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Why Does the Noise Figure of my Passive Structure not Match the Insertion Loss?


It is a common misconception that the noise figure of a passive structure is equal to the absolute vaule of the insertion loss of the network. For example, the S21 of a network is -9 dB, the designer expects the measured noise figure to equal to 9 dB. This situation is only true when the network is perfectly matched. In reality, the noise figure of a passive structure is equal to the available gain of the network.

When using data files, the AWRDE must test to see if the data is passive or active to determine how to calculate the noise for the data file. If passive, it uses the available gain. If active, it uses additional information in the data file to describe the noise. If the AWRDE detects the network is active and there are no noise parameters, an error is issued that the network will be treated as noiseless. In some cases, the network is a passive network but for some reason (deembedding errors from measurements, EM simulator numerical problems, etc.) it produce slightly active s-parameters. In this case, you can tell the simulator to treat the network as passive for noise analysis. To do this:

  1. Right click on the data file in the project tree and select Options.

  2. From the Interpolation/Passivity tab, select the Consider Passive for Noise Simulation option.