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# microstrip line characteristic impedance is not 50 ohm in source term matching

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#### wangzhao

##### Newbie level 5
hi everyone, i'm getting through a tutorial of RF power gain design.
In the source side, the guidence changes the microstrip line characteristic impedance to 8 ohm, but why it doesn't follow 50 ohm?

From your diagrams it appears your goal is to match impedance between 8 ohms and 50 ohms at 960 MHz. That is, to convert volts and Amps so as to produce optimal power transfer.

Your second schematic diverts high frequencies to ground. Therefore it's a low-pass filter. The time-dependent components are capacitors. I don't see how it could provide power gain.

Impedance matching networks generally contain inductors and capacitors.
Although I'm not expert at rf conversion, micro-strip lines or pcb design, I like to contrive simulations. This sort of looks like what you wish to do. By carefully adjusting values it manages a small amount of power gain, theoretically. Incoming signal is 5V amplitude.

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Let me do an advanced explanation.

Two schemetics posted above are sub part of the whole gain circuit. And, the pic2 is the sub-circuit of pic1's [DA_SmithChartMatch_1].

The output imdepance of Amplifier is (2.9-j*1.32) ohm, which is presented by [Term1] and connecting to [P1].

MY GOAL is to match impedance from (2.9-j*1.32) ohm to 50ohm.

Generally, matching circuit uses a constant characteristic impedance upon microstrip line. That means, in my thought, I can keep the characteristic impedance to 50 ohm.

But why [TL1] uses 8 ohm characteristic impedance, rather than 50 ohm?

By the way, thanks for your timing diagram

But why [TL1] uses 8 ohm characteristic impedance, rather than 50 ohm?
With Zline=8 ohm this line gives additional impedance transformation. You will need to check the path in Smith chart when using 8 ohm line vs. 50 ohm.

You will need to check the path in Smith chart when using 8 ohm line vs. 50 ohm.
hi, volker@muehlhaus

I'm following a guidence for beginners, it sets zline characteristic impedance = 8 ohm.

I just want to know what factor effects it to select the first zline characteristic impedance to 8 ohm?

I think the idea is to gradually increase impedance levels from the very low source impedance towards 50 Ohm load impedance. Not sure how "optimized" the 8 ohm value really is, I would expect that you can also design the matching circuit using 12 ohm or 18 ohm line as first element.

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