how to use s-parameter to characterize the RF behaviour?

hie, I am not a RF engineer. but i need some info on s-parameter.

I read in several PCB makers application notes that I can characterize the RF behaviour of the PCB (fully mounted with all components) using s-parameter.

Currently, I have an evaluation board mounted with all the components (all are high frequency components). The die(IC) has also been wire bonded to the board. I have checked the impedance of traces in board and confirmed they have 50 ohms input impedance using TDR probing.

Using a VNA, I am able to obtain the S parameters (S11, S22, S12, S21).
After getting the values, how can I characterize the Rf behaviour of the components in the PCB. Can anyone give me an idea on what analysis can be done using the s parameter. In other words, I want to know how to use s-parameter and what is it used for?

Hi

In short term:
S11 and S22 is the reflection coefficients. These are measured from the point where you connect the cabels from the VNA. They tell you if your circuit is matched, this usually 50 ohms. He VNA sends in a wave(power) and measrure how much is comming back(reflected). The ratio of these two determines S11 and S22. For a good match this should be low or in dBs it should be in the area of at least <-10 dB.

S21 is the transmission from port 1 to port 2. This is the usually the gain of your device depending on how you connect the VNA. This works by sending a wave in to the deviceand measuring the output. If measruring a amplifier this should be greater than 0 dB.

S12 is the same just the other way round.

S-parameters can also be used to calculate the stabillity of a device.

The reason to use s-parameters is the fact that it is difficult at high frequencies to identify the total current and voltages on a line do to the distributed nature of these, which can lead to standing waves. S-parameters relates the incident and the reflected waves into a port.

I would really recommend to look in Pozar or Gonzalez for a detailed explanation.

Regards

hi,
S-parametres absolutely define the response of a circuit. For a 2 port circuit (one output and one input) there are 4 S-parameters refered as to S11,S21, s12 and s22. Sij is defined (in dB) as the power that comes out from port i when signal is entering by port j being all other ports than j (even i) matched (to avoid reflections).
in the example of an amplifier, which is a two port device, S21 would be the forward frequency response (Gain basically). S11 and S22 the input and output matching (or input and output return loss because it`shows reflected power). S12 should be very low in an amplifier.
For complete info on the Sparameters refer to
Pozar, D., "Microwave Engineering". It is really helpful.
hope this helps
regards

Hi,

Like our friends said before me S parameters very important and sometimes only parameter to discuss about it in RF circuits.

About S Parameters I have some paper, that will conclude in general!!

Regards
David

see this