# Mixer noise calculation

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?

• 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... Jul 19, 2022 at 8:48
• 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. Jul 19, 2022 at 10:00
• @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 Jul 19, 2022 at 10:16