OT: impedance matching, transformers

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svofski
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Posted by svofski: Tue. Dec 6, 2005 - 02:00 PM
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Hi,

All my textbooks are at the old place, but with the freaks you're never quite without books.

I have a long line, with 75ohm output cascade, 75ohm cable and I need to make a 75ohm input cascade. I know how to match impedance in unisolated circuits, but I want to make my circuit isolated.

More specifically, I have a HDSL output transformer which has 1:2 turn ratio, suits my frequency range and has primary inductance of 1.2mH - that's all I could gather from manufacturer datasheet (HALO). Passive resistance of windings is very low, below 10ohm. Input signal frequency is approx 6MHz, square wave.

The question is, how do you calculate input impedance of a cascade with a transformer? How can you change it and, therefore, can this transformer be used.

Thank you in advance!

The Dark Boxes are coming.

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AVR Microcontrollers, megaAVR and tinyAVR
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MegaUSBFreak
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Posted by MegaUSBFreak: Tue. Dec 6, 2005 - 02:09 PM
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Hi
afaik the tranformer transformes impeances with ratio^2.

75 Ohms in at 1:2 ratio means 2^2 = 4; 4 x 75 Ohms are 300 Ohms.

Is that what you asked for?

Klaus
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MegaUSBFreak
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Posted by MegaUSBFreak: Tue. Dec 6, 2005 - 02:13 PM
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I forgot to write:

it is independent of inductance

and the DC resistance should be low enough not make much error.
(10 Ohms vs. 300Ohms), but to be exact you have to calculate with it.
--> use a 290 Ohms external resistor

Klaus
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svofski
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Posted by svofski: Tue. Dec 6, 2005 - 02:56 PM
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>--||--    -----+------[ Requiv ]--. 
       )||(     |                  |
       )||(     R1                 |
       )||(     |                  |
>------    -----+------------------'

External resistor plugged so that R1 and Requiv of input cascade together give 300Ohm?

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MegaUSBFreak
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Posted by MegaUSBFreak: Tue. Dec 6, 2005 - 04:01 PM
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I hope i read you picture right:

left is 75 Ohms input.
Voltage ratio of transformer is 1:2 (input: output)
Now i´m not sure: what is R1 and what is Requiv?

Calculate the parallel resistance of both resistors to be 300Ohms.

Klaus
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svofski
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Posted by svofski: Tue. Dec 6, 2005 - 04:13 PM
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R1 is a matching (balance) resistor and Requiv symbolizes input cascade. In parallel they must be 300ohm, that's what I guessed.

Just to verify that I understand the calculation correctly: if I connect windings in reverse: input:output is 2:1. The ratio is therefore 1/2, ratio^2 == 0.25, load impedance must be 0.25*75 = 18.75ohm. Is that so?

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MegaUSBFreak
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Posted by MegaUSBFreak: Tue. Dec 6, 2005 - 05:49 PM
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Quote:
impedance must be 0.25*75 = 18.75ohm. Is that so?

exactely

Klaus
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Look at: www.megausb.de (German)
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svofski
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Posted by svofski: Tue. Dec 6, 2005 - 06:06 PM
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It's clear now. Thank you very much, Klaus!

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