Cadence: VCO Design Using SpectreRF

I was looking for a way I could plot the output frequency of a ring oscillator versus an input tuning voltage.   I did not find anything easy and fast in the literature but I think PSS is the key for the more formal methods.  PSS = Periodic Steady State.  I will look at this later most likely for other applications.  To see what I did do for a useful plot of F versus Vtuning see below.

Research Links

• SpectreRF Workshop: VCO Design Using SpectreRF
• Google: cadence pss analysis tutorial

The current consumption of the ring oscillator is more or less proportional to frequency and thus can be used as a plot of frequency versus voltage.  The tuning voltage goes into a gate of an mosfet.  You can see most of the tuning action occurs just below the Vt of that mosfet since the fet is being used with such low currents.

Data Points

• 500 kHz @ 69 nAmp
• 200 kHz @ 31 nAmp

Assuming current consumption directly proportional to frequency gives 0.000014 nAmp / Hertz.  This checks out.