EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: coppercone2 on May 04, 2021, 06:37:41 am
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So I have a HP impedance match device that does 135, 600 and 900 Ohms. 600 I kind of get, it seems pretty common,
but what the hell are 135 and 900 ohms used for? Does anyone have a history of these impedance values ? Does anything interesting use them?
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I don't think so. They are probably there because it was easy and makes it look like the unit is more versatile.
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Ask a telephone man: Characteristic impedances of 600 Ω for home wiring and 900 Ω for central office wiring. IDSN filters 135 ohms @40kHz
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why does it change like that? I guess ISDN is more similar to 50 Ohm, but why 900 and 600? Then again the ISDN frequency is really low too, so I have no idea why any of this is done.
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why does it change like that? I guess ISDN is more similar to 50 Ohm, but why 900 and 600? Then again the ISDN frequency is really low too, so I have no idea why any of this is done.
The characteristic impedance of a twisted pair telephone cable is typically around the 120-130 \$\Omega\$ range at the higher frequencies where ISDN works. The impedance of a twisted pair cable varies considerably throughout the audio band and is higher at lower frequencies.
The 600 \$\Omega\$ figure was the approximate characteristic impedance for early open wire telephone lines.
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why have 2 different types of cable?
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why have 2 different types of cable?
Open wire lines for putting on top of telephone poles.
Twisted pair cables for burying in the ground.
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Ask a telephone man: Characteristic impedances of 600 Ω for home wiring and 900 Ω for central office wiring. IDSN filters 135 ohms @40kHz
Given the vintage of these devices, I also have one, it would not have been for ISDN but for carrier systems.