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It is not clear for me how a mixer generates noise. Lets say that we take an up-converting mixer there is certain white noise level at LO input - \$N_{LO}\$ and a certain level of a white noise at RF input - \$N_{RF}\$. There are also signal levels \$S_{LO}\$ and \$S_{RF}\$. I understand that the signal is given by conversion loss, so \$S_{IF} = S_{RF} G_{loss}\$. What will be the noise of the IF output, is it simply the biggest of RF or LO noise?

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  • \$\begingroup\$ This is a fairly complex issue to treat on this forum in detail. There are "rules of thumb" for those that need simple estimates of the output IF noise figure, and if you require detailed analysis then an academic textbook would be better to consult. So a good compromise could be the application notes on mixers by various mixer manufacturers. A suitable one might be "A Practical Guide to Noise in Frequency Conversions" on the Marki Microwave website... \$\endgroup\$
    – citizen
    Jul 19, 2022 at 8:48
  • \$\begingroup\$ The noise at the IF output will not be the 'biggest of the sources', but the 'sum of the sources'. For sources when only one is dominant, they are approximately the same, but when sources are similar size, especially if there are a lot of them, you need to take the sum. Another useful approximation is that your output noise will be 'at least' or 'not less than' any of the sources you've calculated. \$\endgroup\$
    – Neil_UK
    Jul 19, 2022 at 10:00
  • \$\begingroup\$ @Neil_UK, thank you for your remarks and for the link. There is a problem though that in some simulation software such as for exemple Genesis (vers 2012), it does not take into account the LO noise and it is not clear why. I do understand that physically the output noise should be the sum of RF and LO noise \$\endgroup\$ Jul 19, 2022 at 10:16

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A mixer doesn't necessarily generate noise.

However, there are several ways in which in it can degrade the noise of a system.

  • Via signal attenuation. If the mixer drops the signal level 6 or 7 dB, typical of a standard double balanced diode mixer, then comparing an LNA driven by the input signal with an LNA driven by the output signal, the input LNA can recover a 6 to 7 dB better noise floor.
  • Via leakage from the LO. If an LO has excess noise at the output frequency, and the LO-IF isolation is poor, this noise will appear at the IF output. If the LO has excess noise at the RF frequency and the RF port match is poor, this noise could appear at the RF input.
  • Compression of dynamic range. Strictly speaking, we are never really bothered by noise floor, only dynamic range (though often with LNAs we assume headroom to be infinite). A mixer with a poor distortion performance may force a system designer to reduce signal levels to avoid distortion, thus pushing the small signals down further into the noise.

That said, the components in a mixer will have resistance, which adds Johnson noise.

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