PLL Lock Range vs. Capture Range: Key Differences Explained

This page explains the basic difference between PLL lock range and capture range.

PLL (Phase Locked Loop) is widely used in communication circuits to select the desired frequency channel. It’s also referred to as a frequency synthesizer.

As shown in the figure below, a PLL consists of a phase detector, VCO (Voltage Controlled Oscillator), and LPF (Low Pass Filter). The PLL can convert an FM modulated signal into its original modulating signal.

The phase detector produces a phase error signal between its two inputs. This phase error signal is passed to the LPF, which generates a DC voltage. This DC voltage is then applied to the VCO, which generates a frequency based on the applied DC voltage.

PLL Free Running Frequency

To understand how a PLL circuit works, let’s consider the condition where no input signal is applied. In this case, the outputs of the phase detector and low-pass filter will be zero. At this time, the VCO operates at its free-running frequency. This is the VCO’s normal operating frequency.

The PLL’s free-running frequency is determined by its internal frequency-determining components.

As explained in the figure, when the frequency changes, the phase detector and LPF will produce a new DC voltage. This voltage forces the VCO frequency to change and adapt to the new input frequency. Hence, the PLL is said to be tracking the input. Any input frequency variation will be matched by the change in the VCO frequency, keeping the PLL locked.

PLL Lock Range

The range of frequencies over which the PLL will track the input frequency signal and remain locked is referred to as the PLL Lock Range. The lock range is typically a band of frequencies above and below the PLL’s free-running frequency.

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

PLL Capture Range

As described earlier, when a frequency within the PLL lock range is applied to the circuit, the circuit will adjust and remain in a locked condition. Any subsequent deviation will be adjusted due to the functionalities of the phase detector, LPF, and VCO.

Once the input signal is captured, the PLL will remain in a locked state and track the changes in the input signal as long as it remains within the lock range.

The range of input frequencies over which the PLL will capture the input signal is referred to as the PLL Capture Range. As shown in the figure, it’s much narrower compared to the PLL lock range.

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

The PLL acts as a band-pass filter, effectively eliminating noise and interference from the input signal, if present.