multiplexing (FDM), I think, but
there are still a few out there.

I *know* ! It's actually difficult to make a practical twisted pair
that *isn't* around 100 ohms at HF !


In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...


Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC ,
the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm impedance
is quite beyond me. Batteries wouldnt last very long if cables were
so bloody dreadful.


Kevin Aylward


Two straight pieces of wire have infinite Impedance at DC?

Of course, ideally.

If you
hook them to a car battery and shorted the other ends, no current
would flow?

Completely irrelevant to the characteristic impedance of a transmission
line.

I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Ho hummm.

Do you know what the "characteristic impedance of a line" actually
means?

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Guy Macon wrote:

Robert wrote:

Kevin Aylward wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC
, the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm
impedance is quite beyond me. Batteries wouldnt last very long if
cables were so bloody dreadful.

Kevin Aylward

Two straight pieces of wire have infinite Impedance at DC? If you
hook them to a car battery and shorted the other ends, no current
would flow?

I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Aylward makes a common error among those who have failed to grasp
the true nature of the physical system they are discussing.

Here we go again. Ignorant individuals being unaware of such ignorance..

{drivil sniped}

Those of us who have been doing this for many years will remember
that the older term for characteristic impedance was surge impedance.

Pay attention dude, the characteristic impedance is not the surge
impedance.

If we had kept that term, people like Kevin wouldn't be so confused.

What part of:

Zo = sqrt((R+jwL)/(G+jwC))

did you miss during your 2nd year at university doing your degree?
Oh...that's right, you don't have a degree, that might well explain it.

Look, mate, the subject is "what is the *characteristic* impedance of a
transmission line at DC". Those of us that are not confused like you
are, can actually substitute in w=0 in the general formula above and
produce the correct answer that said undergraduates, and probable most
other reasonably competent hobbyists can, even if they don't understand
the derivation of such formula.

Plese feel free to reference some accredited text that denies such
formula, as quoted above, is the characteristic impedance of a
transmission line. Now, why don't you go and figure out the difference
between surge impedance and general characteristic impedance. Hint: it
has something to do with the relative values of R, wL, G, and wC

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Genome]

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:_Ezxe.61547$Vo6.42011@fe3.news.blueyonder.co.uk...

Guy Macon wrote:
Robert wrote:

Kevin Aylward wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC
, the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm
impedance is quite beyond me. Batteries wouldnt last very long if
cables were so bloody dreadful.

Kevin Aylward

Two straight pieces of wire have infinite Impedance at DC? If you
hook them to a car battery and shorted the other ends, no current
would flow?

I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Aylward makes a common error among those who have failed to grasp
the true nature of the physical system they are discussing.

Here we go again. Ignorant individuals being unaware of such ignorance..

{drivil sniped}


Those of us who have been doing this for many years will remember
that the older term for characteristic impedance was surge impedance.

Pay attention dude, the characteristic impedance is not the surge
impedance.

If we had kept that term, people like Kevin wouldn't be so confused.

What part of:

Zo = sqrt((R+jwL)/(G+jwC))

did you miss during your 2nd year at university doing your degree?
Oh...that's right, you don't have a degree, that might well explain it.

Look, mate, the subject is "what is the *characteristic* impedance of a
transmission line at DC". Those of us that are not confused like you
are, can actually substitute in w=0 in the general formula above and
produce the correct answer that said undergraduates, and probable most
other reasonably competent hobbyists can, even if they don't understand
the derivation of such formula.

Plese feel free to reference some accredited text that denies such
formula, as quoted above, is the characteristic impedance of a
transmission line. Now, why don't you go and figure out the difference
between surge impedance and general characteristic impedance. Hint: it
has something to do with the relative values of R, wL, G, and wC

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

Stange to say... but, in a previous post Guy stated....

"Guy Macon" <_see.web.page_@_www.guymacon.com_> wrote in message
news:11c33e45vljsr58@corp.supernews.com...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Are you sure you don't want to change "DC" to some low AC frequency?
At DC the insulation resistance is a bit higher than 100 Ohms...

Is he just contradicting you (er himself) to score points.

Doesn't appear to have worked.

Anyway I'm plonked and I think you are. You may have plonked me, hey I don't
mind. Anyway we are both shouting into empty halls. That means FatBoy Fat
can carry on deluding himself.

Snurk


DNA

 

[Mac]

On Fri, 01 Jul 2005 20:20:18 -0700, John Larkin wrote:

On Sat, 02 Jul 2005 01:16:25 GMT, Mac <foo@bar.net> wrote:

On Tue, 28 Jun 2005 19:00:11 -0700, Don Bowey wrote:

On 6/28/05 10:26 AM, in article wLfwe.38930$rb6.4094@lakeread07, "John
Perry" <jp@no.spam> wrote, in part:

snip

In fact, twisted pair is around 100 ohms from DC to microwave


You may want to re-think that.

Here are some specs for non-loaded 26G pic cable; a common Exchange Cable
type:

Resistance per mile from 1 Hz to 15kHz = 441 Ohms. Beyond that freq the
skin effect starts increasing the resistance. At 1MHz it's 463 Ohms, and at
5 MHz it's 2044 Ohms.


Approximate Z per mile at 1 Hz is 20,562 Ohms, and is up to 2057 Ohms at 100
Hz.

In the band from 300 Hz to 3 kHz the Z runs from 1189 Ohms to 383 Ohms. At
5Mhz the Z is 96 Ohms/mile.

In rural areas that are still served by copper, rather than a nearby mux,
the cables are H88 loaded, and can be assumed to have a 600 Ohm Z.

Don

An ex-Toll bastard.

I think Don was talking about the characteristic impedance. IIRC, Don is

OOPS! That should have said "John," not "Don." As in John Perry.

probably wrong about the characteristic impedance at low frequencies.
There was a thread here about that some time ago, where Reg Edwards, I
think, set a lot of people straight. But at modest frequencies up to VHF,
a twisted pair is right around 100 Ohms, regardless of length.

At higher frequencies twisted pairs are largely unusable due to
attenuation, but that doesn't mean that the characteristic impedance is
not around 100 Ohms. For all I know, it is.

--Mac


At low frequencies, for tp or coax, the resistive loss starts to make
the impedance go up. At very high frequencies, non-TEM propagation
modes make the impedance a complex function of frequency. At very high
frequencies, coax is squeezed between these effects: you have to make
it small to supress modes, but that makes the ohmic loss go up.
Somewhere in the 50-100 GHz range, coax becomes sort of useless.

John

John (Larkin),

I seem to have mis-read the attributions prior to my original post. The
only point I really wanted to make is that John Perry (although I
mistakenly said Don, originally) was talking about characteristic
impedance, and Don (although I originally said John mistakenly) was
talking about resistive loss. As you know, the two are not at all the
same, though they are both important, and are in some ways related.

And I believe that John Perry was largely correct when he said that
twisted pair is around 100 Ohms from DC to microwave. Well, not DC, maybe,
and not microwave, but some low frequency (say 100 kHz) to VHF, anyway.
;-)

But I want to make sure I understand what YOU are saying. First of
all, you are talking about characteristic impedance of a transmission
line, right? So when you say that the DC resistance makes the impedance go
up, are you essentially saying the same thing as Kevin Aylward elsewhere
in this thread? That is, at DC the characteristic impedance formula
reduces to:

Zdc = R/G ?

Since G is basically zero, we are left with Zdc = infinity.

--Mac

 

[Mac]

On Sat, 02 Jul 2005 14:36:30 +0000, Guy Macon wrote:

Robert wrote:

Kevin Aylward wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC , the
impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm impedance is
quite beyond me. Batteries wouldnt last very long if cables were so bloody
dreadful.

Kevin Aylward

Two straight pieces of wire have infinite Impedance at DC? If you hook them
to a car battery and shorted the other ends, no current would flow?

I suspect reality would contradict your formula and the wires (normal, not
huge) would melt.

Aylward makes a common error among those who have failed to grasp
the true nature of the physical system they are discussing.

Are you endorsing the point of view of the person you are replying to?
That is, do you think that this thought experiment (shorting a car
battery with a twisted pair) proves Kevin wrong? Because if you endorse
that point of view, then you don't understand characteristic impedance at
all. If you don't endorse that view, why chime in here? I realize you
have declared war on Kevin Aylward, but you should chose your allies more
carefully. ;-)

When a transmission line is connected to a DC voltage, it really does
behave like a resistor equal in value to the transmission line's
characteristic impedance, but only for as long as it takes the pulse
to reach the end of the transmission line and return;

Which pulse is that? First you say "DC" then you say "pulse." Better
make up your mind. ;-)

Note that I don't disagree with what you are saying EXCEPT that you are
talking about applying a Voltage step function to a transmission line, not
a DC Voltage, and not a pulse.

at that point in time what is on the end has an effect.. Before that
point in time the laws of physics do not allow whatever is on the end to
have any effect at all on the source.

This is well known. I'm sure Kevin knows this.

Let's assume that the transmission line is a mile long and is open at
the far end. That's roughly 16 microseconds round trip, IIRC. For those
16us, the source sees only the transmission line's characteristic
impedance. Then the infinite impedance at the end starts to have an
effect.

If the cable is infinitely long, the source sees the transmission line's
characteristic impedance forever. Not having an infinitely long cable
at hand, we can get the exact same effect by terminating the end of the
cable with a load that matches the line's characteristic impedance.

[snipped personal attacks against KA]

So, if you take your statute mile long cable, and terminate with an open
circuit (or a resistor with conductance G), then apply a DC Voltage and
wait 'til the transients settle out, how much current is flowing into the
cable? Hmm. It seems to me you haven't proved Kevin wrong yet...

In short, I think Kevin is correct. My textbooks make no mention that the
formula for transmission line impedance does not apply at DC. And this
very topic was treated in this newsgroup some time ago. IIRC, all of the
most experienced people agreed that the low frequency (audio and
thereabouts) characteristic impedance of cables was far different from the
nominal characteristic impedance, (say, above 100 kHz).

This only matters in long cables, of course, because at these low
frequencies, transmission line effects don't come into play in short
cables.

--Mac

 

[Ban]

Genome wrote:

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Are you sure you don't want to change "DC" to some low AC frequency?
At DC the insulation resistance is a bit higher than 100 Ohms...




Is he just contradicting you (er himself) to score points.

Doesn't appear to have worked.

Anyway I'm plonked and I think you are. You may have plonked me, hey
I don't mind. Anyway we are both shouting into empty halls. That
means FatBoy Fat can carry on deluding himself.

Snurk


DNA

Fuck it Genome, what a twat. But you fuckn got him by the balls. Maybe you
should take out that SM-outfit and whip him a bit for his hypochrisy.
--
ciao Ban
Bordighera, Italy

 

[Richard Henry]

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:_Ezxe.61547$Vo6.42011@fe3.news.blueyonder.co.uk...

Guy Macon wrote:
Robert wrote:

Kevin Aylward wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC
, the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm
impedance is quite beyond me. Batteries wouldnt last very long if
cables were so bloody dreadful.

Kevin Aylward

Two straight pieces of wire have infinite Impedance at DC? If you
hook them to a car battery and shorted the other ends, no current
would flow?

I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Aylward makes a common error among those who have failed to grasp
the true nature of the physical system they are discussing.

Here we go again. Ignorant individuals being unaware of such ignorance..

{drivil sniped}


Those of us who have been doing this for many years will remember
that the older term for characteristic impedance was surge impedance.

Pay attention dude, the characteristic impedance is not the surge
impedance.

If we had kept that term, people like Kevin wouldn't be so confused.

What part of:

Zo = sqrt((R+jwL)/(G+jwC))

did you miss during your 2nd year at university doing your degree?
Oh...that's right, you don't have a degree, that might well explain it.

Look, mate, the subject is "what is the *characteristic* impedance of a
transmission line at DC". Those of us that are not confused like you
are, can actually substitute in w=0 in the general formula above and
produce the correct answer that said undergraduates, and probable most
other reasonably competent hobbyists can, even if they don't understand
the derivation of such formula.

Plese feel free to reference some accredited text that denies such
formula, as quoted above, is the characteristic impedance of a
transmission line. Now, why don't you go and figure out the difference
between surge impedance and general characteristic impedance. Hint: it
has something to do with the relative values of R, wL, G, and wC

How about a google search? First item for "characteristic impedance" is

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

and includes the formula

Zo = (L/C)^(1/2)

derived with a DC battery and a switch.

 

[Genome]

"Ban" <bansuri@web.de> wrote in message
news:BMBxe.105516$75.4713227@news4.tin.it...

Genome wrote:
John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Are you sure you don't want to change "DC" to some low AC frequency?
At DC the insulation resistance is a bit higher than 100 Ohms...




Is he just contradicting you (er himself) to score points.

Doesn't appear to have worked.

Anyway I'm plonked and I think you are. You may have plonked me, hey
I don't mind. Anyway we are both shouting into empty halls. That
means FatBoy Fat can carry on deluding himself.

Snurk


DNA

Fuck it Genome, what a twat. But you fuckn got him by the balls. Maybe you
should take out that SM-outfit and whip him a bit for his hypochrisy.
--
ciao Ban
Bordighera, Italy

For fucks sake you Eyetie idiot stop drinking what you are drinking and
quote your bum correctly. Someone might get the wrong impression.

"Guy Macon" <_see.web.page_@_www.guymacon.com_> wrote in message
news:11c33e45vljsr58@corp.supernews.com...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Are you sure you don't want to change "DC" to some low AC frequency?
At DC the insulation resistance is a bit higher than 100 Ohms...

Note for posterity. John corrected himself later (I think)..

Guy farted...... and you wouldn't have believed the wobble on those
buttocks.
We are talking tectonic plate subharmonics here.

DNA

 

[Genome]

"Richard Henry" <rphenry@home.com> wrote in message
news:7SBxe.3907$Qo.195@fed1read01...

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:_Ezxe.61547$Vo6.42011@fe3.news.blueyonder.co.uk...
Guy Macon wrote:
Robert wrote:

Kevin Aylward wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC
, the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm
impedance is quite beyond me. Batteries wouldnt last very long if
cables were so bloody dreadful.

Kevin Aylward

Two straight pieces of wire have infinite Impedance at DC? If you
hook them to a car battery and shorted the other ends, no current
would flow?

I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Aylward makes a common error among those who have failed to grasp
the true nature of the physical system they are discussing.

Here we go again. Ignorant individuals being unaware of such ignorance..

{drivil sniped}


Those of us who have been doing this for many years will remember
that the older term for characteristic impedance was surge impedance.

Pay attention dude, the characteristic impedance is not the surge
impedance.

If we had kept that term, people like Kevin wouldn't be so confused.

What part of:

Zo = sqrt((R+jwL)/(G+jwC))

did you miss during your 2nd year at university doing your degree?
Oh...that's right, you don't have a degree, that might well explain it.

Look, mate, the subject is "what is the *characteristic* impedance of a
transmission line at DC". Those of us that are not confused like you
are, can actually substitute in w=0 in the general formula above and
produce the correct answer that said undergraduates, and probable most
other reasonably competent hobbyists can, even if they don't understand
the derivation of such formula.

Plese feel free to reference some accredited text that denies such
formula, as quoted above, is the characteristic impedance of a
transmission line. Now, why don't you go and figure out the difference
between surge impedance and general characteristic impedance. Hint: it
has something to do with the relative values of R, wL, G, and wC

How about a google search? First item for "characteristic impedance" is

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

and includes the formula

Zo = (L/C)^(1/2)

derived with a DC battery and a switch.

Ooooh that's the impedance of the bits in a resonant circuit at resonance.
Add a resistance of that and the circuit might be critically damped.

I could be wrong though.

DNA

 

[John Larkin]

On Sat, 02 Jul 2005 17:27:34 GMT, Mac <foo@bar.net> wrote:

On Fri, 01 Jul 2005 20:20:18 -0700, John Larkin wrote:

On Sat, 02 Jul 2005 01:16:25 GMT, Mac <foo@bar.net> wrote:

On Tue, 28 Jun 2005 19:00:11 -0700, Don Bowey wrote:

On 6/28/05 10:26 AM, in article wLfwe.38930$rb6.4094@lakeread07, "John
Perry" <jp@no.spam> wrote, in part:

snip

In fact, twisted pair is around 100 ohms from DC to microwave


You may want to re-think that.

Here are some specs for non-loaded 26G pic cable; a common Exchange Cable
type:

Resistance per mile from 1 Hz to 15kHz = 441 Ohms. Beyond that freq the
skin effect starts increasing the resistance. At 1MHz it's 463 Ohms, and at
5 MHz it's 2044 Ohms.


Approximate Z per mile at 1 Hz is 20,562 Ohms, and is up to 2057 Ohms at 100
Hz.

In the band from 300 Hz to 3 kHz the Z runs from 1189 Ohms to 383 Ohms. At
5Mhz the Z is 96 Ohms/mile.

In rural areas that are still served by copper, rather than a nearby mux,
the cables are H88 loaded, and can be assumed to have a 600 Ohm Z.

Don

An ex-Toll bastard.

I think Don was talking about the characteristic impedance. IIRC, Don is

OOPS! That should have said "John," not "Don." As in John Perry.

probably wrong about the characteristic impedance at low frequencies.
There was a thread here about that some time ago, where Reg Edwards, I
think, set a lot of people straight. But at modest frequencies up to VHF,
a twisted pair is right around 100 Ohms, regardless of length.

At higher frequencies twisted pairs are largely unusable due to
attenuation, but that doesn't mean that the characteristic impedance is
not around 100 Ohms. For all I know, it is.

--Mac


At low frequencies, for tp or coax, the resistive loss starts to make
the impedance go up. At very high frequencies, non-TEM propagation
modes make the impedance a complex function of frequency. At very high
frequencies, coax is squeezed between these effects: you have to make
it small to supress modes, but that makes the ohmic loss go up.
Somewhere in the 50-100 GHz range, coax becomes sort of useless.

John

John (Larkin),

I seem to have mis-read the attributions prior to my original post. The
only point I really wanted to make is that John Perry (although I
mistakenly said Don, originally) was talking about characteristic
impedance, and Don (although I originally said John mistakenly) was
talking about resistive loss. As you know, the two are not at all the
same, though they are both important, and are in some ways related.

And I believe that John Perry was largely correct when he said that
twisted pair is around 100 Ohms from DC to microwave. Well, not DC, maybe,
and not microwave, but some low frequency (say 100 kHz) to VHF, anyway.
;-)

For a real tp, it's probably not 100 ohms at such a low frequency. 100
KHz corresponds to 10 usec corresponds to maybe 7000 feet, which will
have a substantial resistance. Use Kev's formula to factor in R.

But I want to make sure I understand what YOU are saying. First of
all, you are talking about characteristic impedance of a transmission
line, right?

Depends on how you define "characteristic impedance." I don't care to
argue about definitions, but I'd assume that means the short-length
impedance, where short means the resistance is negligable.

So when you say that the DC resistance makes the impedance go
up, are you essentially saying the same thing as Kevin Aylward elsewhere
in this thread? That is, at DC the characteristic impedance formula
reduces to:

Zdc = R/G ?

Since G is basically zero, we are left with Zdc = infinity.

Yup. If you apply a voltage step to a long transmission line, it will
immediately draw current like a resistive load of sqrt(L/C), but then
current drops with time as the resistive losses become significant
relative to length.

If you, say, apply 1 volt and graph current vs time, I'm not sure what
the exact expression for I(t) would be... does anybody know? I guess
you could spice it, using the lossy line model.

John

 

[Robert]

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:qxzxe.61511$Vo6.50653@fe3.news.blueyonder.co.uk...

Robert wrote:
"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:Zzuxe.59226$Vo6.31925@fe3.news.blueyonder.co.uk...
John Perry wrote:
Pooh Bear (and others) wrote:


In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...


Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC ,
the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm impedance
is quite beyond me. Batteries wouldnt last very long if cables were
so bloody dreadful.


Kevin Aylward


Two straight pieces of wire have infinite Impedance at DC?

Of course, ideally.

If you
hook them to a car battery and shorted the other ends, no current
would flow?

Completely irrelevant to the characteristic impedance of a transmission
line.


I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Ho hummm.

Do you know what the "characteristic impedance of a line" actually means?

Kevin Aylward

Why yes, I do.

Robert

 

[Guy Macon]

Mac wrote:

Are you endorsing the point of view of the person you are replying to?
That is, do you think that this thought experiment (shorting a car
battery with a twisted pair) proves Kevin wrong?

No. I disagree strongly and wrote a paragraph refuting the thought
experiment, but then I noticed that he had quoted Kevin (who is in my
killfile for being personally abusive to those who disagree with him)
had made what appears to be an error. Given the histrionics I often
encounter here, it was my considered judgement that disagreeing with
two people in one post would confuse the flamers. I figured that
someone else would pick up on it and post a correction.

For the record, in the thought experiment with the zero ohm load at
the end, the source would see the characteristic impedance until
the first out-and-back reflection, but would soon settle at whatever
the resistance of the cable is, just as the thought experiment with
the open at the other end would see the characteristic impedance until
the first out-and-back reflection, but would soon settle at whatever
the insulation resistance of the cable is.

Because if you endorse that point of view, then you don't understand
characteristic impedance at all.

I agree. I don't endorse that point of view.

If you don't endorse that view, why chime in here?

It's the only place where I saw the Aylward quote. If not for the
fact that I am dealing with a mixed group of rational science-based
engineers and a group of vocal emotion-based flamer engineers here,
I would have simply trimmed all but what I was replying to. Alas,
that would have wasted everyone's time reading a bunch of accusations
of false plonking.

I realize you have declared war on Kevin Aylward,

Actually, if you check the record, Kevin Aylward declared war on me.
My response was to killfile him. I don't see his posts other than
when someone quotes one of them.

but you should chose your allies more carefully. ;-)

Please don't confuse silence with alliance.

When a transmission line is connected to a DC voltage, it really does
behave like a resistor equal in value to the transmission line's
characteristic impedance, but only for as long as it takes the pulse
to reach the end of the transmission line and return;

Which pulse is that? First you say "DC" then you say "pulse." Better
make up your mind. ;-)

Well, yes; in a very real sense, DC does not exist. No DC signal has
the attribute of having a fixed voltage that has been there for an
infinite amount of time and will be there for an infinite amount of
time. All signals that we call "DC" are actually pulses with the pulse
width limited by the beginning and end of the universe. One could argue
that our infinite transmission line is also limited by the size of the
universe, but in the case of a transmission line we can cheat by putting
a matched terminator on the end of a billion-parsec cable.

Note that I don't disagree with what you are saying EXCEPT that you are
talking about applying a Voltage step function to a transmission line, not
a DC Voltage, and not a pulse.

I agree. You have identified a major ambiguity in my argument.
This was, by the way, what I was hoping for; I welcome being corrected
by someone who uses rational arguments rather than personal attacks.

at that point in time what is on the end has an effect.. Before that
point in time the laws of physics do not allow whatever is on the end to
have any effect at all on the source.

This is well known. I'm sure Kevin knows this.

I am sure of that as well. I probably should have addressed the point
above to the fellow who did the thought experiment with the zero ohm
load at the end; Kevin gets many things right but makes an occasional
error (This describes me as well, but I hope that I don't become
abusive when someone points out one of *my* many errors), but the
fellow with the with the zero ohm load at the end thought experiment
seems to have completely missed it.

Let's assume that the transmission line is a mile long and is open at
the far end. That's roughly 16 microseconds round trip, IIRC. For those
16us, the source sees only the transmission line's characteristic
impedance. Then the infinite impedance at the end starts to have an
effect.

If the cable is infinitely long, the source sees the transmission line's
characteristic impedance forever. Not having an infinitely long cable
at hand, we can get the exact same effect by terminating the end of the
cable with a load that matches the line's characteristic impedance.

So, if you take your statute mile long cable, and terminate with an open
circuit (or a resistor with conductance G), then apply a DC Voltage and
wait 'til the transients settle out, how much current is flowing into the
cable? Hmm. It seems to me you haven't proved Kevin wrong yet...

In short, I think Kevin is correct. My textbooks make no mention that the
formula for transmission line impedance does not apply at DC. And this
very topic was treated in this newsgroup some time ago. IIRC, all of the
most experienced people agreed that the low frequency (audio and
thereabouts) characteristic impedance of cables was far different from the
nominal characteristic impedance, (say, above 100 kHz).

As I remember that discussion, the conclusion about low frequency
characteristic impedance of cables was limited to real-world finite-
length cables, and that all of the most experienced people agreed
that an infinite perfect transmission line has the same characteristic
impedance at all frequencies.

[snipped personal attacks against KA]

I am looking forward to seeing "[snipped personal attacks against GM]"
in your responses to KA.









keith wrote:

Aylward may be a lot of things, but ignorant of the relevant
physics isn't one of them.

Pompous ass.

There is no need to call Aylward a Pompous ass, Keith.

------------------------

RST Engineering (jw) wrote:

It has nothing at all to do with your analysis (which I find rather
complete and correct)

See, Kieth? It's possible to agree with me without calling
Aylward a pompous ass.

but a "radar mile" (out and back) is 10.7
microseconds. Just to pick the flyspecs out of the pepper {;-)

I am still working from memory (I am on location and away from my
reference books, but as I recall it, the speed is lower in a cable.
Don't quote me, though; I could be mis-remembering.

------------------------

John Larkin wrote:

keith wrote:

Guy Macon wrote:

Those of us who have been doing this for many years will
remember that the older term for characteristic impedance
was surge impedance. If we had kept that term, people like
Kevin wouldn't be so confused.

Pompous ass.

I disagree. I think "fathead" is the more precise term here.

Shame on you, John! If you disagree with Kevin, please present
your case using logic and evidence. Childish name-calling only
makes you look like someone who thinks that personal attacks are
an acceptable substitute for a civil discussion of the topic
at hand.

------------------------

In case any of the flamers missed it, early on I made an error
when I wrote:

From: Guy Macon <_see.web.page_@_www.guymacon.com_>
Newsgroups: sci.electronics.design
Subject: Re: The truth about decibels
Date: Tue, 28 Jun 2005 17:49:50 +0000
Message-ID: <11c33e45vljsr58@corp.supernews.com>

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Are you sure you don't want to change "DC" to some low AC frequency?
At DC the insulation resistance is a bit higher than 100 Ohms...



I then corrected my error when I wrote:

From: Guy Macon <http://www.guymacon.com/>
Newsgroups: sci.electronics.design
Subject: Re: The truth about decibels
Date: Wed, 29 Jun 2005 02:15:31 +0000
Message-ID: <11c412ah1g7aac3@corp.supernews.com>

Larry Brasfield wrote:

Guy Macon wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Are you sure you don't want to change "DC" to some low AC frequency?
At DC the insulation resistance is a bit higher than 100 Ohms...

Cable impedance at DC is a perfectly sensible concept. To measure
it is simple: Apply your Ohm meter to an infinitely long sample of it.

(Homer Simpson voice) D'OH!

As soon as I read the above I realized that you are correct and that
I was assuming a finite cable length.

If I am not mistaken (again), if you spend, say, 10 minutes or less
making your measurement, you cannot distinguish between an infinitely
long cable and one that is a mere 11 light-minutes long.

I wouldn't want the flamers to miss any of my errors...

 

[Mac]

On Sun, 03 Jul 2005 00:22:02 +0000, Guy Macon wrote:

Mac wrote:

Are you endorsing the point of view of the person you are replying to?
That is, do you think that this thought experiment (shorting a car
battery with a twisted pair) proves Kevin wrong?

No. I disagree strongly and wrote a paragraph refuting the thought
experiment, but then I noticed that he had quoted Kevin (who is in my
killfile for being personally abusive to those who disagree with him)
had made what appears to be an error. Given the histrionics I often
encounter here, it was my considered judgement that disagreeing with
two people in one post would confuse the flamers. I figured that
someone else would pick up on it and post a correction.

[snip good explanation of transmission line effects]

If you don't endorse that view, why chime in here?

It's the only place where I saw the Aylward quote. If not for the fact
that I am dealing with a mixed group of rational science-based engineers
and a group of vocal emotion-based flamer engineers here,

That would be a nice joke business card: "John Doe: flamer engineer."

I would have
simply trimmed all but what I was replying to. Alas, that would have
wasted everyone's time reading a bunch of accusations of false plonking.

Yes. Unfortunately, Guy Macon draws a LOT of fire. ;-)

I realize you have declared war on Kevin Aylward,

Actually, if you check the record, Kevin Aylward declared war on me. My
response was to killfile him. I don't see his posts other than when
someone quotes one of them.

Oh, OK. I don't really care who started it. I apologize for accusing you,

though.

but you should chose your allies more carefully. ;-)

Please don't confuse silence with alliance.

OK, OK, I get it. I mean, I knew you were too smart to miss the error in

that guy's post. It seemed to me that you were so eager to attack (or
badger, at least) KA, that you didn't care about being technically correct.

[snip]

[snipped personal attacks against KA]

I am looking forward to seeing "[snipped personal attacks against GM]"
in your responses to KA.

It seldom seems necessary to reply to KA, but if I do, I will, if
applicable.

[snipped some older comments]

I don't think Kevin Aylward is likely to change his ways. Kevin has
obviously spent a lot of time building up his electronics worldview, and
it is largely (entirely?) self-consistent and consistent with reality. It
is also sometimes a bit quirky, but the bottom line is that he knows his
stuff.

When someone fails to understand what he says, or, worse, "corrects"
him (almost always incorrectly, by the way), he seems to take it badly.
That is when the accusations of ignorance will fly.

To Kevin, I think this is a virtue, not a vice. He would probably say that
he doesn't suffer fools gladly or something like that.

--Mac

 

[Roger Johansson]

Guy Macon wrote:

Well, yes; in a very real sense, DC does not exist. No DC signal has
the attribute of having a fixed voltage that has been there for an
infinite amount of time and will be there for an infinite amount of
time. All signals that we call "DC" are actually pulses with the
pulse

DC is a theoretical model, a characterization which we often get close
enough to in practice to have a use for, and a name for.

This quality, direct current, is an asymptote, a value we often get
very near but never fully reach.
http://mathworld.wolfram.com/Asymptote.html

In spite of the fact that we never find absolutely perfect direct
current anywhere we can often use the formulas which assume that DC
exists, like Ohm's law (the DC version of Ohm's law, U=I*R).

A hundred years ago DC meant current flowing only one way.
Today it means a current which doesn't change.

We have learned to use electronic components to build mathematical and
logic models and machines.
Hardware has become theoretical tools.
You need to use other tools for DC and AC, that is the most important
difference for us.
Transformers, for example, work only on signals which change.



--
Roger J.

 

[Kevin Aylward]

Robert wrote:

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:qxzxe.61511$Vo6.50653@fe3.news.blueyonder.co.uk...
Robert wrote:
"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:Zzuxe.59226$Vo6.31925@fe3.news.blueyonder.co.uk...
John Perry wrote:
Pooh Bear (and others) wrote:


In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...


Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC
, the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm
impedance is quite beyond me. Batteries wouldnt last very long if
cables were so bloody dreadful.


Kevin Aylward


Two straight pieces of wire have infinite Impedance at DC?

Of course, ideally.

If you
hook them to a car battery and shorted the other ends, no current
would flow?

Completely irrelevant to the characteristic impedance of a
transmission line.


I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Ho hummm.

Do you know what the "characteristic impedance of a line" actually
means? Kevin Aylward

Why yes, I do.

Clerly you dont. The characteristic impedance of a line does not depend
on the line terminating impedance. Therefor you comment on s/c the line
has no relevence. Of course, the input impedance of a line does depend
on the terminating impedance.

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Richard Henry wrote:

"Kevin Aylward" <see_website@anasoft.co.uk> wrote in message
news:_Ezxe.61547$Vo6.42011@fe3.news.blueyonder.co.uk...
Guy Macon wrote:
Robert wrote:

Kevin Aylward wrote...

John Perry wrote:

In fact, twisted pair is around 100 ohms from DC to microwave,

Ahmmm...

Zo = sqrt((R+jwL)/(G+jwC))

at w=0, i.e. DC, its:

Zo=sqrt(R/G)

DC conductance, i.e. leakage resistance, is essential 0. So, at DC
, the impedance is, essentially, infinite.

Why anyone should think a bit of cable at DC has a 100 ohm
impedance is quite beyond me. Batteries wouldnt last very long if
cables were so bloody dreadful.

Kevin Aylward

Two straight pieces of wire have infinite Impedance at DC? If you
hook them to a car battery and shorted the other ends, no current
would flow?

I suspect reality would contradict your formula and the wires
(normal, not huge) would melt.

Aylward makes a common error among those who have failed to grasp
the true nature of the physical system they are discussing.

Here we go again. Ignorant individuals being unaware of such
ignorance..

{drivil sniped}


Those of us who have been doing this for many years will remember
that the older term for characteristic impedance was surge
impedance.

Pay attention dude, the characteristic impedance is not the surge
impedance.

If we had kept that term, people like Kevin wouldn't be so confused.

What part of:

Zo = sqrt((R+jwL)/(G+jwC))

did you miss during your 2nd year at university doing your degree?
Oh...that's right, you don't have a degree, that might well explain
it.

Look, mate, the subject is "what is the *characteristic* impedance
of a transmission line at DC". Those of us that are not confused
like you are, can actually substitute in w=0 in the general formula
above and produce the correct answer that said undergraduates, and
probable most other reasonably competent hobbyists can, even if they
don't understand the derivation of such formula.

Plese feel free to reference some accredited text that denies such
formula, as quoted above, is the characteristic impedance of a
transmission line. Now, why don't you go and figure out the
difference between surge impedance and general characteristic
impedance. Hint: it has something to do with the relative values of
R, wL, G, and wC

How about a google search? First item for "characteristic impedance"
is

http://www.allaboutcircuits.com/vol_2/chpt_13/3.html

and includes the formula

Zo = (L/C)^(1/2)

derived with a DC battery and a switch.

You need to understand that many short referances are short. They only
give approximate results that are not true in general.

The correct, and full derivation of the transmission line formula is
quite specific.

For example, http://www.sm.luth.se/~urban/master/Theory/4.html

Note the definition of Zo, the "characteristic impedance".

Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Mac wrote:

On Sun, 03 Jul 2005 00:22:02 +0000, Guy Macon wrote:



Yes. Unfortunately, Guy Macon draws a LOT of fire. ;-)

I realize you have declared war on Kevin Aylward,

Actually, if you check the record, Kevin Aylward declared war on me.
My response was to killfile him. I don't see his posts other than
when someone quotes one of them.

Oh, OK. I don't really care who started it. I apologize for accusing
you, though.

I wouldn't. Mr Macon is indeed the instigator.

but you should chose your allies more carefully. ;-)

Please don't confuse silence with alliance.

OK, OK, I get it. I mean, I knew you were too smart to miss the error
in that guy's post. It seemed to me that you were so eager to attack
(or badger, at least) KA, that you didn't care about being
technically correct.

[snip]


[snipped personal attacks against KA]

I am looking forward to seeing "[snipped personal attacks against
GM]" in your responses to KA.


It seldom seems necessary to reply to KA, but if I do, I will, if
applicable.

[snipped some older comments]

I don't think Kevin Aylward is likely to change his ways. Kevin has
obviously spent a lot of time building up his electronics worldview,
and it is largely (entirely?) self-consistent and consistent with
reality. It is also sometimes a bit quirky, but the bottom line is
that he knows his stuff.

When someone fails to understand what he says, or, worse, "corrects"
him (almost always incorrectly, by the way), he seems to take it
badly. That is when the accusations of ignorance will fly.

Ignorance is not a personal derogatory comment. It is statement that
someone lacks knowledge.

In this case, Mr. Macon does not understand the correct definition of
"characteristic impedance of a line". He confuses the impedance of a
line under pulse conditions, with the techncal definition of
characteristic impedance.

See for example, the 1st referance I located,
http://www.sm.luth.se/~urban/master/Theory/4.html


Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Kevin Aylward]

Guy Macon wrote:

Mac wrote:

Are you endorsing the point of view of the person you are replying
to? That is, do you think that this thought experiment (shorting a
car battery with a twisted pair) proves Kevin wrong?

No. I disagree strongly and wrote a paragraph refuting the thought
experiment, but then I noticed that he had quoted Kevin (who is in my
killfile for being personally abusive to those who disagree with him)
had made what appears to be an error. Given the histrionics I often
encounter here, it was my considered judgement that disagreeing with
two people in one post would confuse the flamers. I figured that
someone else would pick up on it and post a correction.

For the record, in the thought experiment with the zero ohm load at
the end, the source would see the characteristic impedance until
the first out-and-back reflection, but would soon settle at whatever
the resistance of the cable is, just as the thought experiment with
the open at the other end would see the characteristic impedance until
the first out-and-back reflection, but would soon settle at whatever
the insulation resistance of the cable is.

At lim. f->0, the characteristic impedance is sqrt(R/G), not sqrt(L/C)

sqrt(L/C) is only approximately valid at high frequencies.

The bottom line is that sqrt(L/C) is *not* the characteristic impedance
of a line at low frequencies.

http://www.sm.luth.se/~urban/master/Theory/4.html

I realize you have declared war on Kevin Aylward,

Actually, if you check the record, Kevin Aylward declared war on me.
My response was to killfile him. I don't see his posts other than
when someone quotes one of them.

but you should chose your allies more carefully. ;-)

Please don't confuse silence with alliance.

When a transmission line is connected to a DC voltage, it really
does behave like a resistor equal in value to the transmission
line's characteristic impedance, but only for as long as it takes
the pulse to reach the end of the transmission line and return;

Which pulse is that? First you say "DC" then you say "pulse." Better
make up your mind. ;-)

Well, yes; in a very real sense, DC does not exist. No DC signal has
the attribute of having a fixed voltage that has been there for an
infinite amount of time and will be there for an infinite amount of
time. All signals that we call "DC" are actually pulses with the
pulse width limited by the beginning and end of the universe.

Indeed. We analyse the system from the view of the lim f->0 for DC
conditions.

One
could argue that our infinite transmission line is also limited by
the size of the universe, but in the case of a transmission line we
can cheat by putting a matched terminator on the end of a
billion-parsec cable.

You error is that you believe that sqrt(L/C) is the characteristic
impedance at low frequencies. It isn't.

In short, I think Kevin is correct. My textbooks make no mention
that the formula for transmission line impedance does not apply at
DC. And this very topic was treated in this newsgroup some time ago.
IIRC, all of the most experienced people agreed that the low
frequency (audio and thereabouts) characteristic impedance of cables
was far different from the nominal characteristic impedance, (say,
above 100 kHz).

As I remember that discussion, the conclusion about low frequency
characteristic impedance of cables was limited to real-world finite-
length cables, and that all of the most experienced people agreed
that an infinite perfect transmission line has the same characteristic
impedance at all frequencies.

They are mistaken.


Kevin Aylward
informationEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.