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Is it possible to obtain the S-parameters of a circuit with only S11 measurements (S11 measurement with a VNA)?

  1. My gut tells me that it should be possible by connecting open, short and characteristic impedance to the system but for my life I cannot figure how.
  2. What is the DUT has unknown characteristic impedance (or is a complex circuit of lumped elements)? Then I cannot terminate with Z0 exactly. Can I just use a known reference resistor?

Background: I have a PCB with a single SMA as input; the first part is a delay line, then a matching network and then VNA input. I want to design the L-match on the PCB properly.

PS: frequency is 900 MHz (UHF). I can desolder/resolder components on the PCB which are 0402 (for example, I can remove the L-match)

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  • $\begingroup$ > I have a PCB with a single SMA as input; the first part is a delay line, then a matching network and then VNA input Isn't really clear. It sounds like you need to optimise S11, not S21. Can you reword the statement of the problem? But yes if you have a 1-port VNA and some known loads, you can measure (something like) S21*S12 fairly easily. Separating them would only be possible if you know it's a passive network. $\endgroup$
    – tomnexus
    Oct 30, 2020 at 7:36
  • $\begingroup$ Obtain the S parameters of what circuit? If you have a 1-port device then with S11 you've already obtained all the S parameters. So, yes? $\endgroup$
    – Phil Frost - W8II
    Nov 1, 2020 at 15:57

1 Answer 1

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S11 is one of the S-parameters. It uses 1 port on the VNA to look into something and look at the reflection coming back to determine reflection coefficient / VSWR. If the device was calibrated properly then it can also tell you the complex impedance of what it is "seeing". To calibrate you have to use open, short, and load (99% this is 50 ohm), but these have to take the place of the DUT. Then after you calibrate each, the DUT takes their place. The closer you can get it the more accurate. You can use a resistor on your PCB or right before it enters, use 2x 100 ohm resistors to reduce inductance and get ones that are accurate too. of course the circuit (DUT) has to be disconnected for calibration. lot's of info online go check it out. If you want S21 parameter you need 2 ports, one is tx the other one is rx, it will tell you how much power gets through vs frequency. It will take a 4th calibration step which is a 'through' if you want to go that route.

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  • $\begingroup$ So I assume it goes, "Sab" where a=ports and b=? S11 is 1 port, S21 is 2. Just wondering what the sub-digit means. $\endgroup$
    – t252
    Oct 31, 2020 at 21:29
  • $\begingroup$ The do it backwards, the first digit is the receiving port and the second digit is the transmitting port. S11 both the same port so you're looking for reflection, S21 is from port 1 to port 2, you're looking at how much power gets through at the various frequencies. $\endgroup$
    – pgibbons
    Nov 8, 2020 at 3:39

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