aaron_do
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Hi eng,
if you have such a large switch, you may need to worry about loading the VCO, but at 400 MHz it may not be such a problem.
Also, if you only have a 1 V supply, you are bound to have problems with headroom when using so many devices stacked between VDD and ground. I assume you have a tail current, GM transistors, switching transistors and resistor/PMOS loads. The means you have about 0.25 V headroom for each "level".
It seems your Vbias really is too low, otherwise you wouldn't need such large switching transistors.
Anyway just thought i'd throw out the idea of using a passive mixer. You don't need to worry about headroom at all in that case, and I haven't found any really convincing arguements on why an active mixer is better anyway.
cheers, Aaron
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