• 88mis, Explained!

    From G8MNY@21:5/101 to WW on Fri Aug 23 14:55:50 2019
    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)
  • From G8MNY@21:5/101 to WW on Fri Aug 23 23:09:46 2019
    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)