Printed (microstrippe) balun question

printed balun

Hi, everyone. I have a question about the printed (microstrippe) balun simulation and measurement.

Usually, if printed dipole is fed by microstripp or CPW line, a printed balun is needed. And some parameters are used to represent the balun performance, such as Insertion Loss, Return Loss, Amplitude and Phase Balance.

My question is in commercial EM software such as IE3D or HFSS, how can I get this parameters value. Do I need to define one port at microstrip line as conventional port and the other at balance output as differential ports? If yes, how can I extract the Phase Balance from S parameters?

The other question is how to used Vector Network Analyzer to do the measurement?
Since the balanced ports are differential, how can I connect them to SMA connector?

Any information will be appreciated, thanks.

balun + hfss

antenna_abc,

I have done such measurement/simulations in the past. Although the entire analysis can be done in HFSS, I normally do it in ADS. This is done by using the following procedure:

1) In HFSS, use three ports, one for the unbalanced port and one each to ground on the balanced ports.

2) Run the simulation. Once it is done, export the s-parameter matrix (3x3) into a circuit simulator such as ADS.

3) In ADS, connect your unbalanced port with a port to ground and a balanced port across the other two terminals of your data block. Assign a net name to the + and - terminal of the balanced ports. Run an AC analysis (not linear, which does not create data for the nets).

4) In the data window, you can determine the balance by using the appropriate equations:
Amplitude balance = dB(V+)-dB(V-)
Phase balance = phase(V+)-phase(V-)
(V+ and V- are the names I'm using here to describe the nets assigned above)

For lab measurements, you should be okay as most network analyzers with more than two ports support these type of measurements. If not, you can use the same procedure described above...

Hope this helps!

-john kitt.

Hi, antenna_abc:

I think there are multiple situations.

1. If the all the 3-terminals of your balun have ground references, I think what John said is correct. You can define 3 ports between each terminal and its ground. From the 3 ports, you can monitor how good the 2 output ports are balanced.

2. For some baluns, you may not have ground reference for each terminal for the output. You only have 1 pair of differential terminals there. In such a case, you can't define 2 ports on it. For such a situation, monitoring how good the balun is a little bit difficult. You may have to monitor the current distribution on both arms to see how good they balance.

Best regards.