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Design >> Analog Design >> A question about ring VCO in gm-c filter automatic
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Message started by smellyegg on Mar 6^th, 2007, 10:07pm

Title: A question about ring VCO in gm-c filter automatic
Post by smellyegg on Mar 6^th, 2007, 10:07pm

The following VCO schematic( in the attachment) is used for gm-c filter automatic tuning, however, it doesnot oscillate. According to the theory, the last transconductor is used as a negative resistor, which maintains the oscillation, while the first two transconductors as well as the two capacitors are working as an inductor. When I adjust the value of C1 and C2, the output waveform doesnot change at all, however it should change, as the frequency of the output waveform is determined by gm/c.

Are there some special attentions that should be paid to while designing the three transconductors? Thanks.

Title: Re: A question about ring VCO in gm-c filter autom
Post by fonseca.ha on Mar 8^th, 2007, 8:32am

maybe it needs an initial condition that puts it out of equilibrium, and trigger the oscillation,
I think that both capacitors should have the same value, try bracking it in parts to check where the problem comes from.
If you remove the OTA in the right hand side and put an initial condition on the left circuit you should observe a dumped oscillation, does that happen?

Title: Re: A question about ring VCO in gm-c filter autom
Post by smellyegg on Mar 8^th, 2007, 5:36pm

Thank you very much. Yes, it needs an initial condition which triggers the oscillation. When I set the initial voltage of Vo2 to be 2 volts, it becomes much easier to oscillate. And there is another problem with this circuit, which lies on the common mode feedback circuit of the OTA. The W/L values of the input transistors of the CMFB are set to be too large, and when it is set to be smaller, the oscillation also occurs. Thanks again for your help. May you a nice day.^_^