Dear asrf, [quote userid="463108" url="~/cadence_technology_forums/f/custom-ic-design/43924/vco-design-problem"][/quote] hen I use this kind of biasing and sweep my ideal capacitor to change the frequency, freq varies but VCO output swing changes as well !! This is not an unusual issue aarf. What you need to consider is that the steady-state amplitude of the VCO is governed by the amplitude that provides a negative real impedance that is equal to the absolute value of the resonator - in your case inductor and varactor. Hence, as you change your ideal capacitor, you are changing its impedance which will require the sustaining amplifer to adjust the amplitude of its waveform. [quote userid="463108" url="~/cadence_technology_forums/f/custom-ic-design/43924/vco-design-problem"][/quote] nd also I dont want to want full swing for this VCO since if I have full swing the Vgd max will be hig h I want to have swing of 1v pp so: VD swing : (0.9 --> 1.9) VG swing : (0.2 --> 1.2) Without an AGC, it will be very hard to set the amplitude to a fixed amplitude (1 V in your case as you state) over all process and environments AND over all frequencies of VCO operation. I hope this brief explanation provide some intuitive understanding of what you are observing aarf, Shawn
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