Getting matching transformer from telephone

[Ross Herbert]

On Mon, 29 Dec 2008 16:07:31 -0900, [email protected] (Floyd L. Davidson) wrote:

:>"Floyd L. Davidson"
:>> Eeyore
:>>>
:>>>> Historically they *all* had such a transformer, and even
:>>>> today many of them do (it's cheap).
:>>>
:>>>Transformers are not cheap and I haven't ever seen a phone with one in,
:>>>even
:>>>going back 30+ years. There's simply no need.
:>>>
:>>
:>> Get a diagram of any given telset you wish that works
:>> without active components (amplifiers), and you *will*
:>> find a transformer.
:>>
:>> And trust me, they *are* cheap!
:>>
:>> Going back 30+ years, they *all* had transformers.
:>
:>** But not the 1:1 matching/ isolating kind the OP asked about - dickhead.
:
:I'm sorry, but the nickname your mother uses for you is
:not appropriate in reference to other people.
:
lease not that I have not said it is a matching
:transformer, nor have I said that it is necessarily
:appropriate for the OP. All I did was properly describe
:what it actually is.
:
:> The " hybrid " circuit couples the earphone and mic to the line but is
:>generally not even isolating.
:
:Wrong again. It provides isolation. As I mentioned in
:another post, a POTS loop needs to isolate the VF signal
:from the DC signal, and also to isolate the transmit
:signal from the receive signal.
:
:It's primary purpose is isolation. It does not provide
:impedance matching (in typical telsets, though I assure
:you there are special ones that do have exactly that
:function built into the hybrid network).


I have to agree with Phil on this one. The so called "transformer" you are
referring to in a POTS telephone is NOT for isolation at all. In fact in its
main function is to act as a hybrid matching transformer. Most of these
transformers will have 3 inductive windings and may also include non-inductive
windings to act as resistors (such as for the balance arm of the hybrid). The
windings are not generally galvanically isolated from one another and the device
generally conforms as a type of "auto-transformer".

Here is some text from an old BPO training document which describes the function
of the hybrid transformer in a POTS telephone. Despite it being old doesn't mean
that the theory is outdated. Incidentally, the schematic included shows all 3
windings connected in series, ie. NOT galvanically isolated.

QUOTE:
It is only possible for the transmitter, receiver, and line of a telephone
circuit to be interconnected so that each is electrically matched to the
remainder of the circuit, thus ensuring maximum transfer of sent and received
power, when an additional power consuming impedance is included in the
circuit. The power consuming impedance is termed a 'balance'.

The sense of the three windings of the induction coil and the electrical
characteristics of the other components are such that, ideally

(a) when transmitting, the output power is shared only between the line and
balance,

and;

(b) when receiving, the output power from the line is shared only between the
receiver and the transmitter.
UNQUOTE

 

[Ross Herbert]

On Mon, 29 Dec 2008 16:03:16 -0900, [email protected] (Floyd L. Davidson) wrote:

:>Archimedes' Lever wrote:
:>
:>> [email protected] (Floyd L. Davidson) wrote:
:>> >
:>> >As far as it being a matching transformer, the line
:>> >impedance varies typically from perhaps 100 Ohms all the
:>> >way up to perhaps 2000 Ohms... but you will not find
:>> >anything in a telset to adjust it to match. That's because
:>> >nobody cares if it is even close to matching the line
:>> >impedance.
:>>
:>> Which is why I stated its primary purpose as being that of isolation
:>
:>What exactly needs isolating from what ? Have you noticed they tend to be
:>made of plastic ?
:
:The transformers are made of plastic??? (I don't think so...)
:
:A telephone loop has both VF (voice frequency) and DC
:signals going in both directions, all of which need to
:be "isolated" from each other. The transmit and receive
:VF signals also have to be isolated from each other
which is why the transformer is commonly called a
:"Hybrid Network").

The hybrid in a POTS phone can have all windings effectively connected in
series. It is the polarity of the windings and the relative impedances and the
connections of the transmitter, receiver and balance impedance which effectively
provide for SIGNAL ISOLATION (ie. NOT galvanic isolation) depending upon whether
the telephone is transmitting or receiving. The windings don't have to be
galvanically isolated to do this.

 

[Ross Herbert]

On Mon, 29 Dec 2008 16:07:31 -0900, [email protected] (Floyd L. Davidson) wrote:

:
:It's primary purpose is isolation. It does not provide
:impedance matching (in typical telsets, though I assure
:you there are special ones that do have exactly that
:function built into the hybrid network).


WRONG! It does provide impedance matching between the line, transmitter,
receiver and balance network. If these elements are not effectively balanced
during the various transmission and receiving phases then signal power transfer
would be severely degraded and efficiency would be poor.

Do NOT confuse SIGNAL isolation with GALVANIC isolation. The hybrid in a phone
provides the former but NOT the latter.

 

[Archimedes' Lever]

"Floyd Lunatic Davidson"


** Don't be such a pompous bloody prick.

Of course he meant the phone itself.




** Don't be such a pompous bloody prick !!

Any "isolation " transformer has " galvanic isolation" .

Look it up on Google, if you are still completely clueless.

Wot a Wanker.

I do not recall giving it the moniker of "isolation transformer". I
merely mentioned the function of electrical isolation.

 

[Tomi Holger Engdahl]

Twisted pair cable as used for telecoms has a nominal 100-110 ohm
inpedance. See the ADSL specs.

The twisted pair cable as used for telecoms has a nominal 100-120 ohm
impedance for high frequecies the ADSL system uses. The ADSL
system uses frequencies between 138 kHz and 1.1 MHz for downstream
data (and 25 kHz to 138 kHz ofr upstream). This 100-120 ohm
impedance holds pretty well for those frequencies above 100 kHz.

For lower frequencies the the impedance of the telephone cable
is not anymore that 100-120 ohm, but something else.
For voice frequencies used on on normal telephone (300-3400 Hz)
the impedance is normally considerable higher than 120 ohms.

Normal telephone subscriber lines in USA (0.4-0,6mm subscriber PE
insulated vaseline filled cable) are 770 ohm resistor (with 2uf series
capacitor) and 47nF parallel capacity.


2 uF
||
----+-----||--------+
| || |
| | |
--- | | 770 ohms
--- 47 nF | |
| |
----+---------------+

This diagram is referred to 800Hz, but impedance is rather complex,
and varies from high value at low frequency and drops to ca. 150 ohm
on 10kHz and 120-125 ohm above 100kHz.

Some telephone lines can have higher impedance (typically 1100 ohms in
lines with loading coils or telephone air cables).

In european specifications (for Finland etc..) I have seen this
that complex reference impedance Z = 270 + (750 //150 nF)

750 ohm
_____
270 ohm +--|_____|--+
_____ | |
--|_____|---+ +-----
| || |
+----||-----+
||
150 nF

Typical cable used in for subscriber lines has following
characteristics: 0.5 mm diameter wire, loop
resistance 182 ohm/km and pair capacitance 39 nf/km.


TRANSMISSION SYSTEMS FOR COMMUNICATIONS, revised 4th edition, Bell
Telephone Laboratories (1971) gives the followign information on
typical cable characteristics:

"The primary constants of twisted pair cables are subject to
manufacturing deviations, and change with the physical environment
such as temperature, moisture, and mechanical stress. The inductance,
L, is of the order 1 mH/mile for low frequencies and the capacitance,
C, has two standard values of 0.066 and 0.083 uF per mile although
lower capacitance cables are under development.

Of the primary constants, only C is relatively independent of
frequency; L decreases to about 70 percent of its initial value as
frequency increases from 50 kHZ to 1 MHz and is stable beyond; G is
very small for PIC (polyethylene insulated cables) and roughly
proportional to frequency for pulp insulation; and R, approximately
constant over the voiceband, is proportional to the square root of
frequency at higher frequencies where skin effect and proximity effect
dominate."

600 ohms is an irrelevant historical nonsense from the days when they used
telegraph wires for phone circuits.

600 Ohms is somewhat of a compromise between different real-life
impedances that could be seen. Normal telephone line connections are
theoreticallydesigned to be 600 ohm resistive impedance. This 600
ohm is kept as international reference for designing telephone line
equipment (typically the signal powers are measured to 600 ohm load).
In practice the telephone line does lot look like pure 600 ohm
resistance.

Telephone equipment which is designed to operate with 600 ohm loads
will operate with those real-life lines, but it's performance is worse
than in ideal situation. Typically the modems are designed for 600 ohm
reference impedance because they can handle the sidetone.

The return loss of the terminal equipment must be greater than 10 dB
when compared to 600 ohm reference. This measurement applies to
telephones, modems and other terminal equipments. NET4 technical specs
are European specs and they are used in many European countries (NET4
is actually a collection of different specs in use in different countries).

For best performance the telephones are designed to the exact line
impedance. Matching the hybrid circuit to the real line impedance
(instead of 600 ohm) will improve the feedback typically by
3-6dB. 20dB sidetone is easy to achieve, but 30dB is also not too
difficult provided you can measure the line impedance and take steps
to build a correct balancing network.

 

[Archimedes' Lever]

Which has nothing to do with the information I am giving here.

I have come across your attitude in other groups where americans don't
seem to understand that their way of doing things isn't universal
throughout the rest of the world.

Hey! Floyd doesn't have an attitude, and is pretty darned good at
accepting other standards and practices, once they are brought to his
attention. I am sure he will concur with you once he realizes from your
post, that the geography and era were different than that he had his
mindset in.

 

[Phil Allison]

" Floyd LUNATIC AUTISTIC Davidson"


** Wot a brain dead, lying, know nothing ****.

Gotta be another demented, ASD fucked code scribbler.




...... Phil

 

[Phil Allison]

"Archimedes' Lever"


** Piss off - you stupid wanker.



.... Phil

 

[John Livingston]

You also apparently don't know what "repeater stations"
are when dealing with analog carrier systems.

Repeater Stations were not limited to FDM carrier systems. "Repeater"
refers primarily to amplification. Plenty baseband amplified circuits as
well (2 wire and 4 wire).

Correct. I've worked on circuit equalisation at Burghead end. It was
baseband audio - not carrier.

You sent them that distance via FDM carrier systems, not
via landlines, even in the 1930's.

Very rarely - most carrier channels were 300-3400Hz. The noise and
distortion figures came nowhere near the requirements for broadcast
audio. ISTR there were attempts to use carrier, but with all sorts of
problems, not the least being that the CCITT carrier frequency plans
(Groups-Supergroups-Hypergroups) are based on 4KHz channel spacing - not
10KHz. Single Sideband translations are also problematic with music
(broadcast) circuits, as the frequency stability requirements are MUCH
higher than telephone-quality speech. Master oscillators DID go off
frequency ....

Twisted pair cables are not 600 Ohms.

WRONG!!!!!

The characteristic impedance of a transmission line(Zo)varies by
frequency. At audio the influence of series resistance and parallel
conductance outweighs the ratio of inductive to capacitive reactance,
and the Zo rises as the frequency falls. A twisted pair will be
typically 600 ohms at 800Hz.
At RF the reactances become much more significant than the
conductance/resistance, and the Zo will level out at the figure usually
quoted as Characteristic Impedance. The same twisted pair will be about
140 ohms at RF.

Open wire might be though... ;-)

See above.
"rep-coils"???? WTF ??

Known as "Transformers" in ever repeater station I ever worked in .....

By and large? And 75 Ohms??? (Please don't try buzz words...)

Been there. Done it.

John

 

[Don]

In

What was the "test match" effect?

I'm intruiged! Go on - tell us!

 

[Archimedes' Lever]

"Archimedes' Lever"


** Piss off - you stupid wanker.



... Phil

An 'isolation transformer' is a device which gets attached to the power
feed. It is for power conversion and isolation on AC systems.

It has NOTHING to do with discussions about isolation in a telephone
circuit, you retarded ****.

So you piss off, you stupid fucking motherless bastard.

 

[Phil Allison]

"Archimedes' Wanker"

An 'isolation transformer' is a device which gets attached to the power
feed. It is for power conversion and isolation on AC systems.

It has NOTHING to do with discussions about isolation in a telephone
circuit,

** God, you are one STUPID PIG IGNORANT PUKE !

http://en.wikipedia.org/wiki/Isolation_transformer

http://en.wikipedia.org/wiki/Galvanic_isolation



....... Phil



http://en.wikipedia.org/wiki/Isolation_transformer

 

[Eeyore]

So now you finally agree too, eh? Who does that make
into a "TWAT"?


An ignorant statement, to say the least.

An informed one actually. I have NO phone with a transformer. Some analogue
modems yes but even there they found a way round it with optocouplers.

Graham

 

[Eeyore]

Now, you are catching on. Eeyore is a british idiot

Your ignorance stands out like a shining beacon on a dark night.

There never was or has ever been a need for a transformer in a phone.

Graham

 

[Eeyore]

Not much of a difference, eh?

What meds are YOU on ?

 

[Eeyore]

For some cables. Not for others.

WRONG ! For all practical cables used on the 'local run' for telecoms it's
100-100 ohms.

It might, for example, be as high as a couple thousand Ohms too.

Utter tripe.

Not at all true. 600 Ohms is somewhat of a compromise,
between the low impedance of an unloaded cable pair and
the higher impedance if loading coils are used.

You haven't a FUCKING CLUE what you're talking about.

Graham

 

[Eeyore]

600 ohms was the theoretical impedance of an infinite length of GPO
telephone cable. In the early days of broadcast radio (1930s), when
program was sent to the transmitters by GPO cable, assuming a line
impedance of 600 ohms and matching to it, was found to give the best
overall results. When you're sending program 600 miles from London to a
transmitter in Scotland, it matters!

They weren't using something that resembles a single pair of Cat 5 back then.

As I said, it's HISTORICAL only and of no practical value.

Graham

 

[Eeyore]

What is "GPO" ???

General Post Office.

 

[Eeyore]

Telephone cable is twisted pair

Which is why it's 100 ohms or so just like Cat 5 etc.

Graham

 

[Eeyore]

Hey! Floyd doesn't have an attitude, and is pretty darned good at
accepting other standards and practices, once they are brought to his
attention. I am sure he will concur with you once he realizes from your
post, that the geography and era were different than that he had his
mindset in.

Unfortunately has has NO CLUE about the characteristic impedance of twisted pair
cable as used for telecoms.

http://en.wikipedia.org/wiki/Twisted_pair#Unshielded_twisted_pair_.28UTP.29

"UTP is also finding increasing use in video applications, primarily in security
cameras. Many middle to high-end cameras include a UTP output with setscrew
terminals. This is made possible by the fact that UTP cable bandwidth has
improved to match the baseband of television signals. While the video recorder
most likely still has unbalanced BNC connectors for standard coaxial cable, a
balun is used to convert from 100-ohm balanced UTP to 75-ohm unbalanced."

*** 100-ohm balanced UTP ***

Graham