tm123
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Chicago, IL
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Aaron,
I agree with Raj that a divider can be used to scale a VCO frequency down to measure its noise. However, remember that the phase noise at the output of the divider will be the following: PN_VCO(dBc/Hz)-10*log(N^2)+PN_DIV(dBc/Hz) where PN_VCO is your VCO output phase noise, N is your divider ratio, and PN_DIV is the output phase noise of the divider itself. For example, if you use a divide by 2, N=2 and the VCO phase noise will be 10*log(4)=6dB lower at the divider output. It is important to note the PN_DIV because that will probably set the phase noise floor at offset frequencies far away from the carrier and you will be measuring the divider phase noise not the VCO phase noise. I think the noise floor of the spectrum analyzer would have the same effect as output phase noise of the divider, it will eventually set the phase noise floor at offset frequencies far from the carrier. Of course, 'far from the carrier' is a relative term based on how good your VCO is and how bad the divider/spectrum analyzer are.
Hope this helps.
Tim
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