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Noise floor after mixer

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Newbie level 2
Feb 19, 2013
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I have to do some noise floor calculations and don't understand how to do it with mixer in the rf chain.
Lets take an example: The mixer is HMC558LC3B of Hittite. Conversion loss~ 8dB . LO to RF isolation ~ 40dB.
LO data : Power = 15dBm , Noise floor -135dBc/Hz = -120dBm/Hz
IF data : Power = 7dBm, Noise floor -150dBc/Hz = -143dBm/Hz
I have to know the RF Noise floor that i resive after the mixer.
As i see it there are two options :
1) The first one: In this example the IF noise floor is negligible because it is much lower then the LO noise floor and so the RF noise floor is ~ -135dBc/Hz = -136dBm/Hz (after 8dB conversion loss) .
2) The second one : The LO noise floor influences the RF noise floor only after taking into account the LO to RF isolation , and so we should add the IF noise floor after conversion loss -150dBc/Hz =-151dBm/Hz to LO noise floor after isolation -135dBc/Hz = -160dBm/Hz and here -160dBm/Hz < -151dBm/Hz so the noise floor is ~ -150dBc/Hz = -151dBm/Hz.

Maybe i am wrong in both of the options , please help me to decide.
Thank you in advance.


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Sorry if I don't give a direct answer to your question. But looking at the datasheet, with an input P1dB = 10 to 14dBm (the input level at which output is 1dB lower than calculated, due to compression), and with a conversion loss = 7dB + 1dB due to compression, you can't have as high as 7dBm at IF.
Second, I am not well acquainted with noise floor calculations, but I have never seen that a LO noise floor is taken into account. The LO just turns the mixer switches on and off, its noise is not added to the signal as in a linear circuit. What is very taken into account is the phase noise with regards to signals that should fall outside the IF bandpass, but it is a different ball game.
Also remember that in a simple superhet process (not the image rejection one), the noise figure of the receiver is not just the NF of the IF strip plus the mixer loss, there is an additional 3dB because the noise at the image frequency gets also converted to the IF.
Again, I am not giving the answer you requested, but I do hope this can be useful to you.

Thank you very much for your answer.
It was very helpfull.

I use the IF port as the input signal and get output signal at RF port. So i asume that if the P1dB is 10 to 14dBm, there is no problem to use 7dBm signal as IF.

I will take into account the 3dB addition to the noise floor because of the image frequency conversion
and that the LO doesn't influence the output noise floor as i understand from your answer.

Thank you again.

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