Any experience with negative impedance?

[[email protected]]

Romex is a brand name. Not all brands of cable have the paper. It's
not intended as insulation, it is there to make it easier to strip the
outer jacket without damaging the insulation on each wire. You make a
cut next to the ground wire, then pull it to the side till the tear is
long enough. Then use dikes or an electrician's knife to trim the
unwanted jacket. The brands without the paper are cheaper, but require
more time to prep, and sometimes the outer jacket is stuck to the
insulation on the wires, which wastes a lot of time to carefully peel
them apart.

I have a pair of 12/14 AWG strippers that cut the outer as well as
inner insulation (one pass for each). Basically, it just nicks the
outer insulation, all the way around. Then you just pull the end and
the outer insulation just slides off on the paper (sometimes the paper
is left but usually not). They're great for electrical work (I'm just
doing my basement now - 50-60 duplex outlets and a couple of dozen
light fixtures).

 

[[email protected]]

Try them on some without the paper, and see how they work.

I've never seen Romex without the paper; either paper.

 

[Tauno Voipio]

I can claim to be old enough to have always used it.

So you get forward in furlongs / fortnight?

I'm also old enough, but not from the Isles of Fog,
so the frequency is kHz (not KHz).

 

[Adrian Tuddenham]

So you get forward in furlongs / fortnight?

Ah yes, I remember them from my teenage. Have you heard any news of
how Methuselah has progressed since he was at school with me?

 

[Fred Abse]

I presume you are working with RF. Is there an RF equivalent of dBu,
based on voltage without reference to impedance?

Not as far as I know.

dBmV (ref 1 millivolt) are occasionally used, as are dBKTB.

50 ohm signal sources of whatever frequency are usually calibrated
terminated, whether or not they offer voltage, dBm, or both. Some ancient
signal generators were calibrated "EMF into 50 (sometimes 75) ohms", which
was downright confusing.

A useful thing to remember is that thermal noise power in a 1Hz bandwidth,
at 290K, is -174dBm, irrespective of impedance.

 

[Fred Abse]

A few thousand amps D.C. in conductors about 4 metres apart can do
wonderful things to nearby CRT television monitors and pickup tubes.
The return conductors rails should not be in multiple contact with
earth, but faults can occur and remain unnoticed until someone complains
that they are getting several volts difference along an 'earthed' wire
(or the foundations of a steel-framed building have just been eaten
through).

As I understand it, the London subway system uses two live rails, one
positive, one negative, asymmetrically centered to ground. Something to do
with electrolytic effects in buried metalwork.

 

[josephkk]

The dBm I use are relative 1mW in 50 ohms.

There is something very strange here, i learned 0 dBm as 1 mW in 600 ohms.
Of course, that was about 50 years ago, they might have changed it. I
also learned sometime not long after, that dBu (the "u" is really a Greek
mu) is 1 uV in 50 ohms.

?-)

 

[josephkk]

Not as far as I know.

dBmV (ref 1 millivolt) are occasionally used, as are dBKTB.

50 ohm signal sources of whatever frequency are usually calibrated
terminated, whether or not they offer voltage, dBm, or both. Some ancient
signal generators were calibrated "EMF into 50 (sometimes 75) ohms", which
was downright confusing.

A useful thing to remember is that thermal noise power in a 1Hz bandwidth,
at 290K, is -174dBm, irrespective of impedance.

Actually dB and for that matter bels is about power and power ratios,
always irrespective of impedance.

?-)

 

[Adrian Tuddenham]

There is something very strange here, i learned 0 dBm as 1 mW in 600 ohms.
Of course, that was about 50 years ago, they might have changed it.

There hasn't been any change in the definition although there has been a
lot of argument about it based on misunderstanding. At least one
textbook incorrectly defines decibels in terms of voltage and doesn't
mention power (I hope the author corrects this in folowing editions).

I
also learned sometime not long after, that dBu (the "u" is really a Greek
mu) is 1 uV in 50 ohms.

?-)

Perhaps the 'u' in the 600-ohm voltage useage really is an English 'u',
so as to distinguish it from the 'mu' in the 50-ohm useage?

 

[Adrian Tuddenham]

Thanks for that info. It looks as though I don't have much to worry
about there.



That looks very similar. T&E doesn't have the paper insulation and the
colours are different, but the geometry is almost identical. I was
proposing to use 1.5 sq.mm load conductor size (the earth is slightly
smaller) which has a resistance of 12.10 milliohms per metre of single
conductor. If the inductance doesn't prove to be a problem, I would
prefer to use the load cables to feed the voice coil and the earth wire
as the sensing wire - mainly because that is how the installer will wire
it up, no matter how many times I instruct him to the contrary.

As the client will be needing a drum of cable, I will buy it and make
the necessary tests and measurements before sending it on to be
installed on the premises. That way I shall have the opportunity to
sort out any problems 'in house' before arriving on site.


I can claim to be old enough to have always used it.

Fortunately, help is available: [...]
https://dl.dropbox.com/u/53724080/SED/CONVERT1.GIF

I'll print that out and hang it on the wall for quick reference.

 

[WoolyBully]

Fortunately, help is available: [...]
https://dl.dropbox.com/u/53724080/SED/CONVERT1.GIF

I'll print that out and hang it on the wall for quick reference.

direct linear conversions do not need a log chart axis.

 

[Tauno Voipio]

There is something very strange here, i learned 0 dBm as 1 mW in 600 ohms.
Of course, that was about 50 years ago, they might have changed it. I
also learned sometime not long after, that dBu (the "u" is really a Greek
mu) is 1 uV in 50 ohms.

?-)

It is the normal sloppiness of audiophiles. It seems that audio
is far more art than science, so normal maths do not apply.

 

[Fred Abse]

Taunton had the most unusual answer to electrolytic corrosion, they
effectivley used low frequency A.C. by reversing the polarity of the
system each day. It didn't matter to the tram motors whether the overhead
wire was positive or negative.

Driver starts shift. Switches to "reverse", to get out of the depot, puts
a couple of notches on the controller, and crashes the buffers.

"Oh shit, I forgot it's Tuesday!"

 

[Fred Abse]

TV uses dBmv and references 0 dBmv as one millivolt at 75 ohms.

Source EMF or terminated?

 

[Fred Abse]

I thought that one of the conductor rails was grounded at the sub-station
but not at any other point along its length. That prevented any return
currents from flowing through the earth.

According to:
http://en.wikipedia.org/wiki/Railway_electrification_in_Great_Britain#630_V_DC.2C_fourth_rail

The outer rail is at +420V, and the inner rail at -210V.

Stated to have been chosen to avoid circulating currents in the iron
tunnel linings.

 

[Fred Abse]

Actually dB and for that matter bels is about power and power ratios,
always irrespective of impedance.

?-)

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

As are nepers.

 

[Fred Abse]

There is something very strange here, i learned 0 dBm as 1 mW in 600 ohms.

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.

 

[Adrian Tuddenham]

I doubt that such motors are PM ?:-}

The armatures and fields were in series, so the direction of rotation
only depended on their relative polarity, not on their absolute
polarity.

A problem which recently afflicted an experimantal tram was caused by
the motor-control software being identical at both driving ends. The
driver notched up for the first time and the two powered bogies strained
to go off in opposite directions.

There is a hidden danger in some modern heavy-current D.C. equipment:
some contactors have a permanent-magnet blow-out system for quenching
the arc. If the contactor is installed backwards or the current flows
in reverse for any reason (such as incorrect polarity), the contactor
fails to blows out when breaking a heavy current and the arc propagates
back into its terminals and wiring. The older, safer, contactors had
the frame magnetised by winding a few turns of the load conductor around
it, if the current reversed, so did the blow-out field.

 

[Adrian Tuddenham]

According to:
http://en.wikipedia.org/wiki/Railway_electrification_in_Great_Britain#630_
V_DC.2C_fourth_rail

The outer rail is at +420V, and the inner rail at -210V.

I'm quite surprised to hear that.

Stated to have been chosen to avoid circulating currents in the iron
tunnel linings.

I can't imagine why the voltage relative to earth should affect the
circulating currents, they would only depend on the current changes in
the conductor rails.

 

[Fred Abse]

I doubt that such motors are PM ?:-}

Probably not, but it makes a good story ;-)