Are cable delays frequency dependent?
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect but I can't
turn that into variable delays. Is there a magic term I should google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line. Maybe it was just explaining the specs
for a line amplifier. The idea was that the recovered clock would shift
depending on the frequency of the signal. The frequency depended on the data
pattern so you could harass the clock recovery by picking nasty data patterns.
I think I almost understood it back then when I had the info in front of me.
I've tried to remember or reconstruct it a couple of times over the years,
but I've never been successful.
--
These are my opinions, not necessarily my employer's. I hate spam.
Hi
A lot depends on the structure of the cable it's self. For plastic
dielectric cables the answer is normally "yes the delay is frequency
dependent".
One mechanism is simply that the velocity of propagation it's self is
frequency dependent in the material used. Some of the mechanism's are
pretty crazy. Periodic spacers in air / plastic cables can give some
very odd frequency dependent reflections. Even without something odd
like periodic spacers, the cable's characteristic impedance varies
with frequency. With constant termination impedances, you have a
mismatch. Again, you get reflections that are frequency dependent.
Bob
On Sep 12, 2009, at 12:33 PM, Hal Murray wrote:
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect
but I can't
turn that into variable delays. Is there a magic term I should
google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet
or app
note for clock recovery on a T1 line. Maybe it was just explaining
the specs
for a line amplifier. The idea was that the recovered clock would
shift
depending on the frequency of the signal. The frequency depended on
the data
pattern so you could harass the clock recovery by picking nasty data
patterns.
I think I almost understood it back then when I had the info in
front of me.
I've tried to remember or reconstruct it a couple of times over the
years,
but I've never been successful.
--
These are my opinions, not necessarily my employer's. I hate spam.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
Think of light dispersing through a prism creating a spectrum. The amount of
bending (which is equivalent to the amount of slowing down of the light) is
very frequency
dependent there.
----- Original Message -----
From: "Hal Murray" hmurray@megapathdsl.net
To: time-nuts@febo.com
Sent: Saturday, September 12, 2009 5:33 PM
Subject: [time-nuts] Are cable delays frequency dependent?
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect but I
can't
turn that into variable delays. Is there a magic term I should google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line. Maybe it was just explaining the
specs
for a line amplifier. The idea was that the recovered clock would shift
depending on the frequency of the signal. The frequency depended on the
data
pattern so you could harass the clock recovery by picking nasty data
patterns.
I think I almost understood it back then when I had the info in front of
me.
I've tried to remember or reconstruct it a couple of times over the years,
but I've never been successful.
--
These are my opinions, not necessarily my employer's. I hate spam.
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
No virus found in this incoming message.
Checked by AVG - www.avg.com
Version: 8.5.409 / Virus Database: 270.13.93/2365 - Release Date: 09/12/09
06:37:00
In message 20090912163352.8A958BCF7@ip-64-139-1-69.sjc.megapath.net, Hal Murr
ay writes:
If so, what's the mechanism?
There are several mechanisms.
For microwave cables, I think the short and precise explanation is that
materials act weird once you get into the details.
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line.
This is a lot simpler case:
The cables simply didn't have flat frequency responses.
To overcome this, various compensation circuits/methods where used.
These compensations incurred frequency dependent delays.
The concept you are looking for is "Group Delay", which is simply
"transmission delay dependent on frequency".
The reason it is called Group Delay, is that before digital transmission,
telephone circuits were modulated, like radio, to higher frequencies,
so that the same cable could carry multiple calls.
A bundle of 12 circuits, modulated on carriers 4kHz apart was called
a group, and was the maximum you could realistically transmit over a
4-wire twisted pair circuit.
Group delay was originally simply the difference between the slowest
and the fastest circuit, most often the highest and lowest carrier
frequency in such a group.
On Coax lines, multiple groups would again be modulated over each other
in frequency into "supergroups". For instance a 12 MHz coax cable
could carry for instance 10 supergroups of 20 groups of 12 circuits
= 2400 circuits total.
You will notice that 2400*4kHz = 9.6MHz, and if you calculate backwards
that each group of 12 circuits must have occupied 60kHz, the extra
spectrum was used for pilot tones and filter-shapes.
This is why 56kBit became a modem standard speed: It was simply
occupying a group's worth of spectrum.
Anyway, supergroups is where group delay became important:
with two layers of modulation, even using pilot-tones, circuits
carried in frequency ranges where the group-delay changed a lot,
got badly distorted.
Poul-Henning
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
Hal Murray wrote:
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect but I can't
turn that into variable delays. Is there a magic term I should google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line. Maybe it was just explaining the specs
for a line amplifier. The idea was that the recovered clock would shift
depending on the frequency of the signal. The frequency depended on the data
pattern so you could harass the clock recovery by picking nasty data patterns.
I think I almost understood it back then when I had the info in front of me.
I've tried to remember or reconstruct it a couple of times over the years,
but I've never been successful.
Even in the RF region where cables act like distributed LC transmission
lines with a relatively constant characteristic impedance, all
dielectrics are lossy.
A lossy dielectric has a frequency dependent dielectric constant.
This is particularly evident in the vicinity of an absorption edge.
Even remote from an absorption edge the dielectric constant varies with
frequency.
The dielectric constant behaviour as a function of frequency can be
approximated by a Cole-Coles relationship.
This in turn can be approximated by a set of Sellemeier equations, one
per absorption edge.
Dielectric loss (and dispersion) increase with the water content of the
dielectric.
At lower frequencies the cable acts like a distributed RC transmission
line with a strongly frequency dependent characteristic impedance and
propagation delay.
Google telegraphers equations for details.
Bruce
Bruce Griffiths wrote:
Hal Murray wrote:
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect but I can't
turn that into variable delays. Is there a magic term I should google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line. Maybe it was just explaining the specs
for a line amplifier. The idea was that the recovered clock would shift
depending on the frequency of the signal. The frequency depended on the data
pattern so you could harass the clock recovery by picking nasty data patterns.
I think I almost understood it back then when I had the info in front of me.
I've tried to remember or reconstruct it a couple of times over the years,
but I've never been successful.
Even in the RF region where cables act like distributed LC transmission
lines with a relatively constant characteristic impedance, all
dielectrics are lossy.
A lossy dielectric has a frequency dependent dielectric constant.
This is particularly evident in the vicinity of an absorption edge.
Even remote from an absorption edge the dielectric constant varies with
frequency.
The dielectric constant behaviour as a function of frequency can be
approximated by a Cole-Coles relationship.
This in turn can be approximated by a set of Sellemeier equations, one
per absorption edge.
Dielectric loss (and dispersion) increase with the water content of the
dielectric.
At lower frequencies the cable acts like a distributed RC transmission
line with a strongly frequency dependent characteristic impedance and
propagation delay.
Google telegraphers equations for details.
Bruce
For measurements of the dispersion (variation of propagation delay with
frequency) of coaxial cable in the RF region see:
http://www.ece.vt.edu/swe/lwa/memo/lwa0136.pdf
Bruce
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Hal Murray wrote:
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect but I can't
turn that into variable delays. Is there a magic term I should google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line. Maybe it was just explaining the specs
for a line amplifier. The idea was that the recovered clock would shift
depending on the frequency of the signal. The frequency depended on the data
pattern so you could harass the clock recovery by picking nasty data patterns.
I think I almost understood it back then when I had the info in front of me.
I've tried to remember or reconstruct it a couple of times over the years,
but I've never been successful.
Even in the RF region where cables act like distributed LC transmission
lines with a relatively constant characteristic impedance, all
dielectrics are lossy.
A lossy dielectric has a frequency dependent dielectric constant.
This is particularly evident in the vicinity of an absorption edge.
Even remote from an absorption edge the dielectric constant varies with
frequency.
The dielectric constant behaviour as a function of frequency can be
approximated by a Cole-Coles relationship.
This in turn can be approximated by a set of Sellemeier equations, one
per absorption edge.
Dielectric loss (and dispersion) increase with the water content of the
dielectric.
At lower frequencies the cable acts like a distributed RC transmission
line with a strongly frequency dependent characteristic impedance and
propagation delay.
Google telegraphers equations for details.
Bruce
For measurements of the dispersion (variation of propagation delay with
frequency) of coaxial cable in the RF region see:
http://www.ece.vt.edu/swe/lwa/memo/lwa0136.pdf
Bruce
The above paper indicates that where the skin effect dominates the cable
propagation delay dispersion (probably the case for all coax in the RF
region except perhaps when superconductors are used), the dispersion
can be estimated from the variation of cable loss with frequency.
Bruce
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Hal Murray wrote:
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect but I can't
turn that into variable delays. Is there a magic term I should google for
and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data sheet or app
note for clock recovery on a T1 line. Maybe it was just explaining the specs
for a line amplifier. The idea was that the recovered clock would shift
depending on the frequency of the signal. The frequency depended on the data
pattern so you could harass the clock recovery by picking nasty data patterns.
I think I almost understood it back then when I had the info in front of me.
I've tried to remember or reconstruct it a couple of times over the years,
but I've never been successful.
Even in the RF region where cables act like distributed LC transmission
lines with a relatively constant characteristic impedance, all
dielectrics are lossy.
A lossy dielectric has a frequency dependent dielectric constant.
This is particularly evident in the vicinity of an absorption edge.
Even remote from an absorption edge the dielectric constant varies with
frequency.
The dielectric constant behaviour as a function of frequency can be
approximated by a Cole-Coles relationship.
This in turn can be approximated by a set of Sellemeier equations, one
per absorption edge.
Dielectric loss (and dispersion) increase with the water content of the
dielectric.
At lower frequencies the cable acts like a distributed RC transmission
line with a strongly frequency dependent characteristic impedance and
propagation delay.
Google telegraphers equations for details.
Bruce
For measurements of the dispersion (variation of propagation delay with
frequency) of coaxial cable in the RF region see:
http://www.ece.vt.edu/swe/lwa/memo/lwa0136.pdf
Bruce
The above paper indicates that where the skin effect dominates the cable
propagation delay dispersion (probably the case for all coax in the RF
region except perhaps when superconductors are used), the dispersion
can be estimated from the variation of cable loss with frequency.
Skin effect is pretty apparent when doing TDR of sufficient resolution.
The skin effect has the effect that it dies off with time, as the
reverse field is inductive and depends on the change over time, but as
the change stops so does the reverse EMF in the core. This can be seen
when viewing a TDR trace or response of a squarewave of sufficient
transient and with no ringing or impedance mismatch.
Group delay is the derivate of the phase response, and the phase
response varies when amplitude response changes.
It is usually safe to assume the group delay is anything but flat.
(Engineer approach, be safe by assume imperfections)
Cheers,
Magnus
Cheers,
Magnus
Magnus Danielson wrote:
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Hal Murray wrote:
If so, what's the mechanism?
I know that attenuation is frequency dependent due to skin effect
but I can't turn that into variable delays. Is there a magic term
I should google for and/or does anybody have a good URL?
Context is a memory from 20 years ago. I think it was a data
sheet or app note for clock recovery on a T1 line. Maybe it was
just explaining the specs for a line amplifier. The idea was that
the recovered clock would shift depending on the frequency of the
signal. The frequency depended on the data pattern so you could
harass the clock recovery by picking nasty data patterns.
I think I almost understood it back then when I had the info in
front of me. I've tried to remember or reconstruct it a couple of
times over the years, but I've never been successful.
Even in the RF region where cables act like distributed LC
transmission
lines with a relatively constant characteristic impedance, all
dielectrics are lossy.
A lossy dielectric has a frequency dependent dielectric constant.
This is particularly evident in the vicinity of an absorption edge.
Even remote from an absorption edge the dielectric constant varies
with
frequency.
The dielectric constant behaviour as a function of frequency can be
approximated by a Cole-Coles relationship.
This in turn can be approximated by a set of Sellemeier equations, one
per absorption edge.
Dielectric loss (and dispersion) increase with the water content of
the
dielectric.
At lower frequencies the cable acts like a distributed RC transmission
line with a strongly frequency dependent characteristic impedance and
propagation delay.
Google telegraphers equations for details.
Bruce
For measurements of the dispersion (variation of propagation delay with
frequency) of coaxial cable in the RF region see:
http://www.ece.vt.edu/swe/lwa/memo/lwa0136.pdf
Bruce
The above paper indicates that where the skin effect dominates the cable
propagation delay dispersion (probably the case for all coax in the RF
region except perhaps when superconductors are used), the dispersion
can be estimated from the variation of cable loss with frequency.
Skin effect is pretty apparent when doing TDR of sufficient
resolution. The skin effect has the effect that it dies off with time,
as the reverse field is inductive and depends on the change over time,
but as the change stops so does the reverse EMF in the core. This can
be seen when viewing a TDR trace or response of a squarewave of
sufficient transient and with no ringing or impedance mismatch.
Group delay is the derivate of the phase response, and the phase
response varies when amplitude response changes.
It is usually safe to assume the group delay is anything but flat.
(Engineer approach, be safe by assume imperfections)
Cheers,
Magnus
The skin effect contributes a frequency dependent excess (over that when
the cable conductors have infinite conductivity) distributed inductance
to the cable, the variation of the distributed inductance with frequency
means that the cable propagation constant also varies with frequency.
Since the excess distributed inductance due to the skin effect decreases
as the frequency increases, the excess delay due to this excess
inductance decreases as the frequency increases.
Details are in the paper at the previously given URL.
Bruce
At 05:50 PM 9/12/2009, Bruce Griffiths wrote...
Magnus Danielson wrote:
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Bruce Griffiths wrote:
Hal Murray wrote:
We all get every message posted to the list. 5 levels of quoting is
unnecessary.