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Design >> RF Design >> matching questions?
https://designers-guide.org/forum/YaBB.pl?num=1253042910

Message started by liletian on Sep 15^th, 2009, 12:28pm

Title: matching questions?
Post by liletian on Sep 15^th, 2009, 12:28pm

Hi Guys
I had a question on matching. What is the purpose of matching? Well, the following is my calculation.
Suppose we have a perfect matching for 1mW for 50 ohms.
Then the voltage at the input points is
V^2/(2*50)=10^-3,
V=0.316 V.

Well, let's suppose there is a resistor of 10^5,
then the reflection coefficent is gamma=(10^5-50)/(10^5+50)=1-1*10^-3.
So the power at the input point is
(1-gamma^2)*10^-3=2*10^-6.
So the voltage at the input is
V^2/(2*10^5)=2*10^-6
V=0.63V
So in the dismatch case, the voltage is actually twice the input voltage for matching case. I am very confuse about this. Can anyone explain what happened here. In some applications, maximum voltage is better than maximum power, so why do you have to have power match in all the place from antenna to LNA.
Thank you

Title: Re: matching questions?
Post by aaron_do on Sep 16^th, 2009, 6:04am

Hi liletian,

everybody who has ever designed an LNA has probably asked this question. Including me,

http://www.designers-guide.org/Forum/YaBB.pl?num=1195659245

cheers,
Aaron