As a result, the filter output voltage vo increases, and the VCO output frequency
fosc increases until it matches fi, thus keeping the PLL in a locked condition. The
range of frequencies from fi = fmin to fi = fmax where the locked PLL remains in the
locked condition is called the lock range of the PLL. If the PLL is initially locked,
and fi becomes smaller than fmin, or if fi exceeds fmax, the PLL fails to keep fosc equal
to fi, and the PLL becomes unlocked, fosc fi. When the PLL is unlocked, the VCO
oscillates at the frequency fo called the centre frequency, or the free-running
frequency of the VCO. The lock can be established again if the incoming signal
frequency fi gets close enough to fo. The range of frequencies fi = fo – fc to fi = fo + fc
such that the initially unlocked PLL becomes locked is called the capture range of the
PLL. Make sure you understand and can distinguish the definitions of lock range and
capture range! – it will become more obvious when you manage to run the PLL
successfully.