Question about microstrip line width and impedance match.

Hi everyone,

My question may appear strange to you but I am a newbie in RF, that?s why I?m asking for your help.

I have three-port network with port 1 for input, ports2 and 3 for output. All ports are 50 Ohm. Frequency range 1-2 GHz.
In my application I need to split the incoming signal between ports 2 and 3 with -11dB at port 2 and -8dB at port 3.

Theoretically this is a simple task which requires only 4 resistors. I?ve calculated the traces width according to the 50Ohm requirements taking into account the substrate.
But when I start simulating this simple network (S11, S21, S31) taking into account microstrip lines, microstrip via (connection to ground) model, resistors RF models then the results looks strange for me. It is possible to achieve the required -11/-8 dB at ports 2 and 3 but the S11 became too high. When I try to optimize the resistors values for better S11 then I loose the required output levels. The only way I?ve found to get all parameters (S11, S21, S31) in the range is to change the traces width between resistors. But in this case the traces impedance will not be 50 Ohms! So my question is ? does it really matter to have the traces impedance equal to the port impedance if all other parameters will be difficult to adjust?

The only thing that counts is results. The next step is to make one and measure it.

If your circuit input and output impedance is 50 ohm and S11,S21,S31 conform to your requests, you only make one and measure it. Maybe you need to turn it.

The optimal T-line impedance change can be concerned with parasitic effects in Your resistors (series inductance, parallel capasitance etc. if You takes it into account). Also, it can be concerned with mounting pads effects (generally - it is equal to small shunt capasitance). The VIAs usually affects weak on this frequencies. In general: there is not any strange in this situation.

Best regards,
Kit-the-great

Thanks for clarifying this for me.
As far as I understand the reference impedance of each microstrip line inside the network does not really matter. It may be higher or lower then 50Ohm. The only important things are the final values of S11, S21, S31 and possibly input impedance at port 1, as in my case.

I?m wondering if my understanding is correct.

You are making an N port with specific specifications on the S parameters. What happens inside the network is of no concern if the S parameters end up at the specified values.