Common mode oscillation in push-push VCO

Can someone tell me how to understand common mode oscillation in push-push type of oscillator. How can I simulate in ADS and mitigate it? Thanks.

Andrei Grebennikov...Ulrich Rohde
Read their books about oscillators...

no offense but If I had read this in a book, why would I post in this forum. If you know the answer will you be kind enough to share your thoughts. thanks.

I would think that some way of killing the common mode oscillation would either: 1) involve the resonator in such a way that it is very lossy or the wrong phase at the fundamental frequency, but correct at the x2 frequency of the push-push, or 2) kill the gain with either a highpass or lossy mechanism at the lower frequency.

It is not uncommon that oscillators want to oscillate at more than one frequency. Parasitics or bias line resonances often set up oscillations too. You must carefully design the simplest circuit you can.