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[Question] What value of the port impedance should be in a Noise Figure simulation?

xxgeneral

Hi,

When to simulate Noise Figure, the noise source is usually a port, in which the default impedance is 50 Ohm. More importantly, source noise comes from the impedance of the port. So a larger source impedance produces larger source noises. With respect to the Noise Factor equation, N=(No^2-Nl^2)/(Ns^2), larger source noises will make a smaller Noise Factor.

Q1. In a Noise Figure simulation, such as in design of LNA or Mixer, what value of the port impedance should be?

In my opinion, Noise Factor is under the assumption that the transeiver system has a input impedance of 50 Ohm, so is its output impedance. Every module in such a system should consider a 50 Ohm noise source. Otherwise, one cannot get the following equation:

NF = 1 + (NF1-1) + (NF2-1)/G1 + ...

Q2. Am I correct? If anything is wrong, please point it out.

Thanks in advance.

 

Haoran Hou

Regards

Parents

  • Dear Andrew,

    Thanks for your reply.

    As mentioned in Virtuoso® Spectre® Circuit Simulator Reference, F = (No^2 - Nl^2)/Ns^2.

    On the other hand, (No^2 - Nl^2) = Na^2+Ns^2, in which Ns^2 is the output noise due to source noise and Na^2 is the output noise added by the circuit under test(DUT).  So we get F = (Na^2+Ns^2)/(Ns^2) = (Na^2/Ns^2)+1, In this equation, it is seen that F is dependent on Ns^2. And, Ns comes from the impedance of Port source. So I think N is dependent on the impedance of Port source.

    By the way, No, Nl, Ns and Na are all represented in V/sqrt(Hz).

    if there is anything wrong, please point it out. Thanks a lot!

Reply

  • Dear Andrew,

    Thanks for your reply.

    As mentioned in Virtuoso® Spectre® Circuit Simulator Reference, F = (No^2 - Nl^2)/Ns^2.

    On the other hand, (No^2 - Nl^2) = Na^2+Ns^2, in which Ns^2 is the output noise due to source noise and Na^2 is the output noise added by the circuit under test(DUT).  So we get F = (Na^2+Ns^2)/(Ns^2) = (Na^2/Ns^2)+1, In this equation, it is seen that F is dependent on Ns^2. And, Ns comes from the impedance of Port source. So I think N is dependent on the impedance of Port source.

    By the way, No, Nl, Ns and Na are all represented in V/sqrt(Hz).

    if there is anything wrong, please point it out. Thanks a lot!

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