Getting matching transformer from telephone

For some cables. Not for others. It might, for example, be as high as a couple thousand Ohms too.

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.

Telegraph wires were typically a #11 rusty wire, not a twisted pair.

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

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").

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

Impedance for an un-loaded pair varies with frequency. So at DSL frequencies the impedance is 100 to 110 Ohms. But for telephone use They preferred to use loaded pairs and they were designed to be 900 Ohms. And the phone should be a reasonable match to the line to minimize refections which bother the users as echoes

The transformers in some sets was not an isolation transformer but a hybrid and matching transformer. The carbon mic. used in the old phones was powered from the line. And many of the newer phones derive power from the line. So isolation can't be used.

Bill K7NOM

That is one of the best bits of humor I've seen in some time on this group. Nice post. I was rolling on the floor laughing.

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

They are commonly called either "induction coil" or "repeat coil". However, the ones used in the type of telephone you would commonly see (in a home or business) are generally encapsulated, and include a balance network, midpoint capacitor, and possibly other components, and is usually but not always called a "network" of some kind.

Equipment seen in a telephone office would more likely have been built with individual components, and at least up into the 1970's would have had a separate transformer that would indeed have been labeled as "Ind. Coil" or "Rep. Coil". At least for WECo built equipment that would have been true, and others at times did the same or came up with something different just to be different.

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

:Eeyore wrote: :>Archimedes' Lever wrote: :>

:>> snipped-for-privacy@apaflo.com (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.

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!

In the UK these cables went underground from studio, via as many exchanges and repeater stations as necessary, to transmitter and never overhead.

--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk

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 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.

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/

No it is not. (What is "GPO" ???)

Are you aware that inter-office trunking always used 900 Ohms, not 600?

I hate to tell you, but you don't just feed 600 miles of cable with a signal and expect anything to be at the end...

Let me repeat that, just so you'll understand: "Telegraph wires were typically a #11 rusty wire, not a twisted pair." Telephone cable is twisted pair, and sometimes it has been used for teletype service, which might even have been called "telegraph". But when there were wires that were "telegraph wires", they weren't twisted pair.

(Years ago I worked on telegraph systems...)

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

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. Also an inability to read other people's posts and understand what is being said.

For example, you seem to have entirely missed:

^^^^^^^^^^^^^^^^^^^^^

Yes, we did send program feeds from London to Burghead in Scotland via landline.

Yes, although different impedences are *now* used under different circumstances, the cables *were* /all/ 600 ohms twisted pair. In more recent times we've had rep-coils on the incoming lines to match to 150 ohms and 75 ohms but by and large all *main* feeds are now by digital systems.

GPO=General Post Office, the organisation in the UK originally responsible for all telecommunications in the UK.

--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk

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.

It has everything to do with the information we are discussing.

You don't know the difference between telegraph wire and telephone wire, just for starters.

If that was in reference to the previous paragraph of yours, then you should have put the information there. In that case I would have jumped all over your claim that it makes a difference for the 600 mile distance mentioned when in fact it is not 600 miles *per* *section*.

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

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

Twisted pair cables are not 600 Ohms.

Open wire might be though... ;-)

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

Thank you. (I'm familiar with BT, which evolved from the General Post Office.)

Your problem is that you just don't actually understand the telephone system, at all.

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

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.

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 ....

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.

See above.

"rep-coils"???? WTF ??

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

Been there. Done it.

John

It was the term we normally used when referring to the transformer that, by and large, separated our (BBC) wiring from that of the GPO/BT. GPO/BT technicians that I talked to seemed fully conversant with the term.

Depending on the size and layout of the station, it might be at the bottom of a bay or in a seperate "Line termination room"

Plenty of other transformers about at amplifier inputs/outputs.

--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk

But not it seems with practical aspects. If you have you might know about the following

What is a reversible line and how does it work? What was the "test match" effect?

I have a moderate understanding of telephone systems but a much greater one of the use of program lines and why they were 600 ohms.

--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
See: http://www.barndance.org.uk

I'm intruiged! Go on - tell us!

--

Don

--

aioe.org, Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages.

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This should be real interesting...

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com

An example of a typical telephone set designed for customer premise use.

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Floyd L. Davidson 
Ukpeagvik (Barrow, Alaska)              floyd@apaflo.com