Pulse transformers, why?

[ErikBaluba]

Hi,

I'm putting together my own SPI based LAN interface using a cheap Microchip
ethernet chip.

Why are circuits interfaced to Ethernet have pulse transformers? What make
such transformers special? I saw one example schematic for a
pulsetransformer were it was grounded via 2000V capacitors. Is data
transmitted on an Ethernet's twisted-pair cables by generating high voltage
pulses to facilitate longer distance and higher speed because of increased
S/N or something? I tried to find voltage specs for Ethernet signals on
google but couldn't find any.

And then what about such transformers used on the phone lines? I figure they
are necessary to step down the "high" phone line voltages?

erik

 

[DJ Delorie]

Why are circuits interfaced to Ethernet have pulse transformers?

Lightning strikes.

 

[Andrew Holme]

Hi,

I'm putting together my own SPI based LAN interface using a cheap
Microchip
ethernet chip.

Why are circuits interfaced to Ethernet have pulse transformers? What make
such transformers special? I saw one example schematic for a
pulsetransformer were it was grounded via 2000V capacitors. Is data
transmitted on an Ethernet's twisted-pair cables by generating high
voltage
pulses to facilitate longer distance and higher speed because of increased
S/N or something? I tried to find voltage specs for Ethernet signals on
google but couldn't find any.

Ethernet uses low-voltage differential signalling. The transformer is
required to remove common-mode voltages which may result from pick-up on the
cable, or from differences in mains earth potential around the building.
The common-mode signal could easily overload or even destroy the receiver.
Ethernet pulse transformers have a 1:1 ratio.

 

[DJ Delorie]

Ethernet pulse transformers have a 1:1 ratio.

Not all of them. Some are 1:1.414, 1:2, or 1:2.5. I suppose other
ratios are possible too.

 

[DecaturTxCowboy]

Why are circuits interfaced to Ethernet have pulse transformers? What make
such transformers special?

No idea why they use the phrase "pulse" transformer (I guess they could
have just as well used "data" transformer), but its simply a transformer
to float the battery and ground, just like in a telephone system. In a
network card, none of the data in/out pairs are directly connected to a
power supply or ground.

----) || (-----> Telephone line "tip" to subscriber

telephone ) || (
audio ) || (-----> -48 volt power supply "battery"
signals ) ||
) || (-----> Earth ground
) || (

----) || ( -----> Telephone line "ring" to subscriber

And then what about such transformers used on the phone lines? I figure they
are necessary to step down the "high" phone line voltages?

No, see above.

 

[Joerg]

DecaturTxCowboy wrote:

No idea why they use the phrase "pulse" transformer (I guess they could
have just as well used "data" transformer), but its simply a transformer ...

True. It's just a customary term since there are no different words for
small and large transformers in English. In other languages they call
them by what they are really for, like 'Trenn-Uebertrager' in German.
The first part of that means 'separation', as in isolate.
'Transformator' would be their word for the big iron that is connected
to mains. In English a pulse transformer is considered something small,
usually.

Erik: If these transformers weren't there all it would take is one large
inductive spike and the Ethernet card would be gone. For example when
the big air conditioner motor kicks in and the CAT-5 cable happens to
run near its supply line.

Regards, Joerg

 

[Rich Grise]

Hi,

I'm putting together my own SPI based LAN interface using a cheap Microchip
ethernet chip.

Why are circuits interfaced to Ethernet have pulse transformers? What make
such transformers special? I saw one example schematic for a
pulsetransformer were it was grounded via 2000V capacitors. Is data
transmitted on an Ethernet's twisted-pair cables by generating high voltage
pulses to facilitate longer distance and higher speed because of increased
S/N or something? I tried to find voltage specs for Ethernet signals on
google but couldn't find any.

And then what about such transformers used on the phone lines? I figure they
are necessary to step down the "high" phone line voltages?

They're called "pulse transformers" to distinguish them from all of the
other varieties of transformers - here's an interesting read:
http://en.wikipedia.org/wiki/Line_transformer

From my own learnings/experience, I'm speculating that they use pulse
transformers because of their excellent rise-time characteristics, which
would translate into frequency response, and they also can handle enough
power to put a good, solid, reliable signal on the line, plus I intuit
that they have good noise immunity on the receive side, so to speak.

Hope This Helps! (Feh - hope it's accurate! )
Rich

 

[Rich Grise]

Not all of them. Some are 1:1.414, 1:2, or 1:2.5. I suppose other
ratios are possible too.

Any ratio at all that's a ratio of two integers - how do they get 1:1.414?
1000:1414 turns?

Since this is .basics, it should be explained that the turns ratio is
equal to the voltage ratio, and inversely proportional to the current
ratio, and something gets squared to work with impedances, but I slept
or drank my way through that class. ;-)

Cheers!
Rich

 

[DJ Delorie]

Any ratio at all that's a ratio of two integers - how do they get
1:1.414? 1000:1414 turns?

I have no idea. They're labelled as "1 : sqrt(2)". I assume they
don't have trancendental turns in there.

Perhaps they use 5:7 and hope it's close enough? ;-)

 

[Tim Williams]

(sqrt(2) ratio)

Perhaps they use 5:7 and hope it's close enough? ;-)

Bah, just end the last turn a few degrees shy of 7 turns.

Tim