The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> RF Design >> why the input impedance of on-chip balun is not correct?
https://designers-guide.org/forum/YaBB.pl?num=1458956813

Message started by jyk on Mar 25^th, 2016, 6:46pm

Title: why the input impedance of on-chip balun is not correct?
Post by jyk on Mar 25^th, 2016, 6:46pm

Hello,

I designed a 1:1 turn ratio transformer balun in HFSS, with 50 ohm resistance for each of three lumped ports, working at 16GHz. The results shows that S21 and S31 is about -4.5dB, which is normal.
But the Re(Z22）=6 ohm, which is too low. It doen not reflect the 50 ohm impedance on the other side of the balun.
Because the transformer balun's turn ratio is 1:1, I think the differential impedance should be 40~50 ohm.

Could anyone please point out what am I doing wrong with the simulation?
Thans a lot.

Title: Re: why the input impedance of on-chip balun is not correct?
Post by totowo on Apr 8^th, 2016, 5:08pm

1. According to the definition of Z-param, Z22 means the impedance seen from 2nd port when the 1st port OPEN (i1=0). Obviously the 6 Ohm you saw is the parasitic series R of the 2nd coil.
Zm2 is more like what you want (or use S22 to calculate the Zout which should be unique with Zm2);

2. For a real xfmr, the inductance is not infinite.
So the ZM2 sould 50 Ohm parallel with an reactance (j*w*L) even though we assume the parasitic series R=0 and k=1. So you should use serial-parallel-transform to calculate the Re(Zm2) = 50/(1+Q^2)

Best Regards

jyk wrote on Mar 25^th, 2016, 6:46pm: