[help]about VCO centre frequency unstable

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hcseu

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recently I am measureing a CMOS integrated VCO corss couple topology
but I find the centre frequency is unstable
about 100KHz @400Mhz
can anyone tell me what cause the frequency unstable and how to reduce the unstabe freqency
 

The free running frequency of the cross-coupled oscillator depends mostly by the current flowing through the coupling transistors. Also the unknown parasitics of LC tank can affect the stability.
 

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