atcomsystems.ca/forum Forums General General Telephone & Systems Information Correct Install of Ferrite Beads on 25pr into PBX or KSU

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?