[SI-LIST] Re: One question left!!!!!
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: doug@xxxxxxxxxx
- Date: Wed, 19 Mar 2003 22:34:13 -0800
Dear Doug,
Welcome to the wacky world of transmission line transformers. One of the
best references is Jerry Sevick's book entitled, of all things, "Transmission
Line Transformers."
As for your questions, taken out of order:
1) If the transformer is really well balanced, you don't need much shielding.
However, was the glue conductive?
3) The impedance of the ferrite is so high at CATV frequencies that the two
currents are forced to be nearly equal, hence the splitting action. Note that
the energy being split does not flow through the "transformer;" just the
mismatch energy tries to go somewhere in the transformer.
2) The splitter doesn't really require a perfect match at the input; it is
probably much closer to 150 ohms than the 75 ohms impedance of each load.
However, that's a 9.5 dB return loss, not bad for a cheap ferrite and couple
of pieces of wire. That assumes that the wire pair impedance is 150 ohms.
If the impedance is lower, the return loss should be better for the lower
frequency portions of the frequency range, but still no worse than 9.5 dB.
That's all I can find at this hour of the night.
Regards,
Paul
_______________________
Doug Brooks wrote:
> Wow! Thanks guys. I got thousands of answers on the plural of buses (some
> of which were pretty funny!)
>
> To expand on this splitter question:
>
> One splitter I took apart had a single ferrite cylinder. Two small wires
> went from the "input" and went through opposite ends of the cylinder,
> crossing inside. They each then went to one of the "outputs". This was
> supposed to be a CATV wideband splitter. It seems to me there were at least
> three really obvious problems here:
>
> 1. The "lid" was glued on, not soldered. So much for shielding!
>
> 2. I assumed the splitter would match the transmission lines with 75 Ohms
> internal impedance (and at all frequencies). How is that done with a simple
> ferrite? Or is this not necessary?
>
> 3. How much control over signal splitting can you get with a single, simple
> ferrite cylinder?
>
>
> I did use Google, but all the pages I found (and there were a great many)
> did NOT include a schematic or app note on what makes a good splitter, just
> sales and marketing hype, and generalities. That's why I asked if anyone
> know of a web page with a SCHEMATIC.
>
> That's what got my curiosity up.
>
> Thanks for your interest
>
> Doug
>
>
>
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--
Paul Levin
Senior Principal Engineer
Xyratex Storage Systems
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