cross-coupled oscillator frequency variation

Hello everybody! I'm trying to understand what is going on with this oscillator.
I have a cross coupled oscillator with 2 BJT and a current source to bias transistors. The problem I have is that when I increase or decrease the current of this oscillator the frequency changes. Is it normal?
I know that negative resistance depends on DC bias point but I checked here (http://www.seas.ucla.edu/brweb/teach...2013/VCOs1.pdf) that frequency only should depend on tank resonator components. What am I missing here?

Ty very much!

Yes it's normal..
The frequency of the oscillator is not depended only tank circuit, it's also depended on active part of the oscillator.
Tank circuit is dominant on the frequency but parasitic capacitors of active part also contribute to equivalent capacitance of the oscillator.
So, since these parasitic capacitances are bias depended, it's normal to observe a frequency shift on the oscillator frequency.This has been explained on page 10 in your ref. doc.