Getting matching transformer from telephone



Kingston telecoms anyone;)..
...alright not that much of the UK!....
--
Tony Sayer


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Actually I did have them in mind whilst composing my reply but forgot while I was typing. :-)
--
Stuart Winsor

For Barn dances and folk evenings in the Coventry and Warwickshire area
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Eeyore wrote:

Really? Minorities were allowed to use them back then?
http://webpages.charter.net/jamie_5 "
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The definition of on/off hook supervision has not changed, and that makes it exceedingly relevant.

Not necessarily. Some do have transformers.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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"Floyd L. Davidson" wrote:

Such as ?
Graham
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Not true, as has been discussed elsewhere.

Not true, as has been shown conclusively in other posts.

That is absolutely untrue. Loading coils were not uncommon on 1/2 mile loops too! Basically whatever the design plan was for outside plant, it would be used for *all* loops.

Well, there you go! You haven't got enough exposure to how telcom systems function to be telling anyone anything. You should be asking for help!

Then just exactly how do you think the transmitter got power in those old phones?

Do you have a clue? Do you know what the difference between "ground start" and "loop start" is?
Interestingly, the "line card" that supplies loop current, and detects hook supervision, also has a transformer. In the days of mechanical switching systems, it was for isolation only. With modern digital systems, it too is a hybrid.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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"Floyd L. Davidson" wrote:

OLD (and no longer used) is all you seem to know about.
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You falsely claimed it was otherwise. Seems *you* are the one with a lack of "know" about this topic.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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"Floyd L. Davidson" wrote:

What ?
Look, I've designed line interfaces FFS. For Xerox. I've already mentioned it once.
Graham
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You can say anything you like, but when you make up all the ridiculous garbage you are posting to this thread there is little doubt that you haven't got even the faintest clue how telecommuncations equipment works.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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"Floyd L. Davidson" wrote:

Want to to see the schematics ? I'm sure I still have them here somewhere.
Graham
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Yes, I would like to see them. Probably better than the Sunday funny paper.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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Interesting thread (not the childish abuse, the technical stuff!). I didn't know that loading coils were so common in the US, for example.
My twopenn'orth, as someone who has designed line interface circuits in the past:

This is true - someone (not sure who, sorry) commented earlier that this doesn't matter because the speed of light is so high, and that's true for local calls but not for long distance ones. These days there is packet delay to worry about as well. There are echo cancellers but they're not perfect. Mismatches also affect loudness.

It's true that isolation is not important for a well-insulated telephone. It's also true that differential amps are a cheap alternative to transformers for the hybrid part. But I have definitely seen phones with transformers in them in the past (the 80s). And I have designed interfaces myself (for modems) that used transformers in preference to op amps because of their superior common mode rejection, isolation and (electrical) robustness.

I have, from the days before transistors - they were specially constructed to tolerate DC current without saturating the core or overheating.
--
Phil McKerracher
www.mckerracher.net
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"Phil McKerracher"

** So you post more tech BS & childish stuff - great .

** Make sure you get attributitions correct - pal.
- someone (not sure who, sorry) commented earlier that this

** The comment was:
" Audible echoes on a few miles of twisted pair ??? "
Which excludes all longer lines, of course.

** But *audible echoes* on a line are not caused by transmission line behaviour.
They are caused by problems with the hybrid to line impedance match so that signals get retransmitted back to the exchange PLUS there has to be a significant time delay caused by a very long link - ie thousands of miles.

** What you claim you saw without proof is irrelevant
- seen any Martians lately ?
Exceptional cases are also irrelevant to the original matter.

** Modems are not the topic.
Different animals to ordinary phones.
..... Phil
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Actually EC's damned near are perfect!
Years ago, when Data Set Termination cards first started using echo cancellation we had a field technician install one and then have a fit trying to test it! The loopback unit on the customer side of the DST appeared to go into loopback, but the testboard technician did not see a tone coming back. So out came the test equipment, and sure enough the tone was looped back, but it didn't get through this DST. Yet when a tone generator was applied, that tone worked fine. Upon investigating the new type of DST it was discovered that it had EC... and it was literally reducing the level of the looped tone to the point were the test equipment at the testboard didn't even see it.
Whatever, mismatches don't affect loudness so much as the frequency response and transient response. It will make a call sound "tinny", and that is perceived at needing more loudness to understand.
It's true that hearing the echo as an echo requires significant time delay, but hearing the echo as an annoying distortion requires very little delay.

Isolation on a tel line is not related to insulating the user from the device.

Long after the transistor became ubiquitous telephones virtually all used transformers. It wasn't until extremely good, extremely cheap op amps were developed that *any* telephone design went without a transformer. Even today, a lot of telephones have a transformer though, but it is encapsulated and not easily recognized.
Indeed, the difference between other transformers and the hybrid network in a telephone is being able to handle at least 120 mA of current without saturating (I've never heard of one overheating...).
A number of cheap substitutes at various times have tried to get by with less able designs, and many modems have been marketed with transformers that saturate at something far less than is required. It often results in poor data error performance when used on lines with more than 30 mA or so of loop current (which do happen to be very common).
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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Phil McKerracher wrote:

Which go via optical fibre or microwave link, NOT cable any more.
Graham
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That is true, but doesn't mean what you apparently think it does.
A mismatch at the distant end will result in echo, and if the distance (no matter whether it is traversed via metallic cable, fiber, microwave or satellite) is far enough to delay the echo long enough, it will be heard as an echo by the near end user.
Typical local calls usually have a low echo return delay, while long distance will be higher. However, that is not always the case and I can demonstrate examples of local calls that are routed over satellite systems (meaning the echo delay is necessarily very close to 400-500 milliseconds). Those calls require echo cancellation, because even if the echo is 30 dB down, it is still very annoying when there is that much delay.
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska) snipped-for-privacy@apaflo.com
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More fibre these days, BT seems to be taking their large scale microwave equipment's out of service very quickly.
They do however in common with other carriers seem to be using microwave more for last mile applications but for "trunk" applications fibre is the main or only means of transmission...
--
Tony Sayer


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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
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Tomi Holger Engdahl wrote:

True.
But that is NOT the 'characteristic impedance' and will vary depending whether you're 100m from the exchange / C.O. or 5km !
I appreciate your other information but it sure isn't the way we do things here in the UK.
Graham
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