Generally speaking, does anyone know the correct method of installing ferrite beads onto the 25 pair 'tail' into the pbx or ksu?

Should the 25pr be 'wrapped' around the bead or should it be straight through the center?

Should the bead be at the system or at the block?

linkage would be helpful, I've googled till I'm blind and haven't found anything usefull.

Thanks

Usually the ferrite is at the device in order to reduce noise entering the cable along it's journey. I'm sure there is a more thorough explanation for why it's at the device then this as opposed to being at the source (block). This is merely more of an observation then hard facts. I've always seen ferrites used in front of the signalled device, not the signalling one.

Each time the cable passes through the ferrite it increases the impedence (or attenuation? both?) of the cable. So if the idea is to add more impedence to the cable to remove interference picked up from AC sources/EMI/RFI/etc, then I guess you would want to loop it as many times as it takes up to the point where you dont have enough signal strength to do what you need to do.

I'm sure someone else on here has a more in depth explanation (or correct one if I'm mis-remembering it) but I believe that is the general gist of it.

Dunno, I was lead to believe it was to reduce the RF traveling OUT of the ksu. And the ferrites provided with most equipment only allows 1 pass-thru of the cable. My 2 cents worth. John C. (Not Garand)

The number of turns matches the frequency of the interference that you're trying to tune out. i.e., one turn might be 800 - 1000 (AM range), two turns might be 1000 - 1250, etc.

Someone used to have a definitive guide... I thought it was Sandman, but last time I looked, he didn't have it.

Maybe I should mention I was thinking about Mitel SX50 and Panasonic TDA specifically when I posted my previous babble. I've seen the ferrites with 1-2 turns for line cords, etc., but never really investigated them. Used 'em if they came with the equipment, and that's about it. John C. (Not Garand)

The ferrite turns into a choke if it's all "tuned" right. Has to do with wire diameter, wrap rate, core size, etc etc...

Sounds like the question is wether you should use a ferrite core on noisy lines. All I can think of is call the manufacturer? They may have a spec all laid out already. Certain Size ferrite core with a certain rating with a min and max # of wraps/etc.

First, how about you tell us WHY you think you need to do this. We'll take it from there.

-Hal

My original question stems from the fact that some manufacturers include the ferrite beads with the system cabinet, along with instructions to "install on the tails" but no details on how specifically.

My boss thinks that the tails should be wrapped around at least one turn, I think that straight through is better, I used to turn them but was told not to by a former boss though I cannot remember his reasoning.

I always thought the beads were to prevent any 60hz hum from getting into the station ports.

I also understood that 'closer to the source' is better because letting the emi run the length of the cable before choking it out could degrade the signal more than choking it as close to the source as possible.

Since we are talking about choking 60hz noise that means as close to the switch as possible on the tails.

I always like a lively discussion...

Ferrite cores are used in this instance to keep RF noise from radiating FROM the system through the cables. The usually get used because somebody in the original engineering dropped the ball and the system didn't meet FCC part 68 requirements for RF emissions when it was tested. So somebody sticks a ferrite in the cables and it passes. I wouldn't worry much about using them, it will make no difference to the system and might prevent AM radio interference if one is used nearby.

If you do use them they should be placed close to where the cables exit the system. If it looks like you would need two turns (doughnut) then that is what you need to do. But again, I wouldn't worry about them.

No way would they be useful at 60Hz, you would need lots of iron for that.

-Hal

Ferrite beads will attenuate in both directions; whether you need them to reduce noise coming out of the system or going into it is a different matter of course, but simple inductance is not selective in attenuation with regard to direction.

As Hal said, you'd need much larger values of inductance to even start affecting 60Hz, since reactance is a function of both inductance and frequency: X = 2 x pi x f x L.

On the Mitel SX-200 EL/ML, the tech docs say to install the split ferrites 4" from the amphenol connector off the back plane. The 25 pair goes straight through once.

SX50 ferrites are to be inside the cabinet, same for Panasonic TDA. In both cases, if you were successful in wrapping the cable thru twice, there would probably be 1 giant short! John C. (Not Garand)

I was fortunate to have been given an assignment many years ago to investigate ways to reduce RFI emissions as some of our phone switches that were installed in the field were causing RFI problems.
It doesn't really matter who the switch manufacturer was because most phone switches have similar problems. The clocking signal used on the CPU has many harmonics and is the main culprit for emissions. Sometimes the switch mode power supplies can also cause harmonic problems. In our case it was the CPU clock with the odd harmonics that interfered with other pieces of equipment( ie. FBI scanners, police radios and so on).
The use of Ferrites is usually a stop gap measure because the manufacturer does not want to redesign the offending equipment and leaves it up to the field to control the problem.

We found that there were two field modifications that a field tech could use to reduce emissions (the manufacturer has many more, like properly redesign the CPU but that costs money and I digress).
1) Shielded cable
use shielded 25 pair cable and make sure the shield of the cable is bonded to the metal cabinet. All cables leaving the cabinet had to be shielded and bonded for maximum effectiveness. (Don’t have a metal cabinet then you can’t use this method).
2) ferrites
1st of all, there are many different types of ferrite materials. they all have different characteristics that attenuate different frequencies so while one ferrite might work very well to eliminate a noise problem at one location, a different material ferrite might be worthless at the same location. To truly control emission problems, a faraday cage needs to be setup whereas all cables leaving the cabinet needed to be filtered. ( ie. Power, ground wire, CO and station cables) All wires can act as an antenna and radiate the offending signal thus the need to ferrite every cable.

We used a spectrum analyzer to chart our results. We used Ferrite 43 material to reduce the emissions. Our goal was to minimize the odd harmonics between 140 Mhz to 200 Mhz. This used to be the radio spectrum where a lot of Police and FBI frequencies were located. I am not sure if that is still true.
Ferrites had to be placed as the cable left the cabinet. If you were to place the ferrite below the connector and the cable was allowed to run thru the cabinet, the noise would re-radiate onto the cable. So placing the ferrite as it left the cabinet was critical.
The more ferrites you added, the lower the interference that emanated from the cabinet up to a saturation point. We found 4 ferrites in series on a cable gave maximum effectiveness. Additional ferrites made no difference. We found wrapping a cable thru a 4 holed ferrite in series was very effective. The most effective method was the toroid shaped donut. We were able to reduce emissions up to 10 dB. With this setup you had to be sure that the incoming cable on the donut did not touch the outgoing cable on the donut for maximum effectiveness. (there was no giant short as was previously mentioned). In the real world, it was not practical to place 4 ferrites on each cable for installation. It was clumsy and unsightly. We found it better to be sure that all cables leaving the cabinet were wrapped with at least 1 ferrite to control the emissions.
There is a simpler way to test for RFI without using a spectrum analyzer and see if your ferrites are doing the job.
You can buy a scanner. Set it to scan and notice what frequencies it locks on to when your phone switch is turned on. Turn the phone switch off and see if the scanner is still locking on to that frequency. If not, then there is a good chance that is one of your switch’s harmonics. Store that frequency on your scanner and use it to see if your ferrites are working.

In your case Pete, it looks like the manufacturer has one board that does not meet FCC emissions so they are required to have you install a ferrite for this board at every job. The chances of your equipment causing an interference problem is quite remote and rare and in most cases if you install the ferrite or not, nobody is going to really care. (unless the police call you to complain, then I would do something).
Tj

I'm going to memorize TJMCAPS reply amd throw that on my girlfriend the next time I'm anxious. When I get thru with the delivery she will be mine. Whew, I might even take a science class over at the comunity college so I can slip that into an exam and get an A+, maybe ++. Very impressive Obewan, very impressive.

wow! Thanks TJMCAPS

Years ago I installed a electronic key system in an AM radio station. I learned a lot about ferrite beads sheilded cable caps and coils... but the best thing i learned is to never sell a telephone system to an AM radio station. One or two loops, near the ksu or near the block... it is like tuning an old crystal radio there may be a science to it but it's a dark science.

Radio stations used to be 1A2 all the way. Much easier.

Sam

Here's a pic where we used ferrites on a Carrier Access DLC:

Are those all wire wrap terminations?

yes

ps. sorry about the size of the pic. should I delete it?

Just shrink the image on your server. I tried to edit the BBcode to display the picture smaller, but I guess the code isn't supported here.

That's better.