G8MNY TECHNI 23.08.19 06:03l 122 Lines 5338 Bytes #180 (0) @ WW
BID : 18578_GB7CIP
Subj: 88mis, Explained!
Path: OK0NBR OK2PEN CX2SA GB7CIP
Sent: 190823/0900Z @:GB7CIP.#32.GBR.EURO #:18578 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Update Dec 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
After seeing several buls on 88mH coils, I thtI would explain what they
were used for by the millions in the UK telecomms industry.

LINE THEORY
The basic 600Ω 2 wire telephone line system is quite lossy. This is due to too high a capacity between the wires & the copper resistance, this causes high loss & also higher loss at higher frequency than ideal.

Simple Line A Loaded Line section
───┬──R──┬──R──┬──R──┬─── ───┬──((()─R──┬───
600Ω === === === === 600Ω 1200Ω === :::: === 1200Ω
───┴──R──┴──R──┴──R──┴─── ───┴──((()─R──┴───

The capacitance & resistance value of the wires can't be changed. Reducing the load Z flattens the frequency response, but increases the loss (a useful trick for short music circuits!). But adding series inductance spaced evenly along the line, makes the line look like a cut off filter & has several effects...

1/ increases the line impedance (matching transformers need for 600Ω!)
2/ substantially reduces the line loss below the cut off frequency
3/ & flattens the comms band frequency response.

Normal 600Ω 88mH Loaded 1200Ω
Line Loss (e.g. 20 miles) Line Loss
dB dB
30┤ ,' 30┤ ▐
│ .·' │ ▌
20┤ .·' No Sharp 20┤ Flatter response ▌ Sharp
│ __..-~ cut off. │ And Half the Loss ▐ cut off. 10┤------────~~~ 10┤ __/
│ │ -------─────~~~
0┼─┬───┬───┬───┬───┬───┬───┬ Hz 0┼─┬───┬───┬───┬───┬───┬───┬ Hz
100 200 500 1k 2k 5k 10k 100 200 500 1k 2k 5k 10k

The standard for cables between telephone exchanges (office) was to use a loaded cable with an 88mH balanced coil every 2000 yards, starting 1000 yards from the exchange & completing the last 1000 yard pi section with an added capacitor or added capacitor & 44mH terminated inductor half section.

1000 Yards 2000 Yards 1000 Yards
───┬──────((()───────┬──────((()─────┬──── ───┬──────((()─
1200Ω === :::: === :::: === OR === :::: 1200Ω
───┴──────((()───────┴──────((()─────┴──── ───┴──────((()─
88mH 88mH (Made up to) 44mH
(1 section)
- - full section - - - half section -

BACK TO THE COILS
The coils consist of 2 identical bifilar wound windings on a common ferrite or iron core, they are quite high Q, & capable of keeping their inductance with up to 50mA of DC current flowing (which is a lot for a pot core!) which is why they are quite large for a max of 20mW (+13dBm) of AF signals on telecomms lines.

Wire colours are normally Black & White Red/Black & Red/White, & it should be quite easy to identify the 2 line pairs.
_..._
Pot Core _ _ ===== 2 pairs
2cm tall │ ~~~ │===== of wires
2.5cm dia │ │
\_ _/
~~~
Some coils are ally can encapsulated, others plastic & some not at all.

INDUCTANCE VALUE
Watch out for the 44mH ones, as they look much the same other than labelling & have half the 88mH test inductance values below.

Centre Centre
______ Tap Tap ┌────────┐
)::( o )::(o\ / L Cancels
)::( WRONG )::( X so just a
o )::(___ o )::(_/ \___ few mH
│_______88mH │___________ ?


If just 1 winding is used then you have 22mH. By paralleling the 2nd coil you get half the DC resistance, but the phase must be right!

_____ ┌────┬── ┌────────┬──
)::( o )::( o o )::(o\ / L Cancels
)::( 22mH OR )::( 22mH WRONG )::( X so just the
)::(_____ )::( 1/2 DC Ω )::(_/ \ odd mH
└────┴── └────────┴──

USES
So with suitable capacitors they make useful AF filters etc.

They can be used as isolation transformers, not too well isolated, & no good for LF response with the low L value.

One can also be used as an efficient voltage step up or a -rail inverter with just 2 transistors in flip flop. Note the push pull gives nearly 100% output all the time so smoothing is not needed in some applications!

+12V ─┬───┬────┬───┬────┐
55mA │ e\│ 2k2 2k2 │/e
│+ ├──┤ ├──┤ 2x 50V PNP
100u === /│ 22k 22k │\ 100mA
│ │ \ / │
│ │ X │
│ │_____/ \_____│
│ │ │
│ │ ========= ├──┤ ├─┬──┬── -11V 50mA
│ ├─(((()┬(((()─┘ │ │ - (unregulated)
│ └──────)─────────┤ ├─┘ ===100u
0V ──┴──────────┴────────────────┴──


See my buls on "Passive CW Headphone Filter", "AF 2 Tone Test Osc Design" & also "DC Power Conversions".


Why Don't U send an interesting bul?

--- Mystic BBS v1.12 A43 (Linux/32)
* Origin: HAMRADIO telnet slacko.kozow.com (21:5/101)

G8MNY TECHNI 23.08.19 06:03l 122 Lines 5338 Bytes #180 (0) @ WW
BID : 18578_GB7CIP
Read: GUEST
Subj: 88mis, Explained!
Path: OK0NBR OK2PEN CX2SA GB7CIP
Sent: 190823/0900Z @:GB7CIP.#32.GBR.EURO #:18578 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW

By G8MNY (Update Dec 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
After seeing several buls on 88mH coils, I thtI would explain what they
were used for by the millions in the UK telecomms industry.

LINE THEORY
The basic 600Ω 2 wire telephone line system is quite lossy. This is due to too high a capacity between the wires & the copper resistance, this causes high loss & also higher loss at higher frequency than ideal.

Simple Line A Loaded Line section
───┬──R──┬──R──┬──R──┬─── ───┬──((()─R──┬───
600Ω === === === === 600Ω 1200Ω === :::: === 1200Ω
───┴──R──┴──R──┴──R──┴─── ───┴──((()─R──┴───

The capacitance & resistance value of the wires can't be changed. Reducing the load Z flattens the frequency response, but increases the loss (a useful trick for short music circuits!). But adding series inductance spaced evenly along the line, makes the line look like a cut off filter & has several effects...

1/ increases the line impedance (matching transformers need for 600Ω!)
2/ substantially reduces the line loss below the cut off frequency
3/ & flattens the comms band frequency response.

Normal 600Ω 88mH Loaded 1200Ω
Line Loss (e.g. 20 miles) Line Loss
dB dB
30┤ ,' 30┤ ▐
│ .·' │ ▌
20┤ .·' No Sharp 20┤ Flatter response ▌ Sharp
│ __..-~ cut off. │ And Half the Loss ▐ cut off. 10┤------────~~~ 10┤ __/
│ │ -------─────~~~
0┼─┬───┬───┬───┬───┬───┬───┬ Hz 0┼─┬───┬───┬───┬───┬───┬───┬ Hz
100 200 500 1k 2k 5k 10k 100 200 500 1k 2k 5k 10k

The standard for cables between telephone exchanges (office) was to use a loaded cable with an 88mH balanced coil every 2000 yards, starting 1000 yards from the exchange & completing the last 1000 yard pi section with an added capacitor or added capacitor & 44mH terminated inductor half section.

1000 Yards 2000 Yards 1000 Yards
───┬──────((()───────┬──────((()─────┬──── ───┬──────((()─
1200Ω === :::: === :::: === OR === :::: 1200Ω
───┴──────((()───────┴──────((()─────┴──── ───┴──────((()─
88mH 88mH (Made up to) 44mH
(1 section)
- - full section - - - half section -

BACK TO THE COILS
The coils consist of 2 identical bifilar wound windings on a common ferrite or iron core, they are quite high Q, & capable of keeping their inductance with up to 50mA of DC current flowing (which is a lot for a pot core!) which is why they are quite large for a max of 20mW (+13dBm) of AF signals on telecomms lines.

Wire colours are normally Black & White Red/Black & Red/White, & it should be quite easy to identify the 2 line pairs.
_..._
Pot Core _ _ ===== 2 pairs
2cm tall │ ~~~ │===== of wires
2.5cm dia │ │
\_ _/
~~~
Some coils are ally can encapsulated, others plastic & some not at all.

INDUCTANCE VALUE
Watch out for the 44mH ones, as they look much the same other than labelling & have half the 88mH test inductance values below.

Centre Centre
______ Tap Tap ┌────────┐
)::( o )::(o\ / L Cancels
)::( WRONG )::( X so just a
o )::(___ o )::(_/ \___ few mH
│_______88mH │___________ ?


If just 1 winding is used then you have 22mH. By paralleling the 2nd coil you get half the DC resistance, but the phase must be right!

_____ ┌────┬── ┌────────┬──
)::( o )::( o o )::(o\ / L Cancels
)::( 22mH OR )::( 22mH WRONG )::( X so just the
)::(_____ )::( 1/2 DC Ω )::(_/ \ odd mH
└────┴── └────────┴──

USES
So with suitable capacitors they make useful AF filters etc.

They can be used as isolation transformers, not too well isolated, & no good for LF response with the low L value.

One can also be used as an efficient voltage step up or a -rail inverter with just 2 transistors in flip flop. Note the push pull gives nearly 100% output all the time so smoothing is not needed in some applications!

+12V ─┬───┬────┬───┬────┐
55mA │ e\│ 2k2 2k2 │/e
│+ ├──┤ ├──┤ 2x 50V PNP
100u === /│ 22k 22k │\ 100mA
│ │ \ / │
│ │ X │
│ │_____/ \_____│
│ │ │
│ │ ========= ├──┤ ├─┬──┬── -11V 50mA
│ ├─(((()┬(((()─┘ │ │ - (unregulated)
│ └──────)─────────┤ ├─┘ ===100u
0V ──┴──────────┴────────────────┴──


See my buls on "Passive CW Headphone Filter", "AF 2 Tone Test Osc Design" & also "DC Power Conversions".


Why Don't U send an interesting bul?

--- Mystic BBS v1.12 A43 (Linux/32)
* Origin: HAMRADIO telnet slacko.kozow.com (21:5/101)