Two-port network models

1. Oct 9, 2013

agata78

[/SUB]1. The problem statement, all variables and given/known data

Design a ∏ section symmetrical attenuator to provide a voltage attenuation of 15 dB and a characteristic impedance of 600 Ω.

2. Relevant equations

ZS = ZL = Z

R1 = R3 = Z (K + 1 / K -1)

R2 = Z (K2 - 1 / 2 K)

3. The attempt at a solution

Z = 600Ω
K = 15dB = 15(15/20) = 5.6234

R1 = R3 = 600 (5.6234 + 1 / 5.6234 - 1)

= 859.54 Ω

R2 = 600 (5.62342 - 1 / 2 x 5.6234)

= 600 (30.6226 / 11.2468)

= 1633.67 Ω

Can someone please confirm if I am on the right lines? Thanks

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Last edited: Oct 9, 2013
2. Oct 10, 2013

rude man

Not what I got. I don't know where you got your formulas. I just wrote a set of equations to force Zin = 600 ohms from both sides, and an attenuation of 15 dB = gain of 0.1778. My R1 was not too much below yours but my R2 was exactly double yours. Interesting coincidence!

Maybe I'm misunderstanding what the problem asks for. My pi network looks like 600 ohms from both directions and the attenuation is 15 dB. I'm not assuming a source nor a load impedance.

3. Oct 10, 2013

agata78

Hi, when talking about current or voltage, it is db = 20log(ratio).

If you are taking the ratio of powers, then it is dB = 10log(ratio).

We are using first one, which gives us 15db= 20 log (n),

10 (15/20) = n

10 ( 3/4) = n

4√ 10 (3) = 5.6234 = k

I have found similar example but still want to get confirmation is it right or not

4. Oct 10, 2013

rude man

If it's attenuation, it's -15dB, not 15 dB, which is 10^(-15/20) = 01778 = k.

Last edited: Oct 10, 2013
5. Oct 10, 2013

agata78

I know that attenuation means the reduction of signal strength during transmission. But how you can tell its with this example?
Your calculations would be right, but i dont know why is minus there.

6. Oct 10, 2013

agata78

Thanks but it is actually the same example that I was using, but I still can't find a minus there in that example!

Last edited: Oct 10, 2013
7. Oct 10, 2013

rude man

If you go by the link, attenuation is considered a positive quantity.