Q&A

# Measuring Input/Output Impedance with a VNA

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On paper you can measure the input impedance of an amplifier circuit by attaching a voltage source to the input, shorting the output, and calculating the resulting impedance from the voltage and current, or vice versa to find output impedance.
But how would I measure the Input/Output impedance of the amplifiers in the pictures below using a VNA? Do I just connect the VNA to the input, attach a short sma connector to the output, and then observe the smith chart? I don't want to destroy my instruments that's why I'm asking. Thanks.

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The input and output impedance of these amplifier is 50 Ohm. You don't need to measure them. That's because they are intentionally matched to the characteristic impedance of the coaxial cables that connect to their SMA connector, which is always 50 Ohm for most microwave devices, except television cables and likes that are 75 Ohm.

You can check the input impedance of the devices by connecting the input to your VNA (S11 cable). Then in the Smith chart, you'll observe a small spiral around the 50 Ohm central point. Also, if your VNA plots the log(S11) graph, you should see a more or less constant graph at 0 db. I believe the VNA sends a very weak signal to the amplifier, so that it cannot damage the amplifier. But if you want to be sure, check the output power of the VNA, and the maximal acceptable input power of the amplifier.

To test the output impedance of the device is more tricky because the amplifier gives a strong gain and this may damage the VNA. So, you'll need to connect a suitable attenuator (buy attenuators of 5, 10, 20 and 30 db in Ebay or Aliexpress for very cheap). Then connect the output to the S21 cable of the VNA (through the attenuator). In the S21 smith chart, I believe you'll see the output impedance is 50 Ohm (after subtracting the attenuation due to the attenuator).

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