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# A question about phase noise

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#### walkingsun

##### Member level 2
There is an buffered oscillator.

If the phase nosie of oscillator is -140dBc and the phase noise of buffer is -140dBc,

the total phase noise is -134dBc?

Thank you

hmm, I would guess -137 dBc because the two sources aren't correlated.....

But how to define the phase noise spec of a buffer?

good question and one I don't really know the answer to (it is complex for sure) -- and I probably shouldn't have thrown out a guess so quickly.

There are a number of papers from the IEEE-UFFC society that discuss additive phase noise from buffers and such. I would research there.

Lance

the phase nose after the buffer can estimated as the following:
L(fm)=(P_noise*G+KTB(NF-1)*G)/(P_IN*G)=
=L(fm)_in+(KTB(NF-1))/P_IN
P_IN---->THE INPUT POWER;
P_NOISE------>THE 1 HZ POWER fm offset from the FC
G---->GAIN of the buffer;
NF---->noise figure of the buffer
from the above equation, we totify that the KTB(NF-1) is very smaller than the Pin, so the buffer shall not affect the VCO phase noise;but y shall pay attention to the p_1dB point of the buffer, y shall not let the pin*g is greater than the p_1dB!

Very good question! Just to not start a new thread :

I plan to construct a very low phase noise vcxo but his level out it's arround 0dBm and my mixer stage needs +17dBm. Adding 3dB pad before mixer, I need +20dBm from vcxo.

The phase noise of vcxo itself it's arround -145dBm @ 100Hz so can I preserve this figure up to +20dBm ?

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