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VCO How can I lower Phase Noise?

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flames4791

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I have the VCO schematic above. I need a tuning frequency of 2.35G to 2.55G and phase noise to be below -115dbc at 1MHz. Right now I have the Phase Noise at -111dbc. I read somewhere that you can add a capacitor to the drain of the tail transistor to short the AC noise and then add an inductor in series with the transistor and resonate it at 2f0. However, when I tried this it actually made the noise worse. Are there other methods of improving noise? I have minimized the two oscillator transistors already. Any suggestions would be greatly appreciated. Thanks!
 

Anyone have tips on whether nmoscap or pmoscaps have less phase noise and should i use normal nmos or lvt or hvt transistors for the oscillator?
 

having high Q main resonant tank will help reducing phase npise.
 

You can check Leeson formula for phase noise or you can read about the linear time variant theory of phase noise published by Thomas Lee and Ali Hajimiri. Otherwise, you can better the phase noise increasing the Q of your resonant LC circuit, increasing the power dissipation or adjusting the size of your transistors to reduce thermal noise as you did. Maybe it is not a great practice, but you can use the parasitic capacitance of the inductors to tune the frequency of oscillation without the use of capacitors, in order to reduce thermal noise and increasing the Qloaded. Hope it will help
 

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