Any experience with negative impedance?

[Adrian Tuddenham]

It's really just a logarithmic way of expressing a ratio.

As are nepers.

Bels (and dB) are specifically defined as logarithmic ways of expressing
a ratio of power, not of anything else.

However:

1) As power measurements are more difficult to make than (for instance)
voltage, current or sound pressure, it is permissible to use the easier
measurements if they are known to be related to power. Thus it is
possible to measure two voltages and compare them in "dB" if they were
measured sequentially at the same point in the circuit. You are not
really comparing the voltages, only the power they are developing. It
is also reasonable to use the dB to compare voltages in two different
circuits if they have the same impedance. It is not correct to compare
two voltages across different or unspecified impedances in dB.

2) Logarithms may be a convenient way of dealing with large ratios in
other measurements, such as voltages in different impedance circuits;
but, if "dB" is used, the unit must always be correctly suffixed to show
that it is not a true dB, only a convenient application of the
logarithmic principle.

3) Decibels cannot be used as an absolute unit, but can be used to
compare a power measurement (or power derived fom some other
more-measureable unit) with a particular reference power (1 milliwatt in
600 ohms, threshold of hearing etc.). Once again an appropriate suffix
is needed, to show what reference unit its being used.

 

[Jasen Betts]

Source EMF or terminated?

it's into a 75 ohm load.

The meters are have an 'in' and an 'out' port so you can have a terminator on the 'out' port or
put a receiver there.

 

[Phil Allison]

"Adrian Tuddenham"

The requirement is to design a specialist one-off audio amplifier to
drive a loudspeaker at the end of a long cable for experimental
purposes. The voice coil is nominally 15 ohms with a pure resistance of
10 ohms. The loop resistance of the cable will be somewhere between 2
and 4 ohms. It would be desirable to have a damping factor of 10 or
better, so I need to reduce the effect of the cable resistance in some
way.

** See half way down this page:

http://sound.westhost.com/project56.htm

Basically, it is a fool's idea.

FYI:

That 10 ohms of resistance can easily become 15 ohms when the speaker is
driven hard - cos copper wire increases in resistance when it get hot.

Cable resistance of 2 to even 4 ohms is not an issue with a 15 ohm woofer.


..... Phil

 

[josephkk]

It's really just a logarithmic way of expressing a ratio.

As are nepers.

Yep. Also half powers (used in optics and some optical physics
measurements).

?-)

 

[josephkk]

0dBm references 1 milliwatt, nothing more.

1 milliwatt is 1 milliwatt, whether it is dissipated in 50 ohms, 600
ohms,or a megohm.

You only need to quote the impedance if you are referring it to a voltage.

Oops. Yes, you are correct. That is just what i first learned it as.

?-)

 

[John Devereux]

"Adrian Tuddenham"

** See half way down this page:

http://sound.westhost.com/project56.htm

Basically, it is a fool's idea.

FYI:

That 10 ohms of resistance can easily become 15 ohms when the speaker is
driven hard - cos copper wire increases in resistance when it get hot.

JL's three-wire system would take care of that though wouldn't it?

 

[Phil Allison]

"John Devereux"

"Phil Allison"

JL's three-wire system would take care of that though wouldn't it?

** Not part of the OP's question.

It a fool's idea that there IS even a question.

Fuckwit.



..... Phil

 

[[email protected]]

Really? I would of guest voltage into a shunted circuit..

incidentally, my references, book that is, not the net, makes reference
of dBs = 1 mW into 600 ohms (Japanese).

And that so called "0.775" which is actually "0.7746v" above is in
reference to 0 dB = 1 mW into a 600 Ohm R = 0.7746Vrms across 600 ohm
for "Automobiles"

So there you are, a completely messed up system.

oh btw, for TV 0 dB = 1 mW across a 75 ohm R = 1.333x10^8 * W

It's a wonder any one can keep things sorted out.

It has to be the aliens, all this disinformation!

That's the nice thing about standards; there are *so* many to choose
from.

 

[Jamie]

And there is dBs, voltage into a short circuit.

Really? I would of guest voltage into a shunted circuit..

incidentally, my references, book that is, not the net, makes reference
of dBs = 1 mW into 600 ohms (Japanese).

And that so called "0.775" which is actually "0.7746v" above is in
reference to 0 dB = 1 mW into a 600 Ohm R = 0.7746Vrms across 600 ohm
for "Automobiles"

So there you are, a completely messed up system.

oh btw, for TV 0 dB = 1 mW across a 75 ohm R = 1.333x10^8 * W

It's a wonder any one can keep things sorted out.

It has to be the aliens, all this disinformation!


Jamie

 

[josephkk]

I have never seen a meter with a loop through input. CATV/MATV
equipment and even TV transmitters have test ports to allow you to read
a level without disconnecting something else. I currently have four TV
FSM here that were made obsolete by digital TV and none work as you
stated. Every one I've seen, from the early 12 channel meter (Simpson
Model 488) onward have a terminated input. Here is a nice collection of
these meters: <http://theoldcatvequipmentmuseum.org/180/181/index.html>
A return loss bridge has an output, but isn't used to measure signal
levels. You can use a 75 ohm meter with a signal source & home made RLB
to test coax, splitters directional couplers and otter items.

Well, add that one (loop through to load meters) to your toolkit. You
have heard of kill-a-watt power use monitors?

?-)

 

[Adrian Tuddenham]

That's the nice thing about standards; there are *so* many to choose
from.

Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

There is a lot to be said for making sure the fundamentals are taught
and understood before plunging into the real world of bullshit and
jargon which surrounds any technical matters with sales potential.



Typical of the sort of misleading (mis)information which adds to the
confusion is the UK Government's Health & Safety website which includes
the following:

~~~~~~~~~~
How is noise measured?
Noise is measured in decibels (dB). An 'A-weighting' sometimes written
as 'dB(A)', is used to measure average noise levels, and a 'C-weighting'
or 'dB(C)', to measure peak, impact or explosive noises.

~~~~~~~~~~

 

[[email protected]]

Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

Care to cite your "one true standard:?

There is a lot to be said for making sure the fundamentals are taught
and understood before plunging into the real world of bullshit and
jargon which surrounds any technical matters with sales potential.

Bullshit noted.

Typical of the sort of misleading (mis)information which adds to the
confusion is the UK Government's Health & Safety website which includes
the following:

Strawman noted.

~~~~~~~~~~
How is noise measured?
Noise is measured in decibels (dB). An 'A-weighting' sometimes written
as 'dB(A)', is used to measure average noise levels, and a 'C-weighting'
or 'dB(C)', to measure peak, impact or explosive noises.

Your point?

 

[Adrian Tuddenham]

[...]

Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

Care to cite your "one true standard:?

I will quote from an official publication which cites it, as I do not
have acces to the original international agreement:

~~~~~~~~~~~~~

"Admiralty Handbook of Wireless Telegraphy" (H.M.S.O. 1938) Vol. 1
Appendix "A" Section 1.

THE DECIBEL AND THE NEPER.

1. Historical,

The "Decibel" is the 1/10th part of a "bel" (after Alexander Graham
Bell, inventor of the telephone sounder), a unit in which one may
express power ratios and gain or loss ratios of related quantities such
as current and voltage. It originated in line telephony in 1923, when
the American Telephone and Telegraph Company introduced a new unit, then
called the "transmission unit"; this was to replace an older conception
based on a ratio comparison between the decrease in signal strength
produced by a given telephone line and that produced by a "mile of
standard cable." In 1924 an international advisory committee on long
distance telephony in Europe, together with the representatives of the
Bell system, agreed to recommend their countries to adopt as standards

EITHER the "bel," a unit based on logarithms to the base 10, and equal
to 10 of the American Company's "transmission units,"

OR the "neper" (after Napier), a unit based on Naperian
logarithms to the base e.

The growth in popularity of the decibel, since 1929, has been so great
that it is now almost a household word throughout all branches of
Electrical Engineering and Acoustics.


2. Definitions,

THE DECIBEL:- Two powers P1 and P2 are said to differ by N "bels" when-

P1/P2 = 10^N i.e. N=Log[10] P1/P2


[...further examples and definition of neper...]


3. Cables Amplifiers and Attenuators

[...examples of practical application...]


4. Voltage Gain in dBs, - In general, two powers P1 and P2 will be
compared by observing either, the voltage developed across a given
impedance, or, the current through it. **If the input and output
impedances of (say), an amplifier are equal**, the power ratio will be
proportional to the square of the voltage or current ratio.

[... numerical examples...]

The voltage or current ratio relation is very frequently misused to
describe a power ratio without regard to the necessary conditions of
equality between the input and output impedance [...examples
follow...].

~~~~~~~~~~~~~

** The text between asterisks is in bold typeface.

 

[[email protected]]

[...]

Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

Care to cite your "one true standard:?

I will quote from an official publication which cites it, as I do not
have acces to the original international agreement:

"an official publication" ROTFLOL!

<"official publication" snipped>

 

[Adrian Tuddenham]

[...]
Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

Care to cite your "one true standard:?

I will quote from an official publication which cites it, as I do not
have acces to the original international agreement:

"an official publication" ROTFLOL!

<"official publication" snipped>
[reinstated]

"Admiralty Handbook of Wireless Telegraphy" (H.M.S.O. 1938)

I presume that you are feigning ignorance because it suits your cause
-if you did not already know the significance of a publication by
"H.M.S.O." you should have checked before replying.

 

[Adrian Tuddenham]

[...]

Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

Care to cite your "one true standard:?

Martin, W.H., "DeciBel --the new name for the transmission unit. Bell
System Tech. J. January, 1929

 

[Fred Abse]

The "Decibel" is the 1/10th part of a "bel" (after Alexander Graham Bell,
inventor of the telephone sounder)

In which case, it should rightfully be called the DeciMeucci.

About time that guy got something named after him. Bell doesn't deserve
approbation.

 

[Fred Abse]

I presume that you are feigning ignorance because it suits your cause -if
you did not already know the significance of a publication by "H.M.S.O."
you should have checked before replying.

Irrelevant outside of the UK.

 

[Adrian Tuddenham]

[[[.]

Don't blame the standard; there is only one true standard where the dB
is concerned.

What is that?

Martin, W.H., "DeciBel --the new name for the transmission unit. Bell
System Tech. J. January, 1929

Adopted by international agreement.

 

[[email protected]]

[...]
Don't blame the standard; there is only one true standard where the dB
is concerned. The mess-up has been caused by sloppy thinking, bad
teaching (and the ignorance it perpetuates) and failure to see or
explain the larger picture when taking convenient short-cuts.

Care to cite your "one true standard:?

I will quote from an official publication which cites it, as I do not
have acces to the original international agreement:

"an official publication" ROTFLOL!

<"official publication" snipped>
[reinstated]

"Admiralty Handbook of Wireless Telegraphy" (H.M.S.O. 1938)

I presume that you are feigning ignorance because it suits your cause
-if you did not already know the significance of a publication by
"H.M.S.O." you should have checked before replying.

Another stupid Euorpeon heard from. Do you have any idea what the
word "an" means? I want *THE*. ...what an idiot...