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I have a question on hand:

"You have an unknown 2-port device, the black box. Find the operating frequency range. Show how to do this with a VNA. Think of the settings, why?"


VNAs can have a very broadband operating frequency range from DC to e.g., 8 GHz. If the measurements are noisy, then you may see frequency selective characteristics of S11, S12, S21, and S22 parameters. So, how would you judge? This is like hunting in a dark, or? do you have a clever suggestion?

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closed as unclear what you're asking by Andy aka, Dmitry Grigoryev, Dwayne Reid, Finbarr, Lior Bilia Nov 5 '18 at 15:17

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    \$\begingroup\$ A 2 port device has, by default, an operating range from DC to infinity. However, it may not be useful (as intended) below or above a certain frequency but as the device is unknown that is impossible to tell. \$\endgroup\$ – Andy aka Oct 14 '18 at 15:21
  • \$\begingroup\$ Something tells me that you would want to find the -3 dB frequency, or the 45° phase shift frequency. \$\endgroup\$ – Harry Svensson Oct 14 '18 at 15:29
  • \$\begingroup\$ A particularly nasty "unknown" might have very narrow bandwidth, like a crystal. Swept too fast with a VNA, it looks like a small capacitor, because its primary resonant frequency is missed. Only a slow, finely-stepped sweep reveals the proper result - that takes much time. \$\endgroup\$ – glen_geek Oct 14 '18 at 15:52
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There's no magic to this.

There really aren't that many types of 2-port devices. Some things that come to mind:

  • Amplifier
  • Attenuator (fixed or adjustable)
  • Phase shifter / delay line (fixed or adjustable)
  • Filter (many different shapes)
  • Isolator
  • Discrete passives (R,C,L, or a crystal as mentioned by glen_geek)
  • Antenna (A 2 port antenna could be dual polarization or just a dual antenna for MIMO. But this is pushing the definition of a 2-port black box)

Some of these will make their bandwidths obvious. For example, if the device has gain, it's an amplifier, and then it's quite easy to pick out a 1dB or 3dB bandwidth based on the peak gain. If the device has a clear frequency corner, it's probably a filter, and again, easy to see the operating bandwidth (although you have to decide which bandwidth definition to use). An isolator is also obvious because it's the only device without gain that is non-reciprocal (S21 != S12). Once again, an isolator's bandwidth can be easily defined as (e.g.) 1dB extra loss beyond the minimum insertion loss.

The rest of the devices I mentioned are somewhat more ambiguous because they don't tend to have obvious shapes or clean frequency corners, and here it may be difficult to define an operating frequency without knowing the intent of the device. For example, a capacitor might desire minimum insertion loss because it's going to be used as a DC block, while an inductor might desire maximum insertion loss because it's going to be used as an RF choke.

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