Why does the Vs node always oscillate around the second harmonic in a VCO tail filter?
I suggest that you draw the shape of the drain ID,M1 and ID,M2
currents of M1 and M2 in one plot. Now sum these currents
(ID,M1+ID,M2) to get the &quot;tail current&quot; that flows net marked VS. What
is the repitition rate of this current?
Another hint: This is similar to what happens in a bridge rectifier, a
50 Hz sinewave is rectified, however the ripple on the output voltage
has a frequency of .... Hz? See:
elprocus.com/full-wave-rectifier-circuit-working-theory – Bimpelrekkie

Yes, now it makes sense as we can see that the frequency is twice times the fundamental