ACWWong
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whether 2*flo VCO & div2 is better than quadrature VCO (or flo VCO & phase splitter) depends on the frequency, and the nature of the proposed tank.
for example, if the BOM means off chip L is not acceptable, then the VCO power will be dependant on spiral L losses (Q~3 to 10 in CMOS/RFCMOS). so in this case a VCO running at 2GHz will probably be more power efficient and divide-by-2 might be better than a quadrture 1 GHz VCO (as L value needed at 2GHz is lower than 1GHz and Q is usually better for lower L values at higher frequencies). Even off chip L's only really give good Q's and usuable values @ > 500MHz The situation at mid-GHz points (where Lreq is now only low nH or less) gives bondwires as attractive option, moving the tank Q bottle neck to varactor (Q>20), so i think a 4GHz VCO using bondwires to be better than a 2 Ghz using spiral L from the noise viewpoint, without power penalty.
the fact that 2*flo does mean the direct buffering is more power hungry, but i have found that current is better imployed in the VCO and div2 (ie no buffer required) given that the VCO has large swing and div2 is basically a digital circuit. the only reason a buffer would be required is for isolation, but when vco is at flo*2, this may not be a big issue on RX, and for loIF/ZeroIF fVCO≠flo is a must.
remember also the div2 gives a 6dB noise improvement to the VCO phase noise.
hope my comments give you some idea of some of the trade-offs, but basically i think you should play around with your design kit to deduce the best architecture for your receiver..
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