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G8MNY > TECH 08.10.24 08:01l 122 Lines 5347 Bytes #999 (0) @ WW
BID : 17556_GB7CIP
Read: GUEST
Subj: 88mH Coils, Explained!
Path: IZ3LSV<IW8PGT<I3XTY<I0OJJ<GB7CIP
Sent: 241008/0658Z @:GB7CIP.#32.GBR.EURO #:17556 [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 thought I would explain what they
were used for by the millions in the UK telecomms industry.
LINE THEORY
The basic 600ohm 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
ÄÄÄÂÄÄRÄÄÂÄÄRÄÄÂÄÄRÄÄÂÄÄÄ ÄÄÄÂÄÄ((()ÄRÄÄÂÄÄÄ
600R === === === === 600R 1200R === :::: === 1200R
ÄÄÄÁÄÄRÄÄÁÄÄRÄÄÁÄÄRÄÄÁÄÄÄ ÄÄÄÁÄÄ((()ÄRÄÄÁÄÄÄ
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 600ê!)
2/ substantially reduces the line loss below the cut off frequency
3/ & flattens the comms band frequency response.
Normal 600ohm 88mH Loaded 1200ohm
Line Loss (e.g. 20 miles) Line Loss
dB dB
30´ ,' 30´ Þ
³ .ú' ³ Ý
20´ .ú' No Sharp 20´ Flatter response Ý Sharp
³ __..-~ cut off. ³ And Half the Loss Þ cut off.
10´------ÄÄÄÄ~~~ 10´ __/
³ ³ -------ÄÄÄÄÄ~~~
0ÅÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ>Hz 0ÅÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ>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
ÄÄÄÂÄÄÄÄÄÄ((()ÄÄÄÄÄÄÄÂÄÄÄÄÄÄ((()ÄÄÄÄÄÂÄÄÄÄ ÄÄÄÂÄÄÄÄÄÄ((()Ä
1200ê === :::: === :::: === OR === :::: 1200ê
ÄÄÄÁÄÄÄÄÄÄ((()ÄÄÄÄÄÄÄÁÄÄÄÄÄÄ((()ÄÄÄÄÄÁÄÄÄÄ ÄÄÄÁÄÄÄÄÄÄ((()Ä
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 ³ ~~~ ³===== of wires
2.5cm dia ³ ³
\_ _/
~~~
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 ÚÄÄÄÄÄÄÄÄ¿
)::( o )::(o\ / L Cancels
)::( WRONG )::( X so just a
o )::(___ o )::(_/ \___ few mH
³_______88mH ³___________ ?
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!
_____ ÚÄÄÄÄÂÄÄ ÚÄÄÄÄÄÄÄÄÂÄÄ
)::( o )::( o o )::(o\ / L Cancels
)::( 22mH OR )::( 22mH WRONG )::( X so just the
)::(_____ )::( 1/2 DC ê )::(_/ \ odd mH
ÀÄÄÄÄÁÄÄ ÀÄÄÄÄÄÄÄÄÁÄÄ
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 ÄÂÄÄÄÂÄÄÄÄÂÄÄÄÂÄÄÄÄ¿
55mA ³ e\³ 2K2 2K2 ³/e
³+ ÃÄÄ´ ÃÄÄ´ 2x 50V PNP
100u === /³ 22K 22K ³\ 100mA
³ ³ \ / ³
³ ³ X ³
³ ³_____/ \_____³
³ ³ ³
³ ³ ========= ÃÄÄ´<ÃÄÂÄÄÂÄÄ> -11V 50mA
³ ÃÄ(((()Â(((()ÄÙ ³ ³ - (unregulated)
³ ÀÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄ´<ÃÄÙ ===100u
0V ÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄ>
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?
73 De John, G8MNY @ GB7CIP
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