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matching circuit - RF Cafe Forums

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bamatom
Post subject: matching circuit Posted: Tue May 23, 2006 1:56 pm

Lieutenant

Joined: Thu May 18, 2006 9:46 am
Posts: 3
I have a hard time to get a feeling for matching circuits. can anyone explain to me where I am wrong in the following thoughts:
I want to match a real 25 Ohm load to a real 50 Ohm source. I know I can do an LC circuit and I am fine. But, if instead - in theory - I add a 25 Ohm serial resistor, there would be no reflexions back into the source since the total load can be calculated to be 50 Ohms, right? I know, I would waste power in the additional 25 Ohm resistor due to heat. However, if I count the 25 Ohm resistor towards the source, there would be a 75 Ohm source resistance that sees a 25 Ohm load. This is not matched anymore and I would expect reflexions. Where am I thinking in the wrong direction here?
I know this is a very trivail question but I would really appreciate some comments.
Thank you


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Dexter
Post subject: Posted: Tue May 23, 2006 2:28 pm

Captain


Joined: Thu May 04, 2006 7:37 am
Posts: 7
Hello Mr. Bamatom:

In case you haven't seen it, I addressed your question of a few days ago in the Antennas forum, and if you don't mind I'll have a shot at this one, too.

As you suggest, when matching two purely resistlve circuits, only resistors are required. LC is only needed when imaginary parts are involved, or is a "lossless" transformation is required. The lossless case needs basically just a an inductive transformer. The LC solution will, by nature, be frequency dependent wereas the resistive soultion is, theorectically (but not in practice) independent of frequency.

Also as you suspect, attempting to match with a simple 25 ohm series resistor does not solve the problem. What you need is a classic 3-resistor "T" or "pi" formation pad. I just looked it up here on this website and there is an excellent calculator with equations available. There is a minimum attenuation associated with the resistive transformation wich in the case of 25 - to - 50 ohms, works out to 7.66 dB. It allows for unequal soruce and load resistances. You will want to look it over.

https://www.rfcafe.com/references/electr ... uators.htm


Dexter


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yendori
Post subject: Posted: Thu May 25, 2006 3:23 am

General


Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
50ohm source and a 25ohm load.

you add a 25ohm series resistor and the circuit is matched, less some power. Your source now see's 25ohms in series with a 25ohm load = 50ohms.

Sounds good to me, what's the big whoop?

Rod


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Dexter
Post subject: Posted: Thu May 25, 2006 9:43 am

Captain


Joined: Thu May 04, 2006 7:37 am
Posts: 7
yendori wrote:
50ohm source and a 25ohm load.

you add a 25ohm series resistor and the circuit is matched, less some power. Your source now see's 25ohms in series with a 25ohm load = 50ohms.

Sounds good to me, what's the big whoop?

Rod



Well, it depends on whether you want both the source and load to be matched from both of their perspectives. Adding the 25 ohm resistor in series causes the original 25 ohm load to "see" 75 ohms connected to it, while the 50 ohm load does now indeed "see" 50 ohms. The only way to make both the load and the source happy is to insert an impedence transformer (or in this cas a minimum loss resistive pad).

Dexter


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yendori
Post subject: Posted: Sun May 28, 2006 2:18 pm

General


Joined: Thu Sep 25, 2003 1:19 am
Posts: 50
Location: texarcana
yeah, that's what I meant






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