Composite Video Balun: What's needed?

Let's say I was going to DIY a passive composite video balun for use over unshielded CAT5...

I've got a box made by MuxLab sitting in front of me right now, and the video signal is running through a coil of some sort, and that's all there is between the RCA jack and the RJ45 connection. The coil looks identical to the white thing in this photo:


Is that a transformer or some sort of filter?

The specs on this thing say it's for 75ohm input impedance on the RCA and I know that CAT5 is 100ohm. It reads like an open circuit between the tip and the sleeve (and between each conductor of the twisted pair) which leads me to believe that it's not a transformer. There's 5ohms of DCR (on each leg) between the RJ45 and the RCA, which sounds more like an inline filter or choke of some sort.

Obviously I'm not well versed on components like this, or on video standards for that matter.

Anybody know what thing is? I briefly paged through the mouser and digikey catalogs and nothing jumped out at me, and the part number stamped on the side of the one in my hand looks like a proprietary MuxLab part # (ML/27-25A).

a little more research is pointing me towards it being a "common mode choke", which makes sense. Now I just have to find one with similar specs...

http://en.wikipedia.org/wiki/Balun

The folks at Belden Cable have been advocating video over UTP for a while now:

http://www.belden.com/pdfs/Techpprs/VideoandUTP.pdf

(You'll need Adobe Reader to view it, of course)

Hey, something I know about.

It may not be a true balun, but since it's covered in epoxy or whatever that white stuff is, we won't know.

It's most likely a cheap matching transformer than a proper balun.

I've been testing an interesting balun today, the primary has only one pin, there is no ground pin... :shock:

[quote author="Svart"]It may not be a true balun, but since it's covered in epoxy or whatever that white stuff is, we won't know.

It's most likely a cheap matching transformer than a proper balun.[/quote]

Feels like silicone.

It measures out like a common mode choke (similar to a transformer rotated 90 degrees). From what I've read, video baluns should pass a bandwidth all the way down to DC in order to look good, which would eliminate your standard transformers. I've been scouring the net all day trying to figure out what value choke I need for this application.

BTW, thanks for that link magicchord. It's very helpful.

Here is a diagram of a balun for unbalance to balanced conversion. This could be wound on a ferrite core as a 1:1 transformer as the input and output impedances are fairly close. The primary concern is to try and achieve as perfect a balance as possible on the balanced side. The two winding wires can be pretwisted before winding the core to help with this.



The minimum number of turns needed is reflected by the inductive reactance that is required. The rule of thumb shown here should be applied. Use of too many turns, however, will compromise the high frequency response while improving the low frequency response.

A balun system will not have significant DC response. In a composite video situation, the DC restoration at the receiving end will take care of that problem.

I noticed that the unit you show in made by Intelix. They are a spin off of Full Compass Systems, another local company. Intelix is now offering a number of balun products for HD and component video on CAT 5. In one of their FAQ sheets, they quote the losses for using a balun pair. At 1 megaHertz the loss is 6.6db per 1000 feet. RG-59, a common video coax loss is 2.9db per 1000 feet, so, even with all the insertion losses, etc. the performance of baluns/twisted pair is not too bad.

Awesome. Thanks.

I've got a couple of samples coming from various sources. They're all billed as being for this exact purpose, so we'll see how they perform compared to the MuxLab unit I have.