PLL not locked - why is it on the edges?

Hi,
Why is that when PLL not locked, you see the frequency tone on the Spectrum jumps to the edges of the frequency range (back and forth)?
I mean, why isn't it stuck at some other frequency?
Thx.

Because when the PLL breaks lock, there is no longer any restorative force to tune the oscillator correctly. The oscillator usually tunes to one of the supply rails.

Because while the PLL seeks a locking point, it is changing Vtune from low to high being as sawtooth waveform.Therefore frequency is sliding from low to high quickly..

BigBoss,
Is Vtune always a sawtooth waveform?
I think that when using active loop filter configuration, Vtune is not sawtooth? isn't it?

When you consider locking mechanism of a PLL, you will see that PLL willl try to find a locking zone ( including dead zone ) but if it can not find it will increase Vtune till a reset arrives and the loop is started up again.