DJ Dunk

iTrader: (5)

A balun is a device that joins a balanced line (one that has two conductors, with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that has just one conductor and a ground, such as a coaxial cable). A balun is a type of transformer: it's used to convert an unbalanced signal to a balanced one or vice versa. Baluns isolate a transmission line and provide a balanced output. A typical use for a balun is in a television antenna. The term is derived by combining balanced and unbalanced.
In a balun, one pair of terminals is balanced, that is, the currents are equal in magnitude and opposite in phase. The other pair of terminals is unbalanced; one side is connected to electrical ground and the other carries the signal.

Balun transformers can be used between various parts of a wireless or cable communications system.

Some baluns provide impedance transformation in addition to conversion between balanced and unbalanced signal modes; others provide no impedance transformation. For 1:1 baluns (no impedance transformation), the input and output are usually both 50 ohms or 75 ohms. The most common impedance-transformation ratio is 1:4 (alternatively 4:1). Some baluns provide other impedance-transformation ratios, such as 1:9 (and 9:1), 1:10 (and 10:1), or 1:16 (and 16:1). Impedance-transformer baluns having a 1:4 ratio are used between systems with impedances of 50 or 75 ohms (unbalanced) and 200 or 300 ohms (balanced). Most television and FM broadcast receivers are designed for 300-ohm balanced systems, while coaxial cables have characteristic impedances of 50 or 75 ohms. Impedance-transformer baluns with larger ratios are used to match high-impedance balanced antennas to low-impedance unbalanced wireless receivers, transmitters, or transceivers.

In order to function at optimum efficiency, a balun must be used with loads whose impedances present little or no reactance. Such impedances are called "purely resistive." As a general rule, well-designed communications antennas present purely resistive loads of 50, 75, or 300 ohms, although a few antennas have higher resistive impedances.

The "balanced" terminals of some baluns can be connected to an unbalanced system. One terminal of the balanced pair (input or output) is connected to ground, while the other is connected to the active system element. When this is done, the device does not operate as a true balun, because both the input and the output are unbalanced. A balun used in this way has been called an "un-un" (for "unbalanced-to-unbalanced"). Some baluns can work as an impedance transformer between two unbalanced systems if there is little or no reactance. But certain types of baluns do not work properly when connected in this manner. It is best to check the documentation provided with the device, or contact the manufacturer, if "un-un" balun operation is contemplated.



[Edited by DJ Dunk - 9/4/2002 11:04:16 AM]

ozzy

Forget that, it's an RJ45 - BT balun

Stefan

[Edited by ozzy - 9/4/2002 11:44:04 AM]

Dr Hu

I have put in an ISDN router & 2 PC's in a house with a small PABX telephone system (3 incoming ISDN pairs) - the router doesnt recognize the RJ45 socketed ISDN line (no LINK light comes on).

The company who supplied the PABX kit - says I need an ISDN Terminating Balun before it will work.....I have never heard of one of these - and all the kit worked perfectly in the office.

Anybody help? Like what does it do and where can I get one?

TIA

ozzy

Is that a cut 'n paste job Dunk?

Dr Hu

mmmmmm.....thanks........

DJ Dunk

iTrader: (5)

Nah, all from memory - honest guv

DJ Dunk

iTrader: (5)

But seriously, I have no idea, sorry.

Plenty of info on the web tho Albeit totally confusing.

ozzy

Well (thinking logically) it's a telco adapter, so I'd contact a telco company and ask if they can give you more info. Also worth trying are the likes of Black Box, Maplins, RS Components, Farnell, etc..

We use a company called Globalink (0870 845 5655). Ask for Dominic and mention Stefan from RDF Consulting recommended them.

They provide all our PABX support, so should be able to point you in the right direction.

Stefan

Jeff Wiltshire

If you are using an distributed ISDN circuit you must have a terminator on the last port of the LAN (the same as you would have on Co-ax LAN cables)....

I can't remember all the details but I think it's 150Ohm....

Have a search through the RS site....


Jeff

Jeff Wiltshire

Having refreshed my memory somewhat you will need to terminate the S-Bus (RJ45 wall box) correctly...

Have a look at http://www.videkonline.co.uk/SubSect...p?sectionID=21


Jeff

dsmith

All about the S-Bus being properly terminated.

This link gives a few explanations.

To check exactly whats up, need to understand what else is using the ISDN line the router is on and where each device is i.e. distance from NTE (BT-Box). And how its cabled between outlets.

Deano

Dr Hu

Thanks for all the replies guy's!

Bizarrely I had looked through the Videk catalogue thinking they would have this stuff, but I couldnt find it - I have now - its not listed in the Index as ISDN related

So the ISDN Terminator from Videk looks as if its the one I need - thanks Jeff.

Deano - the link to the RS site has timed out

I am about to order one and I'll let you know the outcome!:P

Thanks again

dsmith

Stupid www site

Search for code 208-1742.

Deano