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Basic difference between PLL lock range and capture range

This page describes basic difference between PLL lock range and capture range. PLL is widely used in communication circuits to select the desired frequency channel. PLL is the short form of Phase Locked Loop. It is also referred as frequency synthesizer. As shown in the fig-1 below, PLL consists of phase detector, VCO and LPF elements.

As shown in the fig-1, PLL is used to convert FM modulated signal into original modulating signal. The phase detector produces phase error signal between the two inputs. This phase error signal is passed to LPF which produces DC voltage. This DC voltage is given to VCO which generates frequency based on DC voltage applied. VCO is the short form of Voltage Controlled Oscillator.

PLL free running frequency

To understand the operation of PLL circuit, initially no input signal is applied. Under this condition, phase detector and Low pass filter outputs will be zero. At this time, VCO operates at free running frequency. This is normal operating frequency of VCO. This PLL free running frequency is determined by its internal frequency determining components.

PLL circuit

As explained in fig.1 when frequency changes, phase detector and LPF will produce new DC voltage. This voltage force VCO frequency to change and adopt to the new input frequency. Hence PLL is said to be tracking the input. Any input frequency variation will be matched by the change in the VCO frequency. Hence PLL circuit remains locked.

PLL Lock range

The range of frequencies over which PLL will track the input frequency signal and remains locked is referred as PLL Lock range. The lock range is usually band of frequencies above and below the PLL free running frequency as described earlier.

PLL lock range and capture range
Fig.1 PLL Lock Range

If the frequency of the input signal is outside the PLL lock range than PLL will not be able to lock. Under this condition, VCO frequency jumps to its fundamental free running frequency.

PLL Capture range

As described earlier when the frequency within PLL lock range is applied to the circuit. The circuit will adjust and will remain in locked condition. Any deviation thereafter will be adjusted due to Phase detector, LPF and VCO functionalities. Once input signal is captured, PLL will remain in locked state and will track the changes in the input signal till it remains within lock range.

The range of input frequencies over which PLL will capture the input signal is referred as PLL capture range. As shown in the fig-2, it is much narrower compare to the PLL lock range.

Both PLL lock range and PLL capture range are centered around the VCO free running frequency.

PLL acts as a Band Pass Filter which is effective in eliminating the noise as well as interference from the input signal if present.

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